Are the human placenta chorionic villi and villous tree the synonyms?

Are the human placenta chorionic villi and villous tree the synonyms?

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Are the human placental chorionic villi and the villous tree the synonyms? Or, if not, can I say that these tissues would have practically the same gene expression profile in an RNA microarray experiment?

Morphogenesis of human placental chorionic villi: cytoskeletal, syncytioskeletal and extracellular matrix proteins

Immunocytochemical and ultrastructural methods were used to investigate the distribution of a family of structural proteins in the human placenta near term. These reveal the distribution of cytoskeletal and ‘syncytioskeletal’ components that may account for some of the more obvious micromorphological features of placental structure. In the syncytiotrophoblast a potentially supporting structure ‘the syncytioskeletal layer’ is described. It is an apparently continuous and complex polymeric network covering the villous tree, a surface of the order of 10m 2 in area in the full term placenta (Aherne & Dunnill 1966). It is suggested that this layer plays a part in morphogenesis of the villous tree.

Boyd JD, Hamilton WJ (1970) The Human Placenta. Heffer, Cambridge, pp 1–465

Cantle SJ, Kaufmann P, Luckhardt M, Schweikhart G (1987) Interpretation of syncytial sprouts and bridges in the human placenta. Placenta 8:221–234

Castellucci M, Kaufmann P (1982a) A three-dimensional study of the normal human placental villous core: II. Stromal architecture. Placenta 3:269–285

Castellucci M, Kaufmann P (1982b) Evolution of the stroma in human chorionic villi throughout pregnancy. Bibl Anat 22:40–45

Castellucci M, Zaccheo D, Pescetto G (1980) A three-dimensional study of the normal human placental villous core. I. The Hofbauer cells. Cell Tissue Res 210:235–247

Castellucci M, Schweikhart G, Kaufmann P, Zaccheo D (1984) The stromal architecture of the immature intermediate villus of the human placenta. Functional and clinical implications. Gynecol Obstet Invest 18:95–99

Demir R, Kaufmann P, Castellucci M, Erbengi T, Kotowski A (1989) Fetal vasculogenesis and angiogenesis in human placental villi. Acta Anat (in press)

Jones CJP, Fox H (1977) Syncytial knots and intervillous bridges in the human placenta: an ultrastructural study. J Anat 124:275–286

Kaufmann P (1981) Die Entwicklung der Plazenta. In: Becker V, Schiebler TH, Kubli F (eds) Die Plazenta des Menschen. Georg Thieme, Stuttgart, pp 13–50

Kaufmann P (1982) Development and differentiation of the human placental villous tree. Bibl Anat 22:29–39

Kaufmann P, Sen DK, Schweikhart G (1979) Classification of human placental villi. I. Histology. Cell Tissue Res 200:409–423

Kaufmann P, Nagl W, Fuhrmann B (1983) Die funktionelle Bedeutung der Langhanszellen der menschlichen Placenta. Verh Anat Ges 77:435–436

Kaufmann P, Bruns U, Leiser R, Luckhardt M, Winterhager E (1985) The fetal vascularisation of term human placental villi. II. Intermediate and terminal villi. Anat Embryol 173:203–214

Kaufmann P, Luckhardt M, Schweikhart G, Cantle SJ (1987a) Cross-sectional features and three-dimensional structures of human placenta villi. Placenta 8:235–247

Kaufmann P, Luckhardt M, Leiser R (1987b) Three-dimensional representation of the fetal vessel system in the human placenta. Trophoblast Research 3:107–129

Küstermann W (1981) Über „Proliferationsknoten” und „Syncytialknoten” der menschlichen Plazenta. Anat Anz 150:144–157

Leiser R, Luckhardt M, Kaufmann P, Winterhager E, Bruns U (1985) The fetal vascularisation of term human placental villi. I. Peripheral stem villi. Anat Embryol 173:71–80

Moe N (1971) Mitotic activity in the syncytiotrophoblast of the human chorionic villi. Am J Obstet Gynecol 110:431

