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What is the function of multiple nuclei in syncytial cells?

What is the function of multiple nuclei in syncytial cells?


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What is the function of multiple nuclei in syncytial cells specially in protists with cilia? Is multiple nuclei a special characteristics of only ciliated cells?


A mononucleate's size is limited by its nucleocytoplasmic ratio, and protozoa like being big.

The solution? Multiple copies of the same genes in huge nuclei, or multiple individual nuclei.

Multiple nuclei are common in any large, complex cell. There is nothing specific to the presence of cilia that require multiple nuclei, and ciliates are not the only multinucleate protozoa (the amoeboid Chaos genus can contain up to a thousand). The reason large, complex cells require this is simply because they are so big. Such a huge size requires, in addition to more complex nuclear regulation, a high total protein turnover, which in turn requires a high transcription rate. Even with mRNA being translated by numerous ribosomes, the amplification factor is still limited. Having multiple nuclei increases the rate of mRNA production, which increases the total synthetic capacity of the organism. When only a specific gene is required to be transcribed at a high rate, that individual gene may be duplicated many times. This is often seen in the nucleolus, which contains many copies of the same genetic material required to create the huge amount rRNA needed for ribosome biosynthesis:

Many protozoa are large because they are packing as much complexity into a single cell as they can, even to the point of having a backbone-like structure or simple photosensitive "eyes" (though not all protozoa are large). Many adaptions are required to allow single-celled organisms to grow to this size, such as a more complex cytoskeleton, a dedicated feeding apparatus, a contractile vacuole, etc. Multiple copies of genetic material is just another required adaptation. This adaption can manifest itself in various ways. Some organisms have multiple copies of the same nucleus. Others fuse these nuclei into a single, large nucleus (a macronucleus), keeping a limited number of micronuclei around for sexual reproduction:

Multicellular organisms tend to have multiple nuclei in specific cells for the same reason. Many types of cells, for example, require an extremely high rate of protein turnover and are very large (such as skeletal muscles), so they have multiple nuclei. This occurs when a single nucleus is no longer sufficient, even at peak capacity.

I would like to point out that what you are describing is a coenocyte. A syncytium is a multinucleated cell which has arisen from the fusion of multiple cells and their constituent nuclei (karyogamy), whereas a coenocyte is a cell which had undergone multiple rounds of nuclear division, without accompanying cytokinesis.


Regarding your second question:

Multiple nuclei in a single "cell" is not exclusive to ciliated cells.

Muscle fibers of animals or placenta of mammals undergo cell fusion, resulting in multiple nuclei in one "cell".

Embryos of some animals start with nuclei division without cell division, also resulting (temporarily) in multiple nuclei in one "cell".

Other examples are provided here: https://en.wikipedia.org/wiki/Syncytium


Coenocyte

A coenocyte ( English: / ˈ s iː n ə s aɪ t / ) is a multinucleate cell which can result from multiple nuclear divisions without their accompanying cytokinesis, in contrast to a syncytium, which results from cellular aggregation followed by dissolution of the cell membranes inside the mass. [1] The word syncytium in animal embryology is used to refer to the coenocytic blastoderm of invertebrates. [2] A coenocytic cell is referred to as a coenobium (plural coenobia), and most coenobia are composed of a distinct number of cells, often as a multiple of two (4, 8, etc.). [3]

Research suggests that coenobium formation may be a defense against grazing in some species. [4]


Cell (biology)

Cells can be separated into prokaryotic and eukaryotic categories. Eukaryotic cells contain a nucleus. They comprise protists (single-celled organisms), fungi, plants, and animals, and are generally 5� micrometers in linear dimension. Prokaryotic cells contain no nucleus, are relatively small (1� μm in diameter), and have a simple internal structure. They include two classes of bacteria: eubacteria (including photosynthetic organisms, or cyanobacteria), which are common bacteria inhabiting soil, water, and larger organisms and archaebacteria, which grow under unusual conditions. See Eukaryotae, Prokaryotae

Prokaryotic (bacterial) cells

All eubacteria have an inner (plasma) membrane which serves as a semipermeable barrier allowing small nonpolar and polar molecules such as oxygen, carbon dioxide, and glycerol to diffuse across (down their concentration gradients), but does not allow the diffusion of larger polar molecules (sugars, amino acids, and so on) or inorganic ions such as Na + , K + , Cl - , Ca 2+ (sodium, potassium, chlorine, calcium). The plasma membrane, which is a lipid bilayer, utilizes transmembrane transporter and channel proteins to facilitate the movement of these molecules. Eubacteria can be further separated into two classes based on their ability to retain the dye crystal violet. Gram-positive cells retain the dye their cell surface includes the inner plasma membrane and a cell wall composed of multiple layers of peptidoglycan. Gram-negative bacteria are surrounded by two membranes: the inner (plasma) membrane and an outer membrane that allows the passage of molecules of less than 1000 molecular weight through porin protein channels. Between the inner and outer membranes is the peptidoglycan-rich cell wall and the periplasmic space. See Cell permeability

Eubacteria contain a single circular double-stranded molecule of deoxyribonucleic acid (DNA), or a single chromosome. As prokaryotic cells lack a nucleus, this genomic DNA resides in a central region of the cell called the nucleoid. The bacterial genome contains all the necessary information to maintain the structure and function of the cell.

Many bacteria are able to move from place to place, or are motile. Their motility is based on a helical flagellum composed of interwoven protein called flagellin. The flagellum is attached to the cell surface through a basal body, and propels the bacteria through an aqueous environment by rotating like the propeller on a motor boat. The motor is reversible, allowing the bacteria to move toward chemoattractants and away from chemorepellants.

Eukaryotic cells

In a light microscopic view of a eukaryotic cell, a plasma membrane can be seen which defines the outer boundaries of the cell, surrounding the cell's protoplasm or contents. The protoplasm includes the nucleus, where the cell's DNA is compartmentalized, and the remaining contents of the cell (the cytoplasm). The eukaryotic cell's organelles include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, cytoskeleton, and plasma membrane (Fig. 1). The organelles occupy approximately half the total volume of the cytoplasm. The remaining compartment of cytoplasm (minus organelles) is referred to as the cytosol or cytoplasmic ground substance. Eukaryotic cells also differ from prokaryotic cells in having a cytoskeleton that gives the cell its shape, its capacity to move, and its ability to transport organelles and vesicles from one part of the cell cytoplasm to another. Eukaryotic cells are generally larger than prokaryotic cells and therefore require a cytoskeleton and membrane skeleton to maintain their shape, which is related to their functions.

Eukaryotic cells contain a large amount of DNA (about a thousandfold more than bacterial cells), only approximately 1% of which encodes protein. The remaining DNA is structural (involved in DNA packaging) or regulatory (helping to switch on and off genes).

Plasma membrane

The plasma membrane serves as a selective permeability barrier between a cell's environment and cytoplasm. The fundamental structure of plasma membranes (as well as organelle membranes) is the lipid bilayer, formed due to the tendency of amphipathic phospholipids to bury their hydrophobic fatty acid tails away from water. Human and animal cell plasma membranes contain a varied composition of phospholipids, cholesterol, and glycolipids. See Cell membranes

Cytoskeleton

The cytoskeleton is involved in establishing cell shape, polarity, and motility, and in directing the movement of organelles within the cell. The cytoskeleton includes microfilaments, microtubules, intermediate filaments, and the two-dimensional membrane skeleton that lines the cytoplasmic surface of cell membranes. See Cytoskeleton

Nucleus

One of the most prominent organelles within a eukaryotic cell is the nucleus. The nuclear compartment is separated from the rest of the cell by a specialized membrane complex built from two distinct lipid bilayers, referred to as the nuclear envelope. However, the interior of the nucleus maintains contact with the cell's cytoplasm via nuclear pores. The primary function of the nucleus is to house the genetic apparatus of the cell this genetic machinery is composed of DNA (arranged in linear units called chromosomes), RNA, and proteins. Nuclear proteins aid in the performance of nuclear functions and include polypeptides that have a direct role in the regulation of gene function and those that give structure to the genetic material. See Cell nucleus

Endoplasmic reticulum

The endoplasmic reticulum is composed of membrane-enclosed flattened sacs or cisternae. The enclosed compartment is called the lumen. The endoplasmic reticulum is morphologically separated into rough (RER) and smooth (SER). PER is studded with ribosomes and SER is not. RER is the site of protein synthesis, while lipids are synthesized in both RER and SER. See Endoplasmic reticulum

