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Are animal cells animals themselves?

Are animal cells animals themselves?


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If all life can be divided into several kingdoms, and if the cells that make up organisms are the smallest units of life, does that mean that cells are part of those kingdoms as well? E.g. are animal cells classified as animals themselves too?


Eukariotes are more logically defined as unified multicellular groups in science.

You are right that Eukariote cells share many traits with unicellular organisms, like meiosis division, cell walls, inner metabolism, and it is an intelligent view to consider them as related to unicellular organisms in most ways, although your proposition is mostly of comparative and philosophical us, rather than a scientific biological reasoning and description.

Animal cells share the same chromosomes, which is different from species of micro-organisms, so that eukariote stem cells can differentiate into the specialized cells. Most species definitions contain a strong reproduction framework which applies to all species whereas Eukariote cells start from zygotes and then communicate with each other with signalling hormones to form a specific superstructure which are used to produce more zygotes, which contrasts with procariote definitions of a species.


My opinion is yes, but note this could be subject to debate. This is a semantic question and someone could decide arbitrarily the other way, but I'm not aware of a formal counterargument, and logic seems to demand it…

Consider Devil facial tumor disease. I've linked a Wikipedia article there, and Wikipedia usually is very good about providing taxonomy for organisms and it's a shame they didn't do it here, as they are experts in semantics. NCBI also does not provide a taxonomy. But logically, if we were going to provide one, how could we possibly categorize those cells? They are living, they reproduce, they evolve, they are eukaryotes, and based on their DNA… they are animals!

A less clear-cut instance leading in this direction can be seen with Sacculina carcini, a parasitic barnacle of crabs. That does have a multicellular larva, but it is not a very typical mollusc, and it illustrates that it is the cells and descent of an animal, not its form, which determine its status.

Based on these things, I would say an animal cell can potentially survive, thrive, and in reproducing itself, become recognized as an organism despite its form. However, there is a contrary semantics to be considered, notably in the case of the HeLa cell line. In that instance, in part due to the genetic instability of the tumor cells and their dependence on human intervention, a proposal to name them as a species did not gain traction, while I believe some argued with an eye to legal issues that the cells were still essentially part of the individual Henrietta Lacks. However, in nature the definition of an individual can be fluid - in polyembryony one individual can divide into many, and even in humans one individual can become identical twins, so I wouldn't say that an isolated cell must remain part of the parent individual. Meanwhile, cells of different genetic origins can join into "a" slime mold, though they usually sort themselves out by species. In that instance we have a clear precedent of deciding the taxonomy of individual cells of an organism… but it's not an animal.

Love it or hate it, biology will do whatever it can, without regard for how we choose to think about it.


Information About Animal Cells

Fascinated by various aspects of eukaryotic cell biology? Here's a basket of random interesting information about animal cells that you'll love reading!

Fascinated by various aspects of eukaryotic cell biology? Here’s a basket of random interesting information about animal cells that you’ll love reading!

Before we embark upon our journey towards unveiling sundry interesting facts about animal cells, let’s learn a few details about cells in general. Broadly, cells can be classified under two categories – prokaryotic cells and eukaryotic cells.

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A prokaryotic cell lacks a nucleus, cell membrane and the DNA structure of a prokaryotic cell is not organized in chromosomal order. Eukaryotic cells, on the other hand, are equipped with a nucleus each and all cellular matter including the DNA are contained within cell membranes. The DNA is also organized into chromosomes.

All bacteria – both eubacteria and archaebacteria – are prokaryotes. Every other living organism, in the animal kingdom as well as the plant kingdom, are eukaryotic, meaning they have a cellular structure that consists of cell membrane and nuclear material. Let’s move on to more such information about animal cells.

Random Interesting Facts About Animal Cells

Here is a list of some random information on animal cells that will leave you amazed by the time you’re done reading the last sentence of this article. Here we go!