Schuhmann R (1981) Plazenton: Begriff, Entstehung, funktionelle Anatomie. In: Becker V, Schiebler TH, Kubli F (eds) Die Plazenta des Menschen. Georg Thieme, Stuttgart, pp 191–207

Schweikhart G, Kaufmann P (1977) Zur Abgrenzung normaler artefizieller und pathologischer Strukturen in reifen menschlichen Plazentazotten. I. Ultrastruktur des Syncytiotrophoblasten. Arch Gynäk 222:213–230

Schweikhart G, Kaufmann P, Beck T (1986) Morphology of placental villi after premature delivery and its clinical relevance. Arch Gynecol 239:101–114

Scott WA, Cohn ZA (1982) Secretory products of mononuclear phagocytes. In: Nossel HL, Vogel HJ (eds) Pathobiology of the endothelial cell. Raven Press, New York, pp 351–356

Sen DK, Kaufmann P, Schweikhart G (1979) Classification of human placenta villi. II. Morphometry. Cell Tissue Res 200:425–434

Takemura R, Werb Z (1984) Secretory products of macrophages and their physiological functions. Am J Physiol 246:C1-C9

Tedde G, Tedde-Piras A (1978) Mitotic index of the Langhans' cells in the normal human placenta from the early stages of pregnancy to the term. Acta Anat 100:114–119

Werb Z (1983) How the macrophage regulates its extracellular environment. Am J Anat 166:237–256


In the past 20 years, there have been significant advances in the research and understanding of the biology of the placenta and its derivatives. Initially, the placenta drew attention as an interesting cell source due to its early embryological origin suggesting that cells derived from the placenta could possess unique plasticity and differentiation properties (Bailo et al., 2004). In addition, the placenta displays favorable logistical issues, such as the fact that the human term placenta is readily available at the time of delivery.

We now know that perinatal derivatives are promising for a wide range of regenerative medicine applications due to their differentiation capabilities but mainly due to their unique immune modulatory properties. As a matter of fact, many preclinical studies and initial clinical trials have demonstrated that perinatal derivatives may represent important tools for restoring tissue damage or promoting regeneration and repair of the tissue microenvironment (Caruso et al., 2012 Cirman et al., 2014 Jerman et al., 2014 Silini et al., 2015 Joerger-Messerli et al., 2016 Magatti et al., 2016 Couto et al., 2017 Silini et al., 2017 Bollini et al., 2018 Pogozhykh et al., 2018 Ramuta and Kreft, 2018 Verter et al., 2018 Silini et al., 2019 Ramuta et al., 2020). The term “perinatal” refers to birth-associated tissues that are obtained from term placentas and fetal annexes and more specifically refers to the amniotic/amnionic (herein referred to as amniotic due to its prevalence in literature) membrane, chorionic membrane, chorionic villi, umbilical cord (including Wharton’s jelly), the basal plate (including maternal and fetal cells), and the amniotic fluid. The term �rivatives” is used to refer to the cells isolated from placental tissues, and the factors that these cells release, referred to as their secretome or conditioned medium (including free nucleic acids, soluble proteins, lipids, and extracellular vesicles (such as exosomes, microvesicles and apoptotic bodies). Thus, perinatal derivatives (PnD) include different birth-associated tissues, the cells isolated thereof, and the factors secreted by the cells [fractionated (free-floating factors, extracellular vesicles, extracellular matrix components including proteins, glycosaminoglycans, and glycoconjugates) and unfractionated conditioned medium].

Over a decade ago, in 2008, the consensus from the First International Workshop on Placenta-Derived Stem Cells was published (Parolini et al., 2008). The consensus focused on cells isolated from the amniotic and chorionic parts of the fetal membranes and established the minimal criteria for the definition of mesenchymal stromal cells (MSC) derived from these membranes. In accordance to the criteria established for other MSC sources (Dominici et al., 2006), the criteria established at the First International Workshop on Placenta-Derived Stem Cells focused on adherence to plastic, formation of fibroblast-like colony-forming units, differentiation potential toward one or more lineages, including osteogenic, adipogenic, or chondrogenic lineages, and specific cell surface antigen expression from in vitro passages 2 to 4 (Parolini et al., 2008). In addition, the criteria included one other specific aspect, the determination of the fetal or maternal origin of the perinatal cells (Parolini et al., 2008).