Golgi apparatus

The final posttranslational modifications of proteins and glycolipids occur within a series of flattened membranous sacs called the Golgi apparatus. Vesicles which bud from the endoplasmic reticulum fuse with a specialized region of the cis Golgi compartment called the cis Golgi network. In the trans Golgi network, proteins and lipids are sorted into transport vesicles destined for lysosomes, the plasma membrane, or secretion. See Golgi apparatus

Lysosomes

Lysosomes are membrane-bound organelles with a luminal pH of 5.0, filled with acid hydrolyses. Lysosomes are responsible for degrading materials brought into the cell by endocytosis or phagocytosis, or autophagocytosis of spent cellular material. See Endocytosis, Lysosome

Mitochondria

The mitochondrion contains a double membrane: the outer membrane, which contains a channel-forming protein named porin, and an inner membrane, which contains multiple infolds called cristae. The inner membrane, which contains the protein complexes responsible for electron transport and oxidative phosphorylation, is folded into numerous cristae that increase the surface area per volume of this membrane. The transfer of electrons from nicotinamide adenine dinucleotide (NADH) or flavin adenine dinucleotide (FADH2) down the electron transfer chain to oxygen causes protons to be pumped out of the mitochondrial matrix into the intermembrane space. The resulting proton motive force drives the conversion of ADP plus inorganic orthophosphate (Pi) to ATP by the enzyme ATP synthetase. See Mitochondria

Peroxisomes

Within the peroxisome, hydrogen atoms are removed from organic substrates and hydrogen peroxide is formed. The enzyme catalase can then utilize the hydrogen peroxide to oxidize substrates such as alcohols, formaldehydes, and formic acid in detoxifying reactions. See Peroxisome

Plant cells

Plant cells are distinguished from other eukaryotic cells by various features. Outside their plasma membrane, plant cells have an extremely rigid cell wall. This cell wall is composed of cellulose and other polymers and is distinct in composition from the cell walls found in fungi or bacterial cells. The plant cell wall expands during cell growth, and a new cell wall partition is created between the two daughter cells during cell division. Similar cell walls are not observed in animal cells (Fig. 2).

Most plant cells contain membrane-encapsulated vacuoles as major components of their cytoplasm. These vacuoles contain water, sucrose, ions, nitrogen-containing compounds formed by nitrogen fixation, and waste products.

Chloroplasts are the other major organelle in plant cells that is not found in other eukaryotic cells. Like mitochondria, they are constantly in motion within the cytoplasm. One of the pigments found in chloroplasts is chlorophyll, which is the molecule that absorbs light and gives the green coloration to the chloroplast. Chloroplasts, like mitochondria, have an outer and inner membrane. Within the matrix of the chloroplast there is an intricate internal membrane system. The internal membranes are made up of flattened interconnected vesicles that take on a disc-like structure (thylakoid vesicles). The thylakoid vesicles are stacked to form structures called grana, which are separated by a space called the stroma. Within the stroma, carbon dioxide (CO2) fixation occurs, in which carbon dioxide is converted to various intermediates during the production of sugars. Chlorophyll is found within the thylakoid vesicles it absorbs light and, with the involvement of other pigments and enzymes, generates ATP during photosynthesis. See Plant cell

an elementary living system capable of independent existence and self-replication and development the basis of the structure and life processes of all animals and plants.

Cells exist both as independent organisms (protozoans) and as component elements of multicellular organisms (tissue cells). The term &ldquocell&rdquo was proposed by the English microscopist R. Hooke in 1665. The cell is the object of study of a special branch of biology called cytology. Systematic study of the cell did not begin until the 19th century one of the most important scientific generalizations of that time was the cell theory, which asserted the structural unity of all living things. The study of life at the cellular level is the basis of contemporary biological research.

The structure and functions of each cell show certain characteristics common to all cells, reflecting a common origin from primary organic complexes. The specific features of various cells are the result of specialization in the process of evolution. All cells similarly regulate metabolism, duplicate and use hereditary material, and acquire and utilize energy. At the same time, various unicellular organisms (amoebas, infusorians) differ sharply in terms of size, shape, and behavior. No less diverse are the cells of multicellular organisms. For example, in man, lymphoid cells are small (about 10 microns in diameter) and rounded and participate in immunological reactions, whereas some nerve cells have appendages more than a meter long and perform the body&rsquos basic regulatory functions.

Research methods. The first cytological method was the use of microscopy with living cells. Present-day variants of light microscopy, such as phase-contrast, luminescent, and interference microscopy, make it possible to study the shape of the cell, the general structure of some of its elements, the movement of cells, and cell division. The details of cell structure can be revealed only after special contrasting, which is achieved by staining the killed cell.

A new stage in the study of cell structure is the use of the electron microscope, which has the advantage of considerably greater resolution of cell structures than the light microscope. The chemical composition of the cell is studied by cytochemical and histochemical methods, making it possible to reveal the localization and concentration of various substances in cellular structures, the intensity of synthesis of these substances, and the movement of the substances within the cell. Cytophysiological methods make it possible to study certain functions of the cell, such as excitation and secretion.

General properties. Every cell has two principal parts&mdashthe nucleus and the cytoplasm. Within each of these it is possible to distinguish certain substructures, which differ in shape, size, internal structure, chemical properties, and function. Some of these, called organoids, are vital to the cell and in fact are found in all cells. Others are the products of cell activity and represent temporary formations. The segregation of various biochemical functions takes place by means of these specialized structures, facilitating the accomplishment within the same cell of diverse processes, including the synthesis and decomposition of many substances.

Genetic information on the structure of the proteins characteristic of an organism of a particular species is stored in DNA (deoxyribonucleic acid), the principal component of nuclear organoids called chromosomes. Another, most important property of DNA is its capacity for self-replication, which ensures both the stability of the hereditary information and its continuity&mdash that is, its transmission to succeeding generations.

Ribonucleic acids, the immediate participants in protein synthesis, are synthesized on limited sections of the DNA molecule (embracing several genes), which act as templates. The transcription of the DNA code is accomplished by the synthesis of messenger RNA (m-RNA). Protein synthesis is, in effect, a reading of information from the RNA template. This process, called translation, involves the participation of transfer RNA (t-RNA) and special organoids, called ribosomes, that are formed in the nucleolus. The size of the nucleolus is a function chiefly of the cell&rsquos ribosome requirement the nucleolus is, therefore, especially large in a cell that intensively synthesizes proteins. Protein synthesis, the end result of chromosome function, occurs mainly in the cytoplasm.

In the final analysis, proteins (comprising enzymes, components of cell structures, and the regulators of various processes, including transcription itself) determine all aspects of cell life, permitting the cell to preserve its individuality despite its constantly changing surroundings. Whereas the bacterial cell may synthesize about 1,000 different proteins, almost every human cell synthesizes more than 10,000. Thus, the diversity of intracellular processes increases substantially in the course of evolution.

The sheath surrounding the nucleus, which separates the nuclear content from the cytoplasm, actually consists of two membranes, both of which are perforated by pores&mdashspecialized areas for the transport of certain compounds from the nucleus into the cytoplasm and vice versa. Other substances pass through the membranes by means of diffusion or active transport, the latter process requiring the expenditure of energy.

Many processes occur in the cytoplasm with the participation of the membranes of the endoplasmic reticulum (the principal synthesizing system of the cell), the Golgi apparatus, and the mitochondria. The differences in the membranes of various organoids are determined by the properties of the proteins and lipids that form them. Ribosomes are attached to some of the membranes of the endoplasmic reticulum. These are the site of intensive protein synthesis. This type of granular endoplasmic reticulum is especially well developed in secretory cells or in cells that intensively renew proteins, such as human liver cells, pancreas cells, and nerves. Other biological membranes that are lacking in ribosomes (smooth reticulum) are made up in part of enzymes that participate in the synthesis of carbohydrate-protein and lipid complexes. The products of cell activity may accumulate temporarily in the canals of the endoplasmic reticulum in some cells the substances are actually transported through these canals.

Before being carried out of the cell, substances are concentrated in the lamellar Golgi apparatus, which isolates various cell inclusions, such as secretory or pigment granules, and in which lysosomes are formed (sacs containing hydrolytic enzymes and participating in the intracellular digestion of many substances). The system of canals, vacuoles, and sacs, all surrounded by membranes, acts as an integrated unit the endoplasmic reticulum can, without interruption, connect to the membranes surrounding the nucleus, unite with the cytoplasmic membrane, and form the Golgi complex. However, these connections are not stable. Often (in many cells, usually), the various membranous structures are discrete and exchange substances through the hyaloplasm.