  • Animal cells are nano chemical factories that are completely self-sufficient! The cells themselves manufacture everything that constitutes them. For instance, the cell membrane is produced by an organelle located near the nucleus, known as the Golgi complex.
  • This same organelle also combines proteins, lipids and carbohydrates in a kind of membrane-packed bubble that is then ejected from the cell to be used elsewhere in the body. This is a very important part of macronutrient synthesizing function of cells.
  • Ribosomal RNA, which creates protein from amino acids, which are among the primary building blocks of life, is produced by nucleolus, an organelle contained inside the nucleus.
  • The size of a single, random animal cell can fall anywhere between 1 and 100 micrometers. To put it in other words, no matter how large or small different types of animal cells are with relation to each other, they are still too small to be visible to the naked eyes.
  • Unlike prokaryotic which reproduce via binary fission, eukaryotic or animal cells reproduce either via the process of mitosis or sexually. The latter takes place when gametes of opposite sexual characteristics fuse together. After all, sperms and ova are nothing but gametes or sex cells! This is how new organisms are conceived and born.
  • Tissues are formed when cells of similar structures, composition and characteristics bundle together.
  • Animal cells have an inbuilt self-destruct system which is resorted to when a cell becomes damaged beyond repair or gets severely infected. This cellular suicide is known as apoptosis and when this phenomenon fails to take place, the condition that surfaces is what we know as cancer, where new cells are produced but the older, damaged ones do not die out to make place for them!
  • Contrary to widespread belief, the nucleus of a cell is rarely in the center of the cell! It can be anywhere in the cell but the very epicenter of it, which is mostly the case!
  • A single animal cell has the complete blueprint of all the information that is needed to create a complete organism from it! The genetic matter in the cells is nothing but encrypted information about the biology, psychology, characteristics and personality of the complete organism!
  • The mitochondria contained inside animal cells convert oxygen and other nutrients into energy. This is, thus, the powerhouse that keeps the entire cell up and running!
  • The nucleus is the brain of the cell which stores all genetic information in the form of DNA and it controls and regulates all other cellular functions including growth, metabolism, reproduction, apoptosis, protein synthesis, etc.
  • The plasma membrane that contains all cellular matter inside it is like a semipermeable wall that allows for molecular exchange through it.
  • Most single-celled eukaryotic organisms such as paramecium have external thread and tube-like structures all over them to help in locomotion.

  • The cytoplasm contains a kind of biological scaffolding material, known as cytoskeleton, which is composed of proteins and which helps cells to maintain their shape.
  • Although tiny themselves, cells are not the tiniest particles on earth! Each cell is made up of a number of even tinier particles known as atoms!

Loved it? I’m sure you did! Also, I hope you will leave this page with more knowledge regarding animal cells than what you came here with! As for me, I myself learned a lot while writing this article and I intend to continue my study of animal cells even after I’ve concluded this article!

Related Posts

Animal cells depict various irregular shapes and sizes and are visible only under the microscope. This BiologyWise article elaborates on the definition and the function of the parts of animal&hellip

A plant cell and an animal cell bear certain characteristics in common, as both are eukaryotic in nature. Read this article to gain more information about these similarities.

The following article provides information regarding the structure and functions of various cell organelles belonging to the eukaryotic cell.


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Cells transform themselves in male worms to improve mating

Roundworms mating. Credit: Molina-García, Lloret-Fernández et al

A cell in worms that transforms itself into a completely different type of cell when males mature, to play a key role in mating behavior, has been recently discovered by a team led by UCL researchers. The researchers say their findings, published in eLife, may lead to new clinical applications if scientists can reproduce the mechanism to reprogram cells to adopt new functions.

Co-lead author Dr. Richard Poole (UCL Cell & Developmental Biology) said: "In contrast to behavior, which is characteristically flexible and variable, development needs to be consistent, and once a cell has its own identity with a particular function, this is considered to be fixed throughout the life of the animal. But here, we describe in detail the case of a specialized cell that switches to a new cell type with a completely unrelated function later in the animal's life."