During the last two decades, the literature published on perinatal derivatives has grown exponentially. Specific cells such as MSC have been isolated and characterized from different perinatal tissues, such as the fetal membranes (In ’t Anker et al., 2004 Soncini et al., 2007 Wolbank et al., 2010), chorionic villi (Fukuchi et al., 2004 Igura et al., 2004 Portmann-Lanz et al., 2006 Castrechini et al., 2010), decidua (In ’t Anker et al., 2004 Araújo et al., 2018 Ringden et al., 2018 Guan et al., 2019), and umbilical cord (Wang et al., 2004b Troyer and Weiss, 2008 La Rocca et al., 2009 Hartmann et al., 2010).

The significant increase in acquired knowledge has been paralleled with the evident need for the establishment of updated criteria and consensus policies for the characterization of PnD. Thus, this review aims at providing an updated and extended consensus starting from the policies published in 2008, which were specifically related to cells from fetal membranes (Parolini et al., 2008), and at addressing specific issues related to the proper and transparent definition of PnD, relating not only to fetal membranes but also to all other regions and perinatal tissues.

One issue that must be addressed is related to defining PnD. In its simplest form, this means establishing a reference nomenclature for each derivative that can be isolated from all perinatal, birth-associated tissues. Birth associated or perinatal tissues and organs, such as the human placenta, are complex and are comprised of different tissues (as mentioned above, amniotic membrane, chorionic membrane, chorionic villi, umbilical cord, basal plate including fetal trophoblast cells and maternal uterine cells, and amniotic fluid) (Figure 1). Even today, there is much confusion regarding the identification and location of the specific perinatal tissues and cells. In the current literature the nomenclature used does not necessarily highlight the true differences between cells. At the same time, not all cells can simply be referred to as “placenta-derived stem cells” (Oliveira and Barreto-Filho, 2015), without taking into consideration the exact tissue from which they were derived. A proper and clearly defined nomenclature is absolutely necessary to understand which cells are isolated and used in cell cultures. Incorrect nomenclature and definition of cells ultimately impact the correct identification of the cells and/or derivatives obtained and hinder the direct comparison of results among different research groups.

Figure 1. Architecture of the human term placenta. General overview of the relationship between the basal decidua (maternal side/component of the human placenta) and the fetal side/component of the human placenta represented by the chorion frondosum, the chorionic plate and the fused amniotic membrane (placental portion). The residual portion of the amniotic membrane (reflected portion) adheres to the chorion laeve (so called because it is devoid of villi) which is in touch with the capsular decidua. The amniotic membrane surrounds the amniotic cavity containing amniotic fluid with different types of detached cells. The magnified scheme shows the different parts of the term placental architecture. hBD-MSC, human basal decidua-mesenchymal stromal cells hAFC, human amniotic fluid cells hAFSC, human amniotic fluid stem cells hAF-MSC, human amniotic fluid-mesenchymal stromal cells.

Reference nomenclature should be established followed by a clear indication of the precise localization of cells in perinatal tissues. This should be followed by the next crucial step, the definition of the phenotype of cells and more specifically the markers that will serve as reference standards to identify specific cell types. To this regard, it is important to consider an aspect specifically related to perinatal cells and that is the determination of the fetal or maternal origin (Parolini et al., 2008). This is critical since the detection of maternal cells may depend on cell expansion in in vitro culture. For example, we previously demonstrated that genomic Polymerase Chain Reaction amplification was not able to determine the presence of maternal cells in the freshly isolated mesenchymal fractions of both the amnion and chorion (Soncini et al., 2007). However, after several cell passages in culture, maternal cells were detected in cell populations from the chorionic membrane, while those from the amniotic membrane did not show the presence of maternal alleles (Soncini et al., 2007). Hence, this may well result in working with a mixture of maternal and fetal cells, while the intent was to just work with fetal cells.