Cell energetics depends to a large extent on the work of the mitochondria. The number of mitochondria varies in different types of cells from dozens to several thousand. For example, there are about 2,000 mitochondria in the human liver cell, and their total volume is no less than one-fifth the volume of the cell. The outer membrane of the mitochondrion separates it from the cytoplasm. The basic energy conversions of substances occur on the inner membrane. A result of these conversions is the formation of a compound rich in energy&mdashadenosine triphosphoric acid (ATP)&mdashthe universal carrier of energy in the cell. Mitochondria contain DNA and are capable of self-replication. However, the autonomy of the mitochondria is relative: their reproduction and activities depend on the nucleus. Various syntheses, the transport and excretion of substances, mechanical work, and the regulation of processes in the cell are performed using the energy of ATP.

Certain structures that look like tiny (submicroscopic) tubules participate in cell division and, sometimes, in cell movement. The assembling and functioning of such structures depend on the centrioles. The spindle, operative in cell division, is organized with the participation of the centriole. The spindle, in turn, participates in the translocation of chromosomes and the orientation of the axis of cell division. The basal bodies, derivatives of the centrioles, are necessary for the construction and normal functioning of flagella and cilia, locomotor and sensory formations of the cell whose structure is the same in protozoans and various metazoan cells.

The cell is separated from the extracellular medium by the plasma membrane, through which ions and molecules enter the cell and are excreted from it. The ratio of the cell&rsquos surface to its volume decreases as cell volume increases the larger the cell, the more difficult are its connections with the external medium. The size of a cell, therefore, is necessarily limited.

Active ion transfer, which requires the expenditure of energy and requires special enzymes and, possibly, carrier agents, is characteristic of living cells. Because of the active and selective transfer of certain ions into the cell and the continuous removal of others from it, a difference is created between the ion concentration within the cell and that of the surrounding medium. This effect may be caused by the bonding of ions with cell components. Many ions are necessary as activators of intracellular syntheses and as stabilizers of the structure of the organoids. Reversible changes in the ratio of the ions within the cell to those in the medium are the basis of the cell&rsquos bioelectric activity&mdashone of the most important factors in the transmission of signals from one cell to another.

The plasma membrane, by forming invaginations which then close up and break off as bubbles within the cell, is capable of capturing solutions of large molecules (pinocytosis) or even certain particles with diameters to several microns (phagocytosis). It is in this way that some cells are nourished, that some substances are transferred through the cells, and that bacteria are captured by phagocytes. The cohesive forces that in many cases keep cells close to one another (for example, the integuments of the body or internal organs) are also associated with the properties of the plasma membrane. Cellular cohesion and intercommunication are ensured by the chemical interactions of the membranes and by special structures of the membrane called desmosomes.

The structural scheme of cell, discussed in its general form, is characteristic in its basic features of both animal cells and plant cells. However, there are essential differences in the characteristics of the metabolism and structure of plant cells and animal cells.

Plant cells. As a rule, the plasma membranes of plant cells are covered with a rigid outer sheath (possibly absent only in the germ cells) consisting in the majority of plants mainly of polysaccharides: cellulose, pectic substances, hemicelluloses, and, in fungi and some algae, chitin. These cell walls are supplied with pores through which, by means of cytoplasmic outgrowths, neighboring cells can communicate with one another. The composition and structure of the wall change with the growth and development of the cell. In cells that have ceased growing, the envelope often becomes impregnated with lignin, silica, or other toughening matter.

The cell walls determine the mechanical properties of the plant. The cells of certain plant tissues have especially thick and sturdy walls that retain their skeletal functions even after the death of the cell (wood). Differentiated plant cells have several vacuoles or a single central vacuole that usually occupies the greater part of the cell&rsquos volume. The vacuoles contain a solution of various salts, carbohydrates, organic acids, alkaloids, amino acids, and proteins, as well as a reserve of water. Nutritive substances may also be deposited in the vacuoles. The cytoplasm of plant cells contains special organoids, or plastids, including leucoplasts (in which starch is often deposited), chloroplasts (containing predominantly chlorophyll and responsible for photosynthesis), and chromoplasts (containing carotenoid pigments). Both plastids and mitochondria are capable of self-replication. The Golgi apparatus in plant cells is represented by dictyosomes, which are dispersed throughout the cytoplasm.

Unicellular organisms. The structure and functions of unicellular organisms, or protozoans, combine the features characteristic of any cell with features of independent organisms. Thus, protozoans have the same set of organoids as do the cells of metazoans, the ultrastructure of their organoids is identical, and typical chromosomes are found when protozoans divide. However, the adaptation of protozoans to a variety of modes of life (aquatic or terrestrial, free-living or parasitic) has made for considerable diversity in their structure and physiology. Many protozoans, such as flagellates and infusorians, have a complex motor apparatus and organelles associated with the capture of food and digestion. The study of protozoans is of great interest in uncovering the phylogenetic possibilities of the cell, since evolutionary changes in the organism occur on the cellular level.

Unlike protozoans and the cells of metazoans, bacteria, blue-green algae, and actinomycetes do not have a formed nucleus and chromosomes. Their genetic apparatus, called a nucleoid, is represented by threads of DNA and is not enclosed in a membrane. Even more different from the cells of metazoans and protozoans are the viruses, in which the basic enzymes necessary for metabolism are absent. For this reason viruses can grow and reproduce only by penetrating other cells and making use of their enzyme systems.

Special functions. During the course of metazoan evolution there arose a division of functions between cells, which in turn led to a broadening of the adaptive possibilities of animals and plants to changing environmental conditions. The genetically fixed differences in the shape, size, and aspects of the metabolism of cells are realized during the process of individual development. The principal manifestation of development is cell differentiation&mdashthe structural and functional specialization of the cell. Differentiated cells have the same set of chromosomes as the fertilized egg cell this is proved by the fact that after the nucleus of a differentiated cell is transplanted into an egg cell from which the nucleus has been removed, a complete organism is able to develop. Thus, the differences between differentiated cells are apparently determined by various interrelationships between active and inactive genes, each of which codes the biosynthesis of a particular protein.

Judging by the composition of these proteins, only a small proportion (about 10 percent) of the genes characteristic of the cells of a given species are active (that is, capable of transcription) in differentiated cells. Of these genes, only a few are responsible for the specialized function of the cell, while the rest provide for general cellular functions. For example, in muscle cells, genes are active that code the structure of contractile proteins, and in erythroid cells, genes are active that code the biosynthesis of hemoglobin. However, in every cell the genes that determine the biosynthesis of the substances and structures necessary to all cells, such as the enzymes that participate in energy conversions, must also be active. In the process of cell specialization, certain general cellular functions may develop especially strongly. For example, in gland cells, synthetic activity is most strongly expressed muscle cells are the most contractile nerve cells are the most excitable. Narrowly specialized cells contain structures that are peculiar to themselves (for example, in animals, the myofibrils of muscles, the tonofibrils and cilia of certain integumentary cells, and the neurofibrils of nerve cells in protozoans or the spermatozoa of metazoans, flagella). Sometimes specialization is accompanied by the loss of certain properties (for example, nerve cells lose their capacity to reproduce the mature cell nuclei of mammalian intestinal epithelium cannot synthesize RNA the mature erythrocytes of mammals lack nuclei).

The performance of functions important to the organism sometimes includes the death of the cell. Thus, cells of the epidermis gradually cornify and die but remain for some time in a thin layer protecting the underlying tissues from damage and infection. Cells in the sebaceous glands gradually turn into drops of fat, which is then either utilized by the body or excreted.

Noncellular structures are also formed by the cell in order to perform certain tissue functions. The principal means of their formation are secretion or the transformation of cytoplasmic components. For example, a large proportion of the volume of subcutaneous tissue, cartilage, and bone is interstitial matter&mdash a derivative of connective-tissue cells. Blood cells are found in a fluid medium (blood plasma) that contains proteins, sugars, and other substances produced by various cells of the body. Epithelial cells, which form a sheet, are surrounded by a thin interlayer of diffusely distributed substances. Chief among these are glycoproteins (so-called cement, or supramembranal component). The outer coverings of arthropods and the shells of mollusks are also products of cellular secretion.The interaction of specialized cells is a necessary condition for the life of the organism and often for the cells themselves. Deprived of communication with one another (for example, in culture), cells rapidly lose their characteristic features of specialized function.