This is the second time the research team has identified a new neuron in the C. elegans roundworm. In 2015, they identified a pair of neurons in male worms that allow them to remember and seek sex, even at the expense of food.

Co-lead author Dr. Arantza Barrios (UCL Cell & Developmental Biology) said: "The C. elegans worm has been studied extensively as a model organism, so we were surprised to find once again a type of neuron that had not yet been identified."

The researchers were investigating a glial cell, a non-neuronal cell present in animal nervous systems. This particular glial cell is a key component of an opening in the worm's skin that allows it to sense the external world.

However, the researchers found that, exclusively in males, the cell develops a new identity as the animal nears sexual maturity. The glial cell in males becomes a previously unidentified sensory neuron, in an unusual event called transdifferentiation, or a direct cell identity switch. In its capacity as a neuron, the cell enables the male worms to improve their mating technique.

For the study, the researchers used molecular tools to measure or silence neuronal activity and micro-lasers to remove the neurons in live, active animals. They found that the cell senses the worm's body posture and ensures continuous backward movement along its mate's body during particular steps of the mating sequence. One such step is a newly-identified readjustment movement, when the worm is having trouble inserting its spicules (the equivalent of the worm's penis), which the researchers dubbed the Molina Maneuver (as it was first noticed by one of the study's primary authors, Dr. Laura Molina-García from UCL Cell & Developmental Biology).

Only a few examples of direct cell type switches such as these have been identified in nature. While none have been found in humans, or in any other vertebrates, the researchers say that such switches might be more common throughout the animal kingdom than we realize, and it is likely that some human cells may undergo a similar switch under certain conditions.

Dr. Poole said: "Our findings support the idea that cell identity is more flexible than previously thought. Identifying the mechanisms that regulate this naturally-occurring switch in cell type through future studies will help understand processes such as neural regeneration and injury repair. The findings can also be applied to develop better, more efficient methods for reprogramming cells in the lab, which is a widely used strategy for cell replacement therapies for conditions such as spinal cord injury and neurodegenerative conditions including Parkinson's or Huntington's disease."

Dr. Barrios added: "A deeper understanding of how animal movements are guided by self-sensory feedback could also be useful to robotics, to improve the execution of behavioral sequences."


How Life Made the Leap From Single Cells to Multicellular Animals

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James O'Brien for Quanta magazine

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For billions of years, single-celled creatures had the planet to themselves, floating through the oceans in solitary bliss. Some microorganisms attempted multicellular arrangements, forming small sheets or filaments of cells. But these ventures hit dead ends. The single cell ruled the earth.

* Original story reprinted with permission from Quanta Magazine, an editorially independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.*Then, more than 3 billion years after the appearance of microbes, life got more complicated. Cells organized themselves into new three-dimensional structures. They began to divide up the labor of life, so that some tissues were in charge of moving around, while others managed eating and digesting. They developed new ways for cells to communicate and share resources. These complex multicellular creatures were the first animals, and they were a major success. Soon afterward, roughly 540 million years ago, animal life erupted, diversifying into a kaleidoscope of forms in what’s known as the Cambrian explosion. Prototypes for every animal body plan rapidly emerged, from sea snails to starfish, from insects to crustaceans. Every animal that has lived since then has been a variation on one of the themes that emerged during this time.

How did life make this spectacular leap from unicellular simplicity to multicellular complexity? Nicole King has been fascinated by this question since she began her career in biology. Fossils don’t offer a clear answer: Molecular data indicate that the “Urmetazoan,” the ancestor of all animals, first emerged somewhere between 600 and 800 million years ago, but the first unambiguous fossils of animal bodies don’t show up until 580 million years ago. So King turned to choanoflagellates, microscopic aquatic creatures whose body type and genes place them right next to the base of the animal family tree. “Choanoflagellates are to my mind clearly the organism to look at if you’re looking at animal origins,” King said. In these organisms, which can live either as single cells or as multicellular colonies, she has found much of the molecular toolkit necessary to launch animal life. And to her surprise, she found that bacteria may have played a crucial role in ushering in this new era.