Another important aspect relates to cell culture expansion, because specific characteristics of the cells change, including phenotype and expression of specific proteins. This may result in the impression that one may be working with different cells compared to those isolated from perinatal tissues. In the case of human amniotic membrane MSC (hAMSC) and human amniotic membrane epithelial cells (hAEC) from placenta, it has previously been demonstrated that cell culture up to passage 4 after isolation (passage 0) can induce changes in the expression of cell markers. Such changes include the significant increase in adhesion molecules (e.g., CD49b, CD49d) and the significant decrease of CD14, CD45, and HLA-DR expression on hAMSC, as well as the significant increase of CD13, CD44, CD105, CD146 expression on cultures of hAEC (Stadler et al., 2008 Magatti et al., 2015).

Here, considering the numerous publications and the increasing interest in perinatal derivatives, we address specific issues that are relevant for the clear and precise definition/characterization of perinatal cells, starting from an understanding of the development of the human placenta, its structure, and the different cell populations that can be isolated from the different perinatal tissues. In addition, we describe where the cells are located within the placenta and provide an atlas of the human placenta. We also describe cell morphology and phenotype and propose nomenclature for the cell populations and derivatives discussed herein. The proposed nomenclature will be crucial to lay the foundation for the consistency in the scientific community when referring to PnD. This review is a joint effort from the COST SPRINT Action (CA17116), which broadly aims at approaching consensus for different aspects of PnD research, such as providing inputs for future standards for the processing, in vitro characterization and clinical application of PnD.


All placentas should be handled according to standard biohazard precautions. Gross examination can be performed on fresh specimens, or after 1&ndash2 days of fixation in formalin. Prolonged refrigeration (5&ndash7 days) does not affect histology. We prefer gross examination of fresh placentas, and select tissue samples that are subsequently fixed in formalin overnight before trimming or processing. Placentas from stillborn fetuses should not be fixed in formalin on receipt because placental fibroblasts may be needed for cytogenetic or other metabolic analyses. Microbial cultures should be performed in the labor and delivery suite, and not hours after birth in the pathology laboratory.

Placentas must be weighed and measured. Different approaches to placental weighing (removing cord, removing extraplacental membranes, draining maternal intervillous blood) may be taken, but one method must be used consistently, and documented in the pathology report. The cord length and diameter are recorded. The fetal&ndashplacental weight ratio increases throughout gestation, and abnormally high or low ratios are likely indicators of fetoplacental pathology (see Table 1).

A sample gross dictation form and suggested criteria for selection of placentas for pathologic examination are presented in Tables 2 and 3.

Table 2. Suggested standard dictation for singleton placenta

  1. Received fresh/fixed/not specifically labeled placenta is a _g placenta with a _g attached membranes and umbilical cord.
  2. The placenta is fragmented/round/oval/bilobate/accessory lobes and measures _cm × _cm × _cm in thickness.
  3. Membranes are complete/incomplete and site of rupture is indeterminate/at the placental margin/less than 10 cm/greater than 10 cm, from the nearest placental margin.
  4. Membranes are translucent/thickened/shiny/granular/green/yellow/foul smelling, with squamous metaplasia/amnion nodosum/hemorrhage.
  5. Marginal decidual necrosis is unremarkable/prominent/excessive.
  6. Intramembranous blood vessels are absent /present /intact ruptured and traverse unsupported for _ cm in the membranes.
  7. Membranes are inserted marginally/circummarginate/circumvallate over _% of the placental circumference.
  8. Maximal extent of placenta extrachorales is _cm.
  9. The umbilical cord has central/eccentric/markedly eccentric/marginal /velamentous insertion, _cm from the nearest placental margin and measures _cm in length and _cm in diameter at the fetal end and _cm in diameter at the placental end.
  10. The cord has true knot /hematoma/stricture /torsion/thrombosis not associated with hemostat marks, located_.
  11. The cut surface of the cord shows _ vessels with normal/edematous/decreased Wharton's jelly and no/venous/arterial thrombi.
  12. The chorionic plate vessels are tortuous/ectatic/thrombosed/calcified and subchorionic fibrin deposition is normal/increased/decreased, with the largest plaque measuring _cm × _cm × _cm in thickness.
  13. There is/is not grossly obvious missing placental tissue.
  14. There is fibrinoid deposition decidual/necrosis/calcification/blood clot covering approximately _ % of the maternal surface.
  15. The cut surface is pale/congested/firm/gritty/edematous and shows fibrinoid/infarction/thrombi occupying approximately % of the substance.
  16. Miscellaneous findings.
  17. The standard placental sections are submitted.
  18. Additional sections of grossly identified abnormalities are submitted as follows.
  19. Individual lesions are measured.