Cell division. The basis of the cell&rsquos capacity to reproduce itself is the unique property of DNA to copy itself and the strictly equal division of the reproduced chromosomes in the process of mitosis. The result of division is the formation of two cells that are identical to the original in genetic properties and contain both nucleus and cytoplasm. The processes of chromosomal self-replication and division, formation of two nuclei, and cytoplasmic division are distributed over time in sum, they constitute the mitotic cycle of the cell. If the cell begins to prepare for the next division immediately after dividing, the mitotic cycle coincides with the life cycle of the cell. However, in many cases, after division (and sometimes before it) the cell emerges from the mitotic cycle, becomes differentiated, and fulfills some special function in the body. The contents of such cells may be renewed at the expense of the divisions of only slightly differentiated cells. In some tissues, differentiated cells too are capable of repeatedly entering the mitotic cycle. In nerve tissue, differentiated cells do not divide many of them live throughout the life of the whole organism (that is, in man, several decades). Nevertheless, the nuclei of the nerve cells do not lose their capacity for division transplanted into the cytoplasm of cancerous cells, they synthesize DNA and divide. Experiments with hybrid cells show the influence of the cytoplasm on the manifestation of nuclear functions.

Imperfect preparation for division prevents mitosis or distorts its course. Thus, in some cases, cytoplasmic division fails to take place and a binuclear cell is formed. The outcome of repeated nuclear divisions in a nondividing cell is the appearance of multinuclear cells or complex supracellular structures, or symplasts (for example, in striated muscle). Sometimes reproduction of the cell is limited to reproduction of the chromosomes, and a polyploid cell forms with a double set (compared to the parent cell) of chromosomes. Polyploidy leads to the intensification of synthetic activity and to an increase in the size and mass of the cell.

Cell renewal. In order to work properly over an extended period, every cell must have its worn-out structures restored and the externally induced damages to it eliminated. The restorative processes characteristic of all cells are associated with changes in the permeability of the plasma membrane and are accompanied by the intensification of intracellular synthesis (most importantly, of proteins). In many tissues, stimulation of the renewal processes leads to reproduction of the genetic apparatus and division of the cell this is characteristic, for instance, of the integuments or the hemopoietic system. The processes of intracellular renewal in these tissues are weakly expressed, and their cells live a comparatively short time (for example, cells of the intestinal integument of mammals live only a few days). Intracellular renewal processes attain their maximum expression in nondividing or slowly dividing cell populations, such as nerves. The indicator of the perfection of the internal renewal processes of the cell is the cell&rsquos life-span for many nerve cells this coincides with the life-span of the organism itself.

Mutations. The process of DNA replication usually occurs without deviation and the genetic code remains constant, ensuring the synthesis of the same set of proteins in an enormous number of cell generations. However, in rare instances, mutation may occur&mdashthat is, a change in part of the structure of the gene. The ultimate effect of mutation is a change in the properties of the proteins coded by the mutant genes. If important enzyme systems are affected, the properties of the cell (sometimes of the entire organism) may be changed substantially. For example, mutuation of one of the genes controlling hemoglobin synthesis leads to anemia, a serious condition. Natural selection of useful mutations, on the other hand, is an important mechanism of evolution.

Regulation of cell function. The principal regulatory mechanism of the intracellular processes is associated with various influences on enzymes. Enzymes are highly specific catalysts of biochemical reactions. Regulation may be directly genetic, the composition of the enzymes or the quantity of a given enzyme in the cell being predetermined. In the case of enzyme quantity, regulation may also occur on the level of translation. Another type of regulation is influence on the enzyme itself, as a result of which the enzyme&rsquos activity may be either inhibited or stimulated. Regulation on the structural level influences the assembly of cellular structures (membranes, ribosomes, and so forth). More concrete regulators of intracellular processes include nervous influences, hormones, special substances produced within the cell or by surrounding cells (especially proteins), or the products of the intracellular reactions themselves. In this last case, the effect is accomplished by a feedback principle in which the product of a reaction influences the activity of the enzyme that is the catalyst of the initial reaction. Other regulatory mechanisms include the transport of precursors and ions, influences on template synthesis (RNA, polysomes, synthesis enzymes), and alteration of the form of a regulated enzyme.

The organization and regulation of cell function on the molecular level determine such properties of living systems as spatial compactness and economy of energy. An important property of multicellular organisms&mdashreliability&mdashdepends in great part on the number (interchangeability) of cells of each functional type and on the possibility of their replacement through cellular reproduction and renewal of the components of each cell.

In medicine, changes can be induced in the cell to treat and prevent various diseases. Many medicinal substances change the activity of certain cells. Narcotics, tranquilizers, and analgesics lower the intensity of neural activity stimulants increase it. Some substances stimulate contraction of the muscle cells of the blood vessels. Others stimulate a similar reaction in the muscles of the uterus or heart. Dividing cells can be affected by radiation or cytostatic substances (blocking cell division). Immunization stimulates the activity of the lymphoid cells, which manufacture antibodies against foreign proteins, and thereby prevents a number of diseases.


Introduction

One of the fundamental challenges of cell biology is to define principles of spatial organization of the cell [1], and, in particular, to unravel the mechanisms that control the position, size, and shape of organelles. The nucleus is the principal organelle and organizational center of eukaryotic cells. In textbooks, it is typically depicted in the middle of the cell however, the nucleus’ actual position, (as in the apical/basal position in developing neuroepithelia [2]), depends on the cell’s migratory state, cell cycle stage, and differentiation status [3]. Proper nuclear position is vital for many cell functions, including spatially correct cell division and the direction of cell migration [3].

Multinucleation is one mechanism adopted by cells to generate and sustain large cell sizes. Muscle cells are one of the largest cell types, which are formed by fusion of mononucleated myoblasts and contain up to several tens (invertebrates) to several hundred (vertebrates) nuclei. Myonuclei are typically positioned at the cell’s periphery, and are distributed to maximize internuclear distance. However, in muscles undergoing repair, they are found towards the cell center, and in muscle diseases known as Centronuclear Myopathies, myonuclei are also found to be mispositioned [4, 5]. It has been argued [6], that correct positioning of myonuclei is not only an indicator, but also a cause of muscle diseases. A possible mechanism is provided by the Myonuclear Domain Hypothesis [7, 8], which suggests that each nucleus caters for a particular domain of the cell by making the gene products locally needed. Mispositioned nuclei would consequently not be able to guarantee the correct supply of products to their cytoplasmic domains, affecting muscle function.

In this work, we focus on nuclear positioning mechanisms in multinucleated muscle fibers. Drosophila is a good in vivo model system for investigating muscle development, growth, and homeostasis [9–11], due to the simplicity of its muscle pattern, the ease of genetic manipulation, and the homology of relevant genes and processes to mammalian muscle. Nuclei in newly fused Drosophila embryonic muscle cells undergo an orchestrated series of movements, best described in lateral transverse muscles: after fusion of the myoblasts, the resulting muscle cell is thought to disassemble its centrosomes and redistribute γ-tubulin around each nuclear envelope. The myonuclei initially form a cluster close to the cell center. The cluster splits into two subclusters that then migrate towards the opposing cell poles. Subsequently both clusters break apart, and the nuclei spread out evenly along the cell long axis [12, 13]. As the nuclei spread in the muscle cell, sarcomeres, the fundamental contractile units in muscle, form into myofibrils within each cell, and, at the end of embryogenesis, the nuclei become positioned along the long axis of the cell at its periphery, thereby maximizing internuclear distance. During the subsequent larval stages of development, the muscle cells grow 20-40 fold over the course of 5 days without the addition of new myonuclei [14]. Nevertheless, the myonuclei remain appropriately positioned along the cell, although the mechanisms that are responsible for this are not clear.

While the actomyosin network may be involved in nuclear positioning [15], microtubules (MTs), MAPs (MT Associated Proteins), and MT-based motors, such as kinesin and dynein, have been shown to play a major role [12, 16–18]. As examples, in embryos in which MTs are severed in the muscle cell, the central cluster does not split in many motor mutants, nuclear spreading in the muscle cell is perturbed [19]. However, the precise mechanisms controlling myonuclear positioning remain poorly understood.

Modeling has proven to be very useful in complementing cell biological methods in problems of positioning with, for example, the mitotic spindle [20, 21]. Mathematical modeling focused on multinucleated cells and nuclear positioning is in its infancy. Simple conceptual models of nuclei repelling each other were used in [22] and [23] to show that such models can explain regular distribution of nuclei in muscle cells and in the Drosophila blastoderm syncytium. Detailed mechanical simulations were done in [24] to understand multiple nuclear movements in multinucleate fungus Ashbya gossypii.

Here, we use computational modeling to understand the mechanisms regulating nuclear positioning in Drosophila larva muscle cells. We hypothesize that nuclear positioning is a result of a MT-motor based force balance. Rather than assuming the nature of this force balance, we screened multiple computer-generated forces by comparing the spatial nuclear patterns that they predict to quantitative microscopy data from biological specimens. We then simulated a detailed agent-based model to confirm the predictions of the screen. One model explains all biological data, including many subtle patterns of multi-nuclear positioning. Based on this model we propose that myonuclei are positioned by establishing a force balance via MT-mediated repulsion.