Nicole King, a biologist at the University of California, Berkeley, studies the origins of animals, one of the big mysteries in the history of life.

In a lengthy paper that will be published in a special volume of Cold Spring Harbor Perspectives in Biology in September, King lays out the case for the influence of bacteria on the development of animal life. For starters, bacteria fed our ancient ancestors, and this likely required those proto-animals to develop systems to recognize the best bacterial prey, and to capture and engulf them. All of these mechanisms were repurposed to suit the multicellular lives of the first animals. King’s review joins a broad wave of research that puts bacteria at the center of the story of animal life. “We were obliged to interact intimately with bacteria 600 million years ago,” said King, now an evolutionary biologist at the University of California, Berkeley, and an investigator with the Howard Hughes Medical Institute. “They were here first, they’re abundant, they’re dominant. In retrospect we should’ve expected this.”

Although we tend to take the rise of animals for granted, it is reasonable to ask why they ever emerged at all, given the billions of years of success of unicellular organisms. “For the last 3.5 billion years, bacteria have been around and abundant,” said Michael Hadfield, a professor of biology at the University of Hawaii, Manoa. “Animals never showed up until 700 or 800 million years ago.”

The technical demands of multicellularity are significant. Cells that commit to living together need a whole new set of tools. They have to come up with ways of sticking together, communicating, and sharing oxygen and food. They also need a master developmental program, a way to direct specific cells to take on specialized jobs in different parts of the body.

Nonetheless, during the course of evolution, the transition to multicellularity happened separately as many as 20 different times in lineages from algae to plants to fungi. But animals were the first to develop complex bodies, emerging as the most dramatic example of early multicellular success.

To understand why this might have happened the way it did, King began studying choanoflagellates, the closest living relative to animals, nearly 15 years ago as a postdoc at the University of Wisconsin, Madison. Choanoflagellates are not the most charismatic of creatures, consisting of an oval blob equipped with a single taillike flagellum that propels the organism through the water and also allows it to eat. The tail, thrashing back and forth, drives a current across a rigid, collarlike fringe of thin strands of cell membrane. Bacteria get caught up in the current and stick to the collar, and the choano engulfs them.

What intrigued King about choanoflagellates was their lifestyle flexibility. While many live as single cells, some can also form small multicellular colonies. In the species Salpingoeca rosetta, which lives in coastal estuaries, the cell prepares to divide but stops short of splitting apart, leaving two daughter cells connected by a thin filament. The process repeats, creating rosettes or spheres containing as many as 50 cells in the lab. If this all sounds familiar, there’s a reason for it — animal embryos develop from zygotes in much the same way, and spherical choanoflagellate colonies look uncannily like early-stage animal embryos.

When King began studying S. rosetta, she couldn’t get the cells to consistently form colonies in the lab. But in 2006, a student stumbled on a solution. In preparation for genome sequencing, he doused a culture with antibiotics, and it suddenly bloomed into copious rosettes. When bacteria that had been collected along with the original specimen were added back into a lab culture of single choanoflagellates, they too formed colonies. The likely explanation for this phenomenon is that the student’s antibiotic treatment inadvertently killed off one species of bacteria, allowing another that competes with it to rebound. The trigger for colony formation was a compound produced by a previously unknown species of Algoriphagus bacteria that S. rosetta eats.

S. rosetta seems to interpret the compound as an indication that conditions are favorable for group living. King hypothesizes that something similar could have happened more than 600 million years ago, when the last common ancestor of all animals started its fateful journey toward multicellularity. “My suspicion is that the progenitors of animals were able to become multicellular, but could switch back and forth based on environmental conditions,” King said. Later, multicellularity became fixed in the genes as a developmental program.