Table 3. Which placentas should be examined by a pathologist?

Recommended Maternal Indications for Placental Examination
Systemic disorders with clinical concerns for mother or infant (e.g., severe diabetes, impaired glucose metabolism, hypertensive disorders, collagen disease, seizures, severe anemia (<9 g)
Premature birth at 34 weeks' gestation or earlier
Intrapartum fever and/or infection
Unexplained third trimester bleeding or excessive bleeding >500 mL
History of HIV, syphilis, cytomegalovirus, primary herpes, toxoplasmosis, or rubella infections during this pregnancy
Severe oligohydramnios
History of unexplained or recurrent pregnancy complication fetal growth restriction, stillbirth, spontaneous abortion, premature birth)
Poor pregnancy outcome associated with intrauterine intervention, either diagnostic or therapeutic, for fetal abnormality or induced abortion for fetal abnormality
Abnormal antenatal testing leading to therapeutic intervention

Optional Maternal Indications for Placental Examination
Premature birth at 35&ndash36 weeks' gestation
Severe unexplained polyhydramnios
History of substance abuse

Recommended Fetal/Natal Indications for Placental Examination
Admission to intensive care nursery or transfer to another facility for care
Stillbirth/Perinatal death
Compromised clinical condition defined as any of the following:

Cord blood pH <7.0
Apgar score <7 at 5 minutes
Ventilatory assistance >10 minutes
Severe anemia (hematocrit <35%)
Hydrops fetalis
Birthweight <10th centile
Major congenital anomalies, dysmorphic phenotype, or abnormal karyotype
iscordant twin growth or vanishing twin
Multiple gestation with like sex infants and fused placentas

Optional Fetal/Natal Indications for Placental Examination
Birth weight >95th centile
Discordant growth parameters in any infant
Multiple gestation without other indication

Recommended Placental Indications for Placental Examination
Visible or palpable abnormality (e.g., infarct, mass, vascular thrombosis, retroplacental hematoma, amnion nodosum, abnormal coloration or opacification)
Small-for-gestational-age placental dimensions or weight
Umbilical cord lesions (e.g., thrombosis, torsion, true knot, single artery, absence of Wharton's jelly)
Viscid/thick meconium
Short umbilical cord (<32 cm at term)

Optional Placental Indications for Placental Examination
Large-for-gestational-age placenta
Abnormalities of placental shape
Long umbilical cord (>100 cm)
Marginal or velamentous cord insertion
(Adapted from Langston C, Kaplan C, Macpherson T et al: Practice guideline for examination of the placenta: Developed by the Placental pathology practice Guideline Development Task Force of the College of American Pathologists. Arch Pathol Lab Med 121 (5):449, 1997)

The placental parenchyma is grossly examined by slicing the maternal surface perpendicular to the chorionic plate at approximately 1-cm intervals, leaving the chorionic plate intact. Each surface of each slice is examined for obvious lesions (e.g., infarct, intervillous thrombi) or subtle lesions, such as increased villous granularity (e.g., villous swelling or chronic villitis). Villous tissue should be uniform in color dark or pale areas may mark microscopic pathology.