Distinct architecture: a survey of tissues with extra genomes across nature

In this section, we present a survey of distinct examples of tissues with extra genomes, from mononucleate polyploid to highly multinucleated syncytia. We also discuss the potential benefits and tradeoffs of extra genomes in a number of tissues. A separate discussion of why tissues with extra genomes use different organizational strategies (e.g., mononucleate vs. multinucleated) is presented later, in the section “Form and Function.”

Progenitor syncytia

During progenitor stages of development, nuclei of germ cells and early embryos frequently share cytoplasm. This sub-section highlights the conservation of syncytia in metazoan germ cell lineages and extra-embryonic cells.

Germline syncytia

Nuclear division followed by incomplete cytokinesis (akin to endomitosis) is a widely conserved route to germline cyst formation in many animal species (Fig. 1 Table1 Fawcett et al. 1959 Hime et al. 1996 de Cuevas et al. 1997 Kloc et al. 2004 Maddox et al. 2005 Kosaka et al. 2007 Marlow and Mullins 2008 Lei and Spradling 2013 Amini et al. 2014). This process creates syncytial cyst structures in the female germline of fruit flies (Drosophila melanogaster), clawed frogs (Xenopus laevis), zebrafish (Danio rerio), and mice (Mus musculus). Within these cysts, organelles and other cytoplasmic materials can be shared (Zamboni and Gonndos 1968 Ruby et al. 1969 Gutzeit 1986 Bolívar et al. 2001 Pepling and Spradling 2001 Cox and Spradling 2003 Kloc et al. 2004 Kosaka et al. 2007 Marlow and Mullins 2008 Lei and Spradling 2013, 2016). Cyst cell number is invariantly 16 germ cells in female (and male) Drosophila germ cysts and can reach up to 25 cells in the mouse ovary (Lei and Spradling 2016, reviewed in Greenspan et al. 2015). Frequently, the oocyte is the only cell to survive through oogenesis, a phenomenon known as a meroistic ovary (McCall and Steller 1998 Foley and Cooley 1998 and reviewed in Lu et al. 2017). In such cases, syncytial organization can serve to nourish the growing oocyte.

However, nourishing of a future gamete cannot be the only function of germline syncytia.

For example, death of cyst cells supporting the future oocyte does not always occur in female syncytial germlines, such as in organisms with panoistic ovaries (reviewed in Lu et al. 2017). Similarly, while incomplete cytokinesis leads to syncytial cysts during male germ cell development in several species including in flies and mice, most nuclei of the cyst survive and eventually individualize into mature sperm (reviewed in Yoshida 2016 Yamashita 2018). As in the female syncytial germline, male syncytial germ nuclei can also share gene products (Braun et al. 1989 Caldwell and Handel 1991 Ventelä et al. 2003 Kaufman et al. 2020). Several models for why cytoplasm is shared in developing germ cysts where each germ cell goes on to become a gamete are outlined nicely in previous reviews (Greenbaum et al. 2011 Lu et al. 2017). Briefly, these include (1) the need to synchronize critical events in gamete production such as the onset of meiosis (which is highly synchronized in males), (2) the ability to neutralize a deleterious mutation in a single germ cell of the cyst that might otherwise outcompete neighboring gametes during fertilization, (3) the sharing of X and Y gene products to make early male gametes phenotypically diploid, and (4) increased sensitivity to DNA damage. More recently, study in C. elegans suggests that a syncytium can also compensate for the mechanical stress of oogenesis (Amini et al. 2014 Priti et al. 2018). Future study can determine the extent to which each of these mechanisms contributes to productive function of germline syncytia.

Extra-embryonic syncytia

A common theme across metazoan evolution is the syncytial nature of cells that support the growth of the embryo. Many insects have a syncytial yolk sac as part of the extra-embryonic tissue during embryogenesis (reviewed in Schmidt-Ott and Kwan 2016). As in germline syncytia, these extra-embryonic syncytia are formed by repeated rounds of nuclear division and incomplete cytokinesis of the yolk sac cytoplasm during blastoderm stages. In some insect species including in Drosophila melanogaster, most of these nuclei then migrate out of the future yolk sac to the embryo surface and contribute to the animal body plan (Campos-Ortega et al. 1997, reviewed in Zissler 1992 Davis and Patel 2002). The syncytial yolk nuclei that remain play important roles in embryonic morphogenesis, in part through maintaining integrin-based adhesion with the surrounding embryo (Reed et al. 2004 Benton et al. 2013). In all teleost fishes, the yolk syncytial layer is an extra-embryonic tissue. This tissue is derived from nuclear divisions with incomplete cytokinesis (Lentz and Trinkaus 1967 Kimmel and Law 1985a, b Chu et al. 2012). The zebrafish yolk syncytial layer, which consists of several hundred nuclei (Carvalho et al. 2009), has numerous roles in early fish development, including nourishing the early embryo (Walzer and Schönenberger 1979a, b) and in signaling events that pattern the embryo (reviewed in Carvalho and Heisenberg 2010 Webb and Miller 2013). Relative to other syncytia, the role of yolk syncytia remains poorly understood. Future studies aimed at specifically disrupting the syncytial nature of the yolk in insect and fish species are needed to understand potential roles of yolk syncytia in embryos.

In many mammals, the extra-embryonic placenta that supports the embryo contains a syncytium. Unlike the examples discussed above in the germline and yolk, cell fusion is the mechanism of syncytium formation in the placenta (reviewed in Gerbaud and Pidoux 2015 Soygur and Sati 2016). In humans, there are three distinctly localized syncytial layers of trophoblast cells in the placenta, each thought to play an endocrine and immune barrier function (reviewed in Turco and Moffett 2019 Roberts et al. 2021). Trophoblast layers may be some of the most extreme examples of syncytia in nature, with up to 6 × 10^10 nuclei estimated in a human syncytial trophoblast (Simpson et al. 1992). In mice, genetic ablation of syncytin molecules, which are endogenous retroviral proteins required for syncytiotrophoblast layer formation, leads to a variety of growth phenotypes, including fetal death (Dupressoir et al. 2009, 2011). Further, altered syncytin expression is reported in human placental pathologies (Chen et al. 2006 Vargas et al. 2011 Soygur et al. 2016). A recent study (Buchrieser et al. 2019) revealed an acute sensitivity of trophoblast syncytium formation to immune signaling through interferons. Upon interferon stimulation in pregnant mice, trophoblast syncytium formation is blocked, leading to halted pregnancy. It is interesting to speculate that the formation of an extraembryonic syncytium provides a developmental checkpoint related to immune sensing. Future work in insect, fish, and mammalian models can further reveal the roles of extraembryonic syncytia, such as whether they allow for more rapid transmission of signals that determine whether embryogenesis should proceed.

Somatic polyploidy

Following germ cell and embryo stages, extra genomes are seen again as specific somatic lineages differentiate. One general theme of somatic cells with extra genomes is that they can enable the growth of much larger cells (Edgar and Orr-Weaver 2001). In general, cell size growth occurs according to nutrient availability and metabolic capacity (as reviewed in Melaragno et al. 1993 Lloyd 2013 Schoenfelder and Fox 2015). It follows then that adding additional genome copies can support larger cells due to increased metabolism and secretion. This sub-section presents a survey of mononuclear or multinuclear somatic polyploidy, focusing on tissue examples that are conserved in animals.

Cardiomyocytes

One of the most conserved examples of polyploidy in a somatic tissue is in the cardiac muscle. Cardiomyocytes in the simple heart tube of Drosophila are polyploid (Yu et al. 2013, 2015). In mammals, cardiomyocyte polyploidy appears to have co-evolved with endothermy (Hirose et al. 2019). Fish (including zebrafish), amphibians, and reptiles have little cardiomyocyte polyploidy (Wills et al. 2008 Hirose et al. 2019). Monotremes such as echidna and platypus have moderate levels of cardiomyocyte polyploidy (40–50% polyploid), while rodents, humans, and other mammals have largely (90% +) polyploid cardiomyocytes (Soonpaa et al. 1996 Bergmann et al. 2015 Hirose et al. 2019).

One potential tradeoff to polyploidy in mammals is a loss of regenerative capacity. The diploid zebrafish heart responds to tissue injury using cell division (hyperplasia) to replace dead or missing cells (Poss et al. 2002), and experimentally increasing cardiomyocyte ploidy in this organism decreases regeneration (Gonzalez-Rosa et al. 2018). Instead, polyploid cardiomyocytes in many mammals respond to tissue damage through scarring and cellular and tissue enlargement through further nuclear content addition (hypertrophy, Meckert et al. 2005 Patterson et al. 2017 Hirose et al. 2019 Derks and Bergmann 2020 Han et al. 2020). The low rate of cell division in mammals accompanies scarring, and the hypertrophy that results instead of cell division is insufficient to replace lost cardiomyocytes (Senyo et al. 2013).