King’s persistence in studying this humble organism, which was overlooked by most contemporary biologists, has won her the admiration of many of her fellow scientists (as well as a prestigious MacArthur fellowship). “She strategically picked an organism to gain insight into early animal evolution and systematically studied it,” said Dianne Newman, a biologist at the California Institute of Technology in Pasadena, who studies how bacteria coevolve with their environment. King’s research offers a thrilling glimpse into the past, a rare window into what might have been going on during that mysterious period before the first fossilized animals appeared. The research is a “beautiful example” of how bacteria shape even the simplest forms of complex life, Newman said. “It reminds us that even at that level of animal development, you can expect triggers from the microbial world.” The bacteria system in S. rosetta can now be used to answer more specific questions, such as what the benefit of multicellularity might be — a question King and her collaborators at Berkeley are now working to answer.

The first bacteria may date back as far as 3.5 billion years. But animals, the first complex multicellular life form, took much longer to emerge.


Sound: the Auditory System

The information below was adapted from OpenStax Biology 36.4

Auditory stimuli are sound waves, which are mechanical pressure waves that move through a medium, such as air or water. (There are no sound waves in a vacuum since there are no air molecules to move in waves.) Because sound waves exert pressure, sound is detected by mechanoreceptors.

As is true for all waves, there are four main characteristics of a sound wave: frequency, wavelength, period, and amplitude. Three of these are important for understanding how hearing works:

  • Frequency is the number of waves per unit of time, which is heard as pitch. Frequency is also related to wavelength, where high-frequency (15,000 Hz) sounds are higher-pitched and shorter wavelength than low-frequency, long wavelength (100 Hz) sounds. Most humans can perceive sounds with frequencies between 30 and 20,000 Hz. Dogs detect up to about 40,000 Hz cats, 60,000 Hz bats, 100,000 Hz and dolphins 150,000 Hz, and some fish can hear 180,000 Hz.
  • Amplitude, or the dimension of a wave from peak to trough, in sound is heard as volume. The sound waves of louder sounds have greater amplitude than those of softer sounds. For sound, volume is measured in decibels (dB). In the figure below, the softest sound that a human can hear is the zero point. Humans speak normally at 60 decibels.

For sound waves, wavelength corresponds to pitch. Amplitude of the wave corresponds to volume. The sound wave shown with a dashed line is softer in volume than the sound wave shown with a solid line. (credit: NIH via OpenStax Biology)

  • Outer ear:
    • Sound waves are collected by the external, cartilaginous part of the ear
    • Sound waves then travel through the auditory canal and cause vibration of the ear drum (tympanic membrane)
    • The eardrum transmits sound to the middle ear by vibrating the ossicles, the three small bones of the middle ear which collect force and amplify sounds. The three ossicles are unique to mammals.
    • The ossicles transmit the vibrations to a thin membrane called the oval window, which is the outermost structure of the inner ear .
    • The vibrations of the oval window create pressure waves in the fluid inside the cochlea. The cochlea is a whorled structure, like the shell of a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal
    • Inside the cochlea, the basilar membrane is a flexible membrane that runs the length of the cochlea, and contains the mechanoreceptors called hair cells which transduce sound waves into action potentials. The basilar membrane vibrates in response to sound pressure waves, pressing the hair cells against the tectorial membrane, which physically bends the hair cell cilia, initiating action potentials in the afferent neurons that communicate sounds stimuli to the brain.

    Sound travels through the outer ear to the middle ear, which is bounded on its exterior by the tympanic membrane. The middle ear contains three bones called ossicles that transfer the sound wave to the oval window, the exterior boundary of the inner ear. The organ of Corti, which is the organ of sound transduction, lies inside the cochlea. (credit: OpenStax Biology, modification of work by Lars Chittka, Axel Brockmann)

    When the sound waves in the cochlear fluid contact the basilar membrane, the basilar flexes back and forth. The basilar membrane’s flexibility changes along its length, such that it is thicker, stiffer, and narrower at one end of the chochlea, and thinner, floppier, and broader at the other end. As a result, different regions of the basilar membrane vibrate according to the frequency of the sound wave conducted through the fluid in the cochlea, with the stiffer region vibrating in response to high frequency (higher-pitched) sounds, and the more flexible region vibrating in response to low frequency (lower-pitched) sounds. In other worlds, pitch is detected based on which region of the basilar membrane vibrates in response to a sound (and thus which hair cells are activated).