We suggest the following approach to placental sampling:

  1. One cassette containing one section from the fetal end of the umbilical cord (the end closest to the baby), 2.5 cm from the insertion (to ensure accurate vessel count) and avoiding areas with evidence of cord clamping, and one strip of extraplacental membranes taken from the rim of the site of rupture (if identifiable).
  2. One cassette containing one section from the placental end of the umbilical cord (the end closest to the chorionic plate), and one strip of extraplacental membranes extending from the rim of the site of rupture perpendicular to the placental margin. If a few marginal villi are included in this strip, this piece of membrane will always be able to be distinguished from the site of rupture sample. (Cassettes 1 and 2 may be combined.)
  3. Four sections containing grossly representative villous parenchyma in the central 60% of the chorionic disc (avoiding the margins) and can include areas of pallor and increased granularity. Full-thickness sections (i.e., extending from the chorionic plate to the basal plate) are preferable, but often the placenta is too thick. In such cases, the full-thickness section can often be divided, and the pieces placed in a single cassette. At minimum, these four tissue samples should include:

One cassette containing chorionic plate from an area with minimal subchorionic fibrin, and containing chorionic vessels.
Three cassettes based on the basal plate. Examination of the uteroplacental vascular segments delivered with the placenta can be readily achieved, especially in the fixed placenta, in which they can often be seen as small irregularities or an S-shape on the maternal surface. Several thin slices of the basal plate may be placed in one cassette, and will generally yield at least one uteroplacental vessel for examination. These cassettes also provide chorionic villi from the central areas of the villous parenchyma, representative of the best functioning villous tissue.

Additional sections from lesions.

Four sections of placental parenchyma will yield 95% of villous abnormalities. Adequate placental diagnosis can be done with three to four cassettes, depending on the thickness of the placenta. Uteroplacental vessels should be available for review in all cases, especially when uteroplacental vascular pathology is suspected.


Pregnancy Complication

Altered placental DNA methylation has been studied in the context of pregnancy complications including miscarriage, preeclampsia, intrauterine growth restriction (IUGR), and trisomy (see Table 2). Generally speaking, many changes in DNA methylation are observed with gross placental pathology (i.e., hydatidiform mole, triploidy, early-onset/severe preeclampsia), although more subtle changes occur when there is not a distinct pathology. For example, early-onset/severe preeclampsia is associated with decreased placental perfusion (characterized by increased syncytial knots, aggregates of intervillous fibrin, vascular lesions) and smaller placentas (Redline 2008 Nelson et al. 2014). There are correspondingly widespread and large changes in DNA methylation associated with early-onset preeclampsia (Yuen et al. 2010 Jia et al. 2012 Blair et al. 2013 Anton et al. 2014). Although a subset of these changes overlap sites altered in syncytial trophoblast differentiation and hypoxia exposure (Yuen et al. 2013), the overall relationship of the DNA methylation changes to the observed pathology is unknown and likely complex (Blair et al. 2013). Regardless, DNA methylation profiling should prove clinically useful to group placentas that show similar underlying pathologies as well as to identify DNA methylation marks that could be used to quantify or characterize placental DNA circulating in maternal blood during pregnancy (Yuen et al. 2011b).

DNA methylation changes that have been reported to be associated with IUGR tend to be of small magnitude and have not always been reproduced in other studies. An example is altered DNA methylation associated with the H19/IGF2 ICR in IUGR, which has been reported in some but not all studies (Guo et al. 2008 Bourque et al. 2010 Tabano et al. 2010 Cordeiro et al. 2014). This may be because of both technical aspects and different clinical criteria used in these studies. Many studies have used small-for-gestational age (SGA) (<10th percentile) as a surrogate for IUGR, although only a subset of SGA cases are associated with placental-mediated IUGR. Some studies may also be complicated by inclusion of cases that co-occur with preeclampsia. Larger samples sizes with stricter case criteria may help clarify consistent placental DNA methylation changes associated with IUGR. Genome-wide DNA methylation profiling may prove useful as an approach to classification of cases by etiology in a manner unbiased by clinical presentation.