Cardiomyocytes demonstrate a great range of mononucleate and multinucleate polyploidy throughout the animal kingdom. Within the category of the mammalian heart with 90% + polyploid adult cardiomyocytes, the number of nuclei per cardiomyocyte varies greatly. Mouse and rat cardiomyocytes are almost entirely binucleate (2 nuclei each with 2C DNA content, or 2 × 2C) and are formed by endomitosis with incomplete cytokinesis (Soonpaa et al. 1996 Li et al. 1996, 1997a, b Engel et al. 2006 Liu et al. 2010, 2019). Adult human cardiomyocytes contain both mononucleated and binucleated cardiomyocytes with typical ploidies of 4–8C, with a range of diploid, tetraploid, and octoploid DNA content (Brodsky et al. 1992 Bergmann et al. 2009, 2015 Mollova et al. 2013). Pig cardiomyocytes typically contain anywhere between 4 and 16 diploid nuclei arranged in a line (Velayutham et al. 2020). Giraffe cardiomyocytes contain 4–8 diploid nuclei also organized linearly, although 4 nuclei are more common (Ostergaard et al. 2013). Most other mammals studied, including sheep, rat, dog, cat, and cow, have predominantly binucleate cardiomyocytes, with each nucleus containing diploid DNA content (Bensley et al. 2016 Velayutham et al. 2019). Future study is needed on non-humate primates and other large mammals to determine the evolution of mononucleate versus multinucleate cardiomyocyte polyploidy. Why mammalian cardiomyocytes have variable nuclear numbers is unknown (see section “Form and Function”).

In addition to developmental polyploidization, nuclear content is added in response to heart injury and disease. Human cardiomyocytes reach ploidies up to 32C in hearts hypertrophied in response to infarction, congenital heart defects, rheumatic heart diseases, and significant hypertension (Brodsky et al. 1994 Herget et al. 1997 Steinhauser and Lee 2011 Senyo et al. 2013). Interestingly, roughly equal populations of mononucleated and binucleated cardiomyocytes are retained after hypertrophy (Brodsky et al. 1994). This suggests that hypertrophic growth largely occurs through ploidy increase in individual nuclei and not the addition of new cells in humans.

Hepatocytes

Hepatocytes are the major cell type in the vertebrate liver. Like cardiomyocytes, hepatocytes also exhibit species-specific differences in ploidy and nuclear number. Several reviews explore hepatocyte polyploidy in depth (Gentric and Desdouets 2014 Zhang et al. 2019). Hepatocyte polyploidy and multinucleation varies between species. Rat and mouse hepatocytes are

75–90% polyploid, human hepatocytes are

30–40% polyploid, and guinea pig and woodchucks have very low levels of hepatocyte polyploidy and binucleation (Kreutz et al. 2017). Mammalian livers also exhibit mixed populations of binucleate and mononucleate polyploid cells, which makes the liver an intriguing system to study their differences. Within the

30% polyploid segment of human hepatocytes, binucleate (2 × 2C and 2 × 4C) and mononucleate (1 × 4C, 1 × 8C) polyploid cells are found (Kudryavtsev et al. 1993). The mouse liver contains roughly equal fractions of mononucleate diploid, mononucleate polyploid, binucleate, and binucleate polyploid cells, whereas nuclear number beyond 2 is uncommon (Kreutz et al. 2017). Insects such as Drosophila also have hepatocyte-like cells, known as oenocytes, and these cells also are polyploid (Gutierrez et al. 2006 Cinnamon et al. 2016). The vertebrate liver is highly regenerative, and both diploid hepatocyte division and polyploidy-promoting endocycles or endomitosis are capable of full restoration of liver mass (Davoli et al. 2010 Diril et al. 2012 Miyaoka et al. 2012). In contrast, mice with livers that are incapable of diploid hepatocyte division are prone to a diabetic-like phenotype, inflammation, and fibrosis (Dewhurst et al. 2020 Ow et al. 2020). Further, polyploidy can both promote or suppress liver cancer, depending on the context (Zhang et al. 2018 Lin et al. 2021). Therefore, while multiple hepatocyte ploidies and nuclear numbers can compensate for liver injury, the absence of diploidy can impact hepatocyte metabolism and liver health.

Skeletal muscle

Skeletal muscle provides an interesting example of a syncytial tissue that uses often high levels of multinucleation and, in some cases, nuclear content increase to grow and adjust to changing demands. Skeletal muscle consists of long, multinucleated muscle fibers formed through cell–cell fusion events (reviewed in Deng et al. 2017 Petrany and Millay 2019). In the mouse extensor digitorum longus muscle, there are approximately 220 nuclei per myofiber (Hansson et al. 2020). In some organisms, both cell fusion and ploidy increase within individual nuclei contribute to muscle fiber size growth and function (Windner et al. 2019). One potential hypothesis for why skeletal muscle contains numerous nuclei is the myonuclear domain hypothesis (reviewed in Schwartz 2018), which proposes that each nucleus is responsible for supplying its own territory in the syncytial muscle fiber. The usage of many small nuclei minimizes transport distance of nuclear products to the cytoplasmic domain especially when forming a long cylinder as opposed to a sphere.

Drosophila skeletal muscle employs both multinucleation through cell fusion and nuclear scaling (through regulation of ploidal level) to reach a fixed muscle fiber size (Windner et al. 2019). This suggests that there is coordination between nuclei and sensing of global nuclear content within the muscle fiber. Windner et al. propose a model in which nuclear number, ploidy content, and nucleolus size inform and optimize cell metabolism for the desired muscle fiber size (Windner et al. 2019). A potential advantage of nuclear scaling over cell fusion is the lack of reliance on extrinsic myoblast production. Nuclei already incorporated into the fiber can increase in ploidy in response to signaling. Therefore, the relative contributions of cell fusion and polyploidization may be due to myoblast production potential. The use of both cell fusion and nuclear ploidy increase allows for precise tuning of muscle fiber size and metabolism in Drosophila.

In contrast to fly skeletal muscle, mammalian skeletal muscle appears to only grow through multinucleation. A recent study found that while myonuclear number impacts muscle fiber size within a lower range of nuclear number, the impact of additional myonuclei on fiber size diminishes at a higher range of nuclei. Further, fibers with fewer nuclei can adapt by increasing transcriptional output (Cramer et al. 2020). Skeletal myofibers also experience a limited amount of cellular hypertrophy without DNA replication. However, most adaptive growth after exercise or injury occurs via myonuclear accretion (the addition of nuclei, Goh et al. 2019). The diversity of ploidy organization in tissues such as cardiac and skeletal muscle brings to mind the question—when is mononucleate or multinucleate polyploidy advantageous?


Multiple functional domains and complexes of the two nonstructural proteins of human respiratory syncytial virus contribute to interferon suppression and cellular location

Human respiratory syncytial virus (RSV), a major cause of severe respiratory diseases, efficiently suppresses cellular innate immunity, represented by type I interferon (IFN), using its two unique nonstructural proteins, NS1 and NS2. In a search for their mechanism, NS1 was previously shown to decrease levels of TRAF3 and IKKε, whereas NS2 interacted with RIG-I and decreased TRAF3 and STAT2. Here, we report on the interaction, cellular localization, and functional domains of these two proteins. We show that recombinant NS1 and NS2, expressed in lung epithelial A549 cells, can form homo- as well as heteromers. Interestingly, when expressed alone, substantial amounts of NS1 and NS2 localized to the nuclei and to the mitochondria, respectively. However, when coexpressed with NS2, as in RSV infection, NS1 could be detected in the mitochondria as well, suggesting that the NS1-NS2 heteromer localizes to the mitochondria. The C-terminal tetrapeptide sequence, DLNP, common to both NS1 and NS2, was required for some functions, but not all, whereas only the NS1 N-terminal region was important for IKKε reduction. Finally, NS1 and NS2 both interacted specifically with host microtubule-associated protein 1B (MAP1B). The contribution of MAP1B in NS1 function was not tested, but in NS2 it was essential for STAT2 destruction, suggesting a role of the novel DLNP motif in protein-protein interaction and IFN suppression.