    In the mammalian ear, sound waves cause the stapes to press against the oval window. Vibrations travel up the fluid-filled interior of the cochlea. The basilar membrane that lines the cochlea gets continuously thinner toward the apex of the cochlea. Different thicknesses of membrane vibrate in response to different frequencies of sound. Sound waves then exit through the round window. In the cross section of the cochlea (top right figure), note that in addition to the upper canal and lower canal, the cochlea also has a middle canal. The organ of Corti (bottom image) is the site of sound transduction. Movement of stereocilia on hair cells results in an action potential that travels along the auditory nerve. Image credit: OpenStax Biology.

    The site of transduction from sound waves to action potentials is in the organ of Corti (spiral organ). How is sound transduced from a wave to an action potential? Within the organ of Corti, hair cells are held in place above the basilar membrane with their hair-like stereocilia embedded in the tectorial membrane above them. When a sound wave flexes the basilar membrane:

    1. The hair cells on the basilar membrane are flexed against the tectorial membrane, which bends the stereocilia
    2. The bending of the stereocilia causes potassium ion channels to open in the cell membrane the hair cell is bathed in a solution high in potassium, so potassium rushes into the cell through the open channels, depolarizing the hair cell
    3. Just like in a synaptic terminal, membrane depolarization ultimately causes synaptic vesicles in the hair cell to fuse with the plasma membrane, releasing neurotransmitters into the synaptic cleft between the hair cell and its synapsed afferent neuron
    4. If the resulting depolarization is sufficient, an action potential is transmitted to the chochlear nerve. Intensity (volume) of sound is determined by how many hair cells at a particular location are stimulated, while pitch is determined by which particular hair cells along the basilar membrane are stimulated

    Bending of cilia (yellow) in hair cells in the inner ear in response to sound pressure waves results in opening of potassium channels that depolarize the hair cells, cause release of neurotransmitters on the synapsed sensory neurons (shown in blue), and can trigger action potentials (APs) in the axons of those neurons. Image credit: Modification of work by Thomas.haslwanter – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14585601

    This video provides a quick overview of mammalian cochlear function in hearing:


    Finding alternatives to animal testing

    Researchers at the University of California, Riverside, are part of an ambitious plan at the U.S. Environmental Protection Agency, or EPA, to eliminate animal testing by 2035. Their contribution: developing a way to test whether chemicals cause musculoskeletal birth defects using lab-grown human tissue, not live animals.

    Nicole zur Nieden, an associate professor of molecular, cell, and systems biology, and David Volz, an associate professor of environmental toxicology, are both experts on alternatives to regulatory toxicity testing and chemicals policy and regulation. They received $849,811 to grow human stem cells into bone-like tissue to test industrial and environmental chemicals that might interfere with fetal growth.

    Birth defects that affect musculoskeletal tissues can be caused by chemical ingredients in pesticides, fungicides, paints, and food additives. Harmful chemicals must be identified through testing in order to be regulated. Currently, this testing is done on live animals, usually rodents such as mice.

    Nicole zur Nieden
    Credit: UC Riverside

    The UC Riverside project, led by zur Nieden, will stimulate human pluripotent stem cells, which have the capacity to develop into any sort of cell, with agents that direct them to form bone cells. The cells will pass through the same developmental stages and be subject to the same molecular cues as in a human embryo. The researchers will expose the cells to selected chemicals at critical junctures, then assess them using advanced imaging and next-generation sequencing techniques.