Chromosomally normal first trimester miscarriages show relatively few DNA methylation changes compared with placentas obtained from elective terminations (Hanna et al. 2013), possibly because of the heterogeneous etiology of miscarriage in such samples. Nonetheless, a number of significant differences in DNA methylation have been noted, including an increased number of extreme values at imprinted DMRs (Pliushch et al. 2010 Hanna et al. 2013). It remains to be determined if such changes are causative or a consequence of associated placental changes in these pregnancies (e.g., delayed or arrested development).

Even when a distinct etiology for abnormal placental function is apparent, it is important to consider gestational age effects. This can be illustrated by our studies of placentas with trisomy 16 (Blair et al. 2014). Although widespread changes in placental DNA methylation were found in both first trimester trisomy 16 (ascertained from miscarriages) and third trimester trisomy 16 (in the context of confined placental mosaicism), there was relatively little overlap between the major changes at these different gestational ages. Overall there were a greater number of changes in the third trimester, which may reflect the greater time for changes to accumulate in response to such a genetic insult.

DNA Methylation Changes in Response to Exposure

The placenta has received attention in the field of developmental origins of health and disease (DOHaD) because of its role in moderating the fetal microenvironment. In addition to its immunologic and synthetic roles, placental tissue transfers molecules in the maternal blood to the fetus as well as consuming some itself. By changing structure, cell composition or gene expression, the placenta may respond to, buffer against, or adapt to the contents of maternal blood. For example, smaller and/or lighter placentas have been documented in offspring of mothers who smoke (Anblagan et al. 2013), were pregnant during Ramadan fasting (Alwasel et al. 2013), or were exposed to higher levels of air pollution (van den Hooven et al. 2012). Larger and/or heavier placentas have been documented in offspring of mothers who are obese or endure psychosocial stress during pregnancy (Tegethoff et al. 2010). Furthermore, structural and functional changes in the placenta might correlate with changes in the fetus. Raised blood pressure in offspring was found to be associated with smaller placenta (weight and size) in children born to mothers of short stature and low socioeconomic status (Barker et al. 2010).

Analyzing site-specific and/or genome-wide placental DNA methylation may thus afford a resource for assessing maternal environmental conditions that shape fetal development and postnatal disease. A measured change in placental DNA methylation may reflect one of several effects of an exposure: (1) altered placental morphology (2) changes in gene expression or (3) changes in the establishment/maintenance of DNA methylation. The placenta shows a remarkable degree of developmental plasticity (Yuen and Robinson 2011). Therefore, it is important to consider that evidence of an exposure in the placenta will not necessarily be associated with an effect in the fetus. But while numerous studies have examined maternal exposures by targeting individual genes likely to be involved in placental adaptation, far fewer have examined the placental methylome in association with environmental exposures (Tables 2 and 3). With the exception of chromosomal abnormalities, preeclampsia, and perhaps maternal smoking, little difference in genome-wide placental DNA methylation has been consistently observed. Some of the difficulties facing DOHaD epigenome-wide association studies (EWAS) in placenta include the following.

Study design: Amassing a large group of human samples with a homogeneous exposure is difficult. Heterogeneous genetic background and environmental modifiers further complicate phenotype and necessitate an even larger sample size.

Statistical power and biological significance: Investigators should first consider if there is reason to expect alterations in DNA methylation on the site-specific or genome-wide level, if at all. The power of EWAS studies is heavily burdened by the essential correction for multiple testing. Furthermore, the magnitude of change in DNA methylation that might be consequent to an exposure and/or result in functional changes is unknown and likely to vary across the genome.

Exposure conditions: Study of the blood of individuals born to mothers exposed to the Dutch famine suggests that differential DNA methylation at some imprinted genes may be dependent on the timing of exposure (i.e., periconceptional vs. late gestation) as well as sex of the fetus (Heijmans et al. 2008 Tobi et al. 2009). Variables such as the type, timing, and duration/dose of exposure may result in differential adaptation and in turn differential patterns of DNA methylation.

Studies of genome-wide placental DNA methylation with environmental exposures

With attention to these factors, the placenta should yield valuable information as to the in utero exposures that may affect neonatal and postnatal health. Deciphering the epigenetic record of the placenta is an exciting and growing area of research, and focusing on changes in specific cell populations within the placenta may help distinguish changes in placental growth/structure from changes caused by the epigenome.