Figures

Dissimilar effect of NS1 and…

Dissimilar effect of NS1 and NS2 N-terminal sequences on TRAF3 (A), IKKε (B),…

Luciferase reporter assays of N-terminal…

Luciferase reporter assays of N-terminal NS deletions. (A) IFN activation assay: A549 cells…

(A) Importance of the C-terminal…

(A) Importance of the C-terminal DLNP tetrapeptide of NS2 but not NS1 in…

Formation of homo- and heteromers…

Formation of homo- and heteromers by mutant as well as wild-type NS proteins.…

NS-MAP1B association. A549 cells in…

NS-MAP1B association. A549 cells in 10-cm plates were transfected with 23 μg empty…

MAP1B is required for the…

MAP1B is required for the STAT2-degrading activity of NS2. The indicated range of…

Predominantly nuclear and mitochondrial location,…

Predominantly nuclear and mitochondrial location, respectively, of singly expressed NS2 and NS1. A549…

Modified intracellular location of NS1,…

Modified intracellular location of NS1, when coexpressed with NS2. A549 cells were transfected…

Biochemical confirmation of NS1 and…

Biochemical confirmation of NS1 and NS2 localization in subcellular fractions. Nuclei, mitochondria, and…

Summary model. (A) The various…

Summary model. (A) The various postulated NS species and their intracellular location. Individual…


Spermatozoa

Perhaps one of the best studied cell types with an asymmetric nuclear shape is spermatozoa. There is a dramatic distinction between male and female gametes across metazoa while the ovum is (usually) large, immotile and has a spherical nucleus (e.g., Zuccotti et al. 2005), the spermatozoa are small, highly motile and have an array of shapes. The iconic tadpole-shape is only one of many solutions evolution has crafted in the task of making cells that can swim energetically and carry a streamlined payload of DNA.

During the process of spermiogenesis, histones are replaced with protamines, enabling a greater compaction of the chromatin. The reasons for this extra compaction are debated it likely aids swimming ability, but may also help protect the DNA from damage, and provide an extra level of epigenetic regulation to the paternal genome (Rathke et al. 2014).

As the nucleus compacts, the developing spermatid also sheds most of its cytoplasm. Consequently, the majority of the head is filled by the sperm nucleus, and the shape of the nucleus often closely follows the shape of the sperm head. It seems that the nucleus is an active participant in the development of the final sperm shape—the nucleus condenses and adopts a shape before the cytoplasm is lost and the cell membrane tightens in, rather than the condensing cell squeezing the nucleus into shape see for example the staged spermatids in Russell et al. (1993).

Although all sperm require the ability to swim, it appears there is no single most efficient shape for this. The swimming efficiencies of a given shape also depend on environmental factors, such as the medium through which the sperm will be travelling conditions are quite different for the sperm of sea urchins released into the ocean, to opossum sperm swimming through a viscous fluid and requiring a double-headed sperm to maintain orientation (Moore and Taggart 1995).

Examples of distinctive sperm shapes

Mammalian spermatozoa commonly conform to the stereotypical ‘tadpole’ or ‘paddle’ shape. They possess a head partly covered by an enzyme containing region (the acrosome), a neck, midpiece, a tail of some length, and are dorsoventrally flattened to a degree. However, even within mammals, an assortment of sperm shapes, especially relating to the head, can be observed (examples of sperm head shapes across a variety of taxa are given in Fig. 2) from the ovate-like shape of pig and human sperm, to the falciform sperm head of rodents, the ensiform sperm heads seen in some species of bats (Beguelini et al. 2014), and the more square-headed sperm of orcas and beluga whales (Miller et al. 2002).

A selection of sperm head morphologies from across metazoa acrosomal regions are shaded in grey and nucleus cross-sections denoted by a dashed outline. a The typical ovate or paddle head shape seen in many mammals. b Examples of giant acrosomes (including sagittal cross-sections) and falciform hooks seen in rodents. c Atypical mammalian head shapes. d Examples of morphologies from outside mammalia, including the anomalous sperm head of the Eurasian bullfinch (Pyrrulah pyrrulah), the rounded acrosome-less sperm head of the sea bream (Sparus aurata) and the spiralling acrosome sperm head of the ‘living fossil’, Tubiluchus troglodytes—the nucleus of which also forms a remarkable double spiral in the anterior portion of the sperm head, seen here in cross-section

Sperm heads also vary in the relative shapes and sizes of their functional regions the acrosome is a region located over part of the anterior half of the head, which contains enzymes necessary to engage, disperse and penetrate the strata surrounding the ovum. A number of mammalian species, including the guinea pig (Cavia porcellus) and ground squirrel (Otospermophilus beecheyi) (Fawcett 1970), various species of shrew (Bedford et al. 1994), and the greater bulldog bat (Noctilio leporinus) (Phillips et al. 1997) have been observed to produce sperm with giant, and often curiously shaped, acrosomes. Despite the variation in size and shape, generally, there is a correlation between sperm size, number and fecundity (Gomendio and Roldan 2008).

Sperm shape variation between taxonomic groups

A wealth of scanning electron microscope images of sperm were produced between the 1960s and 1990s. This technique allows for detailed examination of sperm ultrastructure including substructures of the sperm head, such as the nucleus. However, studies comparing sperm shape between species and other taxonomic groups seem surprisingly rare, with work often focused on the detailed examination of the sperm of a single species.

Some studies reveal remarkable examples of outliers in spermatozoan architecture: within passerine birds, the Eurasian bullfinch (Pyrrhula pyrrhula) is identified as an oddity (Birkhead et al. 2007) due to the chunkier tubular shape of its sperm when compared to the typical passerine worm-like, spiralling sperm head shape (see Fig. 2). The greater bulldog bat (Noctilio leporinus) possesses sperm described as ‘unique among mammalian spermatozoa’, owing to their spatulate, ridged and giant sperm acrosome. Unusually, the condensed nucleus only occupies roughly one third of the sperm head (Phillips et al. 1997). The variation of sperm shapes is additionally extraordinary when looking beyond the vertebrates. One remarkable example is the double-helical nucleus of the psudocoelomate worm Tubiluchus troglodytes, around which the acrosome also spirals (Ferraguti and Garbelli 2006).

The origins of sperm head shapes are not always well understood, and with such variation seemingly being generated over a relatively short evolutionary time period, sperm morphology may provide additional clues in the search for the origins and relatedness of even minor taxonomic groups (Rowe et al. 2015). Although uncommon, such detailed and digestible comparisons of sperm shape across taxa include the work of Downing Meisner et al. (2005), who examined sperm from 36 species of aridactylans, perissodactylans and cetaceans and outlined the somewhat subtle variation within the broadly elongate ovate sperm morph of these groups. Other comparisons include fish (Jamieson and Leung 1991), Asian rodents (Breed and Yong 1986 Breed and Musser 1991), and passerine birds (Birkhead et al. 2006) but whilst these works focus primarily on the potential of sperm shape for use in attaining phylogenetic clarity, it is often beyond their scope to do anything more than postulate the origin and functional relevance of nuclear shape variations within these groups, or to consider the processes by which these shapes arise.

Influences on sperm shape

Mating and post-copulatory preferences exert pressure on sperm. Within promiscuous species and species with sperm-storing females, sperm may compete directly with rival male sperm as well as sperm from the same ejaculate. In the eusocial naked mole rat, reproduction is restricted to one dominant female queen and a single breeding male, suggesting limited or no sperm competition between males. Extreme sperm polymorphism is seen within a single ejaculate, including lobed, compressed, double-headed, and miniaturised sperm heads. Sperm exhibit poor motility, sperm concentration is highly varied between males, and defining a ‘normal’ spermatozoa morphology is difficult (van der Horst et al. 2011) this examination of sperm from across the colony structure also suggested that the sperm had irregular and variable chromatin condensation.

The function of the falciform ‘hook-shape’ of the rodent sperm head and nucleus has been the subject of much debate. It has been suggested that the hook facilitates the formation of ‘sperm trains’ (Immler et al. 2007), in which a group of aggregated sperm are able to swim faster than an individual sperm. This is advantageous in species in which a female mates with multiple males in succession. However, directly associating sperm shape with functional advantages has been difficult, due to the wide ranging viscosities of the vaginal fluid in which they swim, the differing components to the seminal fluid, and differences in flagellar length and number (Simmons and Fitzpatrick 2012). Consequently, more research into sperm morphology and function is needed, especially into the processes that drive sperm head shape and by association, sperm nucleus shape.

Associations of shape changes with fertility

Despite the variation in sperm shape, it is clear that there is an impact of shape abnormalities in fertility. Studies of the hydrodynamic efficiency of sperm from a range of different species have shown that sperm with morphological abnormalities are poorer swimmers, such as in humans (Katz et al. 1982 Gillies et al. 2009) and bulls (Ostermeier et al. 2001). Subsequent studies in cattle demonstrated that sperm motility varies between cattle breeds, and also varies depending on the temperature at which the sperm were developing (Rahman et al. 2011).