    Bone cells can develop through three different pathways. zur Nieden will use chemicals known to affect specific routes of bone development to look for patterns in how the chemicals affect these origins. The patterns will serve as blueprints for testing unknown chemicals. Next, the researchers will test unknown chemicals and compare them to previously compiled libraries of compounds that have already been tested in animals to see how accurate the petri dish, or in vitro, tests are for assessing risk.

    A hallmark feature of bone-forming cells is that they make a bony matrix out of little crystals called hydroxyapatite, which eventually form calcium phosphate, the white stuff on the surface of all bones. Cost-saving visual analysis can help identify defects in calcium.

    “Calcium crystals appear white when viewed with your eyes,” said zur Nieden. “But when you view the cultures using phase contrast microscopy, it inverts the light so the normal crystals appear black. Abnormal crystals will have more white and shades of gray. You can use an image analysis algorithm to measure the blackness in images to determine if the calcium has formed correctly or not.”

    Non-animal tests are already commonplace

    Scientists have known for a long time that animals differ from humans in important developmental and physiological ways, and that animal test results are not always reliable for people. Moreover, animal research is expensive and time-consuming, as well as increasingly untenable for ethical reasons. Non-animal alternatives have been in development for nearly 25 years, and some are already standard.

    “To the general public, the EPA’s announcement seemed to come out of nowhere,” said Volz, whose lab will sequence messenger RNA in chemical-exposed bone cells from zur Nieden’s lab to look for changes in gene expression. “It didn’t happen overnight. That train has already left the station.”

    Volz said the EPA’s Science to Achieve Results Program, through which UC Riverside received the new grant, has been funding research on animal alternatives for more than 10 years.

    The EPA’s plan to end animal testing by 2035 follows up on earlier changes to the Toxic Substances Control Act, or TSCA, enacted in 1976. TSCA authorizes the EPA to regulate chemicals found in consumer products such as cleaning agents, furniture, paint, carpeting, clothing, and other consumer goods. Regulation under TSCA does not apply to chemicals in food, drugs, cosmetics, and pesticides, which are regulated under different laws.

    Even after TSCA, thousands of common chemicals used in everything from plastic to sunscreen have never been tested for safety in humans. In 2016, Congress passed the Lautenberg Chemical Safety Act, amending TSCA to close the loophole for industrial chemicals. The law mandated the EPA to evaluate existing chemicals with clear and enforceable deadlines, and to develop risk-based chemical assessments. It promoted the use of non-animal testing methods, a move sought by both industry and animal rights groups.

    Animal alternatives might have limits

    The new EPA plan introduces an aggressive timeline for ramping up development of non-animal tests that can accurately predict toxicity in humans. Volz said the United States lags behind some other countries around the world, which have already greatly reduced animal testing. He said he interacts with fewer and fewer students interested in research involving animal experiments, and that our culture is shifting toward a desire to reduce animal suffering.

    But neither Volz nor zur Nieden are sure animal testing can ever be replaced completely, a position echoed by the EPA memo, which states that after 2035, animal tests will be approved on a case-by-case basis. Some chemicals, for example, are not directly toxic to cells but become toxic after they are metabolized in the body.

    “If your result is that the chemical does not interfere with a human stem cell developing in a dish, how sure can you be that’s not really happening in humans? The best way we have to assess that is an animal experiment,” zur Nieden said. “At the same time, we want to do this in an appropriate way. We need to think about, is this really necessary? Can we look at the question some other way?”

    zur Nieden thinks we need a tiered system, with in vitro tests weeding out the most toxic chemicals first, and animal tests used where in vitro tests don’t reveal toxicity.

    “If you cannot fully replace an animal test with an in vitro method, you can at least decrease suffering of the animal. If you think about a highly toxic chemical that has effects on the mom as she is exposed during pregnancy as well as on the developing embryos, if you can use an in vitro test system to find all these strong toxic chemicals, you will not need to test them in an animal,” she said.

    Previous versions of the test system zur Nieden will use for the new musculoskeletal research have been able to identify embryotoxic chemicals for other tissues, such as heart tissue, with almost 100 percent accuracy.