Morphological abnormalities are well-known contributors in human infertility teratozoospermia, in which >85 % of sperm are morphologically abnormal, is frequently encountered in infertile men. The primary genetic correlates appear to be aneuploidies and DNA fragmentation (Braekeleer et al. 2015 Coutton et al. 2015). Mice with deletions on the long arm of their Y-chromosome exhibit abnormal morphologies, becoming more severe as the size of the deletion grows (Ward and Burgoyne 2006). Interestingly, sperm from males with this deletion also exhibit a sex-ratio skewing in favour of females, indicating that (in mice) there are different developmental effects of sex-linked genes on X-bearing and Y-bearing spermatids (Cocquet et al. 2012).

Clearly, there are important developmental pathways remaining to be elucidated in sperm development, especially those relating to the shaping of the sperm head, within and across taxa.


Syncytial-type cell plates: a novel kind of cell plate involved in endosperm cellularization of Arabidopsis

Cell wall formation in the syncytial endosperm of Arabidopsis was studied by using high-pressure-frozen/freeze-substituted developing seeds and immunocytochemical techniques. The endosperm cellularization process begins at the late globular embryo stage with the synchronous organization of small clusters of oppositely oriented microtubules ( approximately 10 microtubules in each set) into phragmoplast-like structures termed mini-phragmoplasts between both sister and nonsister nuclei. These mini-phragmoplasts produce a novel kind of cell plate, the syncytial-type cell plate, from Golgi-derived vesicles approximately 63 nm in diameter, which fuse by way of hourglass-shaped intermediates into wide ( approximately 45 nm in diameter) tubules. These wide tubules quickly become coated and surrounded by a ribosome-excluding matrix as they grow, they branch and fuse with each other to form wide tubular networks. The mini-phragmoplasts formed between a given pair of nuclei produce aligned tubular networks that grow centrifugally until they merge into a coherent wide tubular network with the mini-phragmoplasts positioned along the network margins. The individual wide tubular networks expand laterally until they meet and eventually fuse with each other at the sites of the future cell corners. Transformation of the wide tubular networks into noncoated, thin ( approximately 27 nm in diameter) tubular networks begins at multiple sites and coincides with the appearance of clathrin-coated budding structures. After fusion with the syncytial cell wall, the thin tubular networks are converted into fenestrated sheets and cell walls. Immunolabeling experiments show that the cell plates and cell walls of the endosperm differ from those of the embryo and maternal tissue in two features: their xyloglucans lack terminal fucose residues on the side chain, and callose persists in the cell walls after the cell plates fuse with the parental plasma membrane. The lack of terminal fucose residues on xyloglucans suggests that these cell wall matrix molecules serve both structural and storage functions.

Figures

Light Microscopy of a Longitudinal…

Light Microscopy of a Longitudinal Section of a Developing Arabidopsis Seed. (A) Overview.…

Syncytial-Type Cell Plates Form between…

Syncytial-Type Cell Plates Form between Nonsister Nuclei at the Onset of Cellularization in…

Examples of Assembly of Mini-Phragmoplasts…

Examples of Assembly of Mini-Phragmoplasts at the Onset of Endosperm Cellularization and Vesicle…

Two Wide Tubular Networks Formed…

Two Wide Tubular Networks Formed in Two Adjacent Mini-Phragmoplasts. WTN, wide tubular network.…

Developmental Stages in Syncytial-Type Cell…

Developmental Stages in Syncytial-Type Cell Plate Formation. (A) Early stages in vesicle aggregation…

Syncytial-Type Phragmoplast and Cell Plate.…

Syncytial-Type Phragmoplast and Cell Plate. (A) Transverse section of a syncytial-type cell plate.…

Syncytial-Type Cell Plates Ready to…

Syncytial-Type Cell Plates Ready to Fuse with a Syncytial Cell Wall in the…

Fusion of Syncytial-Type Cell Plates.…

Fusion of Syncytial-Type Cell Plates. (A) Syncytial-type cell plates (arrows) forming simultaneously around…

Fusion of Syncytial-Type Cell Plates…

Fusion of Syncytial-Type Cell Plates with the Syncytial Cell Wall. (A) Transverse section…

Labeling Cell Plates and Cell…

Labeling Cell Plates and Cell Walls with Anti-Xyloglucan and Anti-Pectic Polysaccharide Antibodies. (A)…

Labeling Cell Plates and Cell…

Labeling Cell Plates and Cell Walls with Anti-Callose Antibodies and CBHI-Gold Probe. (A)…

Diagram Showing Microtubule Organization in…

Diagram Showing Microtubule Organization in the Syncytial Endosperm at the Onset of Cellularization…

Stages of Syncytial-Type Cell Plate…

Stages of Syncytial-Type Cell Plate and Cell Wall Formation. The model depicts the…


What is Coenocyte?

A coenocyte or a coenocytic cell is a multinucleate cell which is a result of multiple nuclear divisions without undergoing cytokinesis. These cells are present in different types of protists such as algae, protozoa, slime molds and alveolates. When considering algae, coenocytic cells are present in red algae, green algae and Xanthophyceae. The entire thallus of siphonous green algae is a single coenocytic cell.

Figure 02: Coenocyte

In plants, the endosperm initiates its growth when one fertilized cell becomes a coenocyte. Different plant species produce many coenocytic cells with a different number of nuclei. Apart from plants, some filamentous fungi contain coenocytic mycelia with multiple nuclei. Those coenocytes functions as a single coordinated unit with multiple cells.


MATERIALS AND METHODS

Here we provide details on the experimental results shown in Figures 1A and 4, A and C.

Fly stocks

All D. melanogaster stocks were grown on standard cornmeal medium at 25°C. The following stocks were used: apterousME-NLS::dsRed (apRed/control) (Richardson et al., 2007), ens swo (Metzger et al., 2012), Mef2-Gal4 (Ranganayakulu et al., 1998), UAS-Ens-HA (Metzger et al., 2012), UAS-robo.myc (gift from the Bashaw lab Bashaw et al., 1998), sns-GAL4 (gift from the Abmayr lab Kocherlakota et al., 2008) and duf-GAL4 (Deng et al., 2015). From the Bloomington Drosophila Stock Center: robo 11/1 (8755 Kidd et al., 1998). To visualize nuclei in the Lateral Transverse muscles, stocks were crossed with apRed (Richardson et al., 2007) which fluorescently labels nuclei. The GAL4-UAS system (Brand and Perrimon, 1993) was used to express the UAS constructs. Embryos were staged according to (Campos-Ortega and Hartenstein, 2013).

Fluorescent antibody staining

Embryos were prepared for staining as previously described (Richardson et al., 2007). Embryos were incubated in primary antibody for 1 h at room temperature or overnight at 4°C and used at the following dilutions: rat anti-Tropomyosin (50567, Abcam, Cambridge, UK), 1:500 rabbit anti-DsRed (632496, Clontech, Mountain View, CA), 1:400 chicken anti-GFP (13970, Abcam), 1:500. Alexa fluor-conjugated secondary antibodies (Invitrogen, Carlsbad, CA) were applied 1:400 for 1 h at room temperature. Fluor-conjugated phalloidin (Life Technologies, Carlsbad, CA) were added with the secondary antibody at 1:100. Samples were mounted in ProLong Gold antifade reagent (Invitrogen).

Imaging

Z-stacks of fixed samples were acquired using a Zeiss LSM700 laser-scanning confocal microscope using a Plan-Apochromatic 20×/0.8-NA M27 objective and PlanNeo 40×/1.3-NA oil-immersion objective and processed in FIJI/ImageJ (NIH).

Quantification of the maximum spread

For analysis of nuclear spread at stage 16, embryos of all genotypes carrying apRed were used to visualize the nuclei in the lateral transverse muscles. Maximum spread was calculated using the segmented line function in FIJI to measure the distances from the dorsalmost nucleus to the dorsal myotube pole, the ventralmost nucleus to the ventral myotube pole, and the total myotube length. Maximum spread was calculated by subtracting the first two values from the third and expressing the difference as a percentage of total myotube length. A minimum of 17 and a maximum of 40 muscles were analyzed.

Quantification of the number of nuclei

To count nuclear number in the LTs, embryos carrying apRed were collected and dechorionated using 50% bleach for 4 min at room temperature. After being rinsed, embryos were quickly heat-fixed in water at 65°C and mounted in halocarbon oil on a glass slide. The number of nuclei was counted per muscle in stage 17 embryos, when it is possible to identify individual myonuclei. A minimum of 26 and a maximum of 49 muscles were analyzed.


Watch the video: 0102Numbers of nuclei in cells (December 2022).