    There are two main types of cell: eukaryotic cells and prokaryotic cells.

    Eukaryotic Cells

    Eukaryotic cells are bigger and more complex than prokaryotic cells. The cells that make up animals, plants and fungi are eukaryotic cells.

    Therefore, animals, plants and fungi can be described as being ‘eukaryotic’.

    Although animals and plants are both eukaryotic, there are differences between animal and plant cells. We’ll learn about these differences further down the page.

    Prokaryotic Cells

    Prokaryotic cells are smaller and simpler than eukaryotic cells. Single-celled organisms such as bacteria and archaea are made up of a single prokaryotic cell. These organisms can be described as being ‘prokaryotic’.


    Animal Cell

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    Tyson Brown, National Geographic Society

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    National Geographic Society

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    Related Resources

    Cell Biology

    A cell is the smallest unit that is typically considered alive and is a fundamental unit of life. All living organisms are composed of cells, from just one (unicellular) to many trillions (multicellular). Cell biology is the study of cells, their physiology, structure, and life cycle. Teach your students about cell biology using these classroom resources.

    History of the Cell: Discovering the Cell

    Initially discovered by Robert Hooke in 1665, the cell has a rich and interesting history that has ultimately given way to many of today&rsquos scientific advancements.

    Comparing Plant and Animal Cells

    Identify the differences between plant and animal cells and how these differences relate to their cellular functions.

    Unicellular vs. Multicellular

    Cells function differently in unicellular and multicellular organisms. A unicellular organism depends upon just one cell for all of its functions while a multicellular organism has cells specialized to perform different functions that collectively support the organism.

    Related Resources

    Cell Biology

    A cell is the smallest unit that is typically considered alive and is a fundamental unit of life. All living organisms are composed of cells, from just one (unicellular) to many trillions (multicellular). Cell biology is the study of cells, their physiology, structure, and life cycle. Teach your students about cell biology using these classroom resources.

    History of the Cell: Discovering the Cell

    Initially discovered by Robert Hooke in 1665, the cell has a rich and interesting history that has ultimately given way to many of today&rsquos scientific advancements.

    Comparing Plant and Animal Cells

    Identify the differences between plant and animal cells and how these differences relate to their cellular functions.

    Unicellular vs. Multicellular

    Cells function differently in unicellular and multicellular organisms. A unicellular organism depends upon just one cell for all of its functions while a multicellular organism has cells specialized to perform different functions that collectively support the organism.


    Summary

    This tutorial began our discussion of Kingdom Animalia. Animals have been around for at least 700 million years, and during this time they have diversified to a remarkable degree. Likely, the ancestor of all animals was a colonial protist. In examining the phylogenetic tree, keep in mind that there are major dichotomous characters that occur at each branch point. In many instances these characters are based on the development of animals.

    Eumetazoan development (as with other sexually reproducing, multicellular eukaryotes) begins with fertilization. The zygote then undergoes a series of cell divisions to produce a mass of cells that has some hollow character (the parazoans do not follow this type of embryology). The first tissue to form is the blastoderm, and this young tissue surrounds the blastocoel. In the next step (gastrulation), cells migrate inward and begin to differentiate into endoderm. Thus, gastrulation marks the embryonic stage where additional tissue types form. In the case of diploblastic animals, there are only two tissue types (ectoderm and endoderm), whereas triploblastic animals have three tissue types (ectoderm, mesoderm, and endoderm). During gastrulation, the primitive gut also forms from the archenteron.

    Depending on the species, a second body cavity (coelom) can form. Thus, animals can be characterized as lacking a second body cavity (acoelomates), having a coelom that is incompletely lined with mesoderm (psuedocoelomates), or having a coelom that is completely lined with mesoderm (coelomates). As we survey the various phyla of Kingdom Animalia, keep in mind the relationship between these major character traits and their relationship with animal development.


    Watch the video: Κύτταρα (September 2022).


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