The perfusion of the intervillous spaces of the placenta with maternal blood allows the transfer of nutrients and oxygen from the mother to the fetus and the transfer of waste products and carbon dioxide back from the fetus to the maternal blood supply. Nutrient transfer to the fetus occurs via both active and passive transport. Active transport systems allow significantly different plasma concentrations of various large molecules to be maintained on the maternal and fetal sides of the placental barrier. [ 9 ]

Adverse pregnancy situations, such as those involving maternal diabetes or obesity, can increase or decrease levels of nutrient transporters in the placenta resulting in overgrowth or restricted growth of the fetus [ citation needed ] .


Waste products excreted from the fetus such as urea, uric acid and creatinine are transferred to the maternal blood by diffusion across the placenta.


IgG antibodies can pass through the human placenta, thereby providing protection to the fetus in utero. [ 10 ]

Furthermore, the placenta functions as a selective maternal-fetal barrier against transmission of microbes to the fetus. However, insufficiency in this function may still cause mother-to-child transmission of infectious diseases.

Endocrine function

In humans, aside from serving as the conduit for oxygen and nutrients for fetus, the placenta secretes hormones (secreted by syncytial layer/syncytiotrophoblast of chorionic villi) that are important during pregnancy.

Human Chorionic Gonadotropin (hCG): The first placental hormone produced is hCG, which can be found in maternal blood and urine as early as the first missed menstrual period (shortly after implantation has occurred) through about the 100th day of pregnancy. This is the hormone analyzed by pregnancy test a false-negative result from a pregnancy test may be obtained before or after this period. Women's blood serum will be completely negative for hCG by one to two weeks after birth. hCG testing is proof that all placental tissue is delivered. hCG is present only during pregnancy because it is secreted by the placenta, which is present only [ 11 ] during pregnancy. hCG also ensures that the corpus luteum continues to secrete progesterone and estrogen. Progesterone is very important during pregnancy because, when its secretion decreases, the endometrial lining will slough off and pregnancy will be lost. hCG suppresses the maternal immunologic response so that placenta is not rejected.

Human Placental Lactogen (hPL [Human Chorionic Somatomammotropin]): This hormone is lactogenic and growth-promoting properties. It promotes mammary gland growth in preparation for lactation in the mother. It also regulates maternal glucose, protein, and fat levels so that this is always available to the fetus.

Estrogen is referred to as the "hormone of women" because it stimulates the development of secondary female sex characteristics. It contributes to the woman's mammary gland development in preparation for lactation and stimulates uterine growth to accommodate growing fetus.

Progesterone is necessary to maintain endometrial lining of the uterus during pregnancy. This hormone prevents preterm labor by reducing myometrial contraction. Levels of progesterone are high during pregnancy.

Cloaking from immune system of mother

The placenta and fetus may be regarded as a foreign allograft inside the mother, and thus must evade from attack by the mother's immune system.

For this purpose, the placenta uses several mechanisms:

  • It secretes Neurokinin B-containing phosphocholine molecules. This is the same mechanism used by parasiticnematodes to avoid detection by the immune system of their host. [ 12 ]
  • There is presence of small lymphocytic suppressor cells in the fetus that inhibit maternal cytotoxic T cells by inhibiting the response to interleukin 2. [ 13 ]

However, the placental barrier is not the sole means to evade the immune system, as foreign foetal cells also persist in the maternal circulation, on the other side of the placental barrier. [ 14 ]

Other functions

The placenta also provides a reservoir of blood for the fetus, delivering blood to it in case of hypotension and vice versa, comparable to a capacitor. [ 15 ]


In conclusion, identification of infection of the placental tissues is essential to investigation of spontaneous abortion, stillbirth, premature delivery, growth retardation, and the sick newborn. In some cases, particularly with Cytomegalovirus and Herpes Simplex Virus, the neonate may be initially asymptomatic, and prompt recognition of infection can inform clinical care. The presence or absence of acute chorioamnionitis and funisitis can aid in care of the premature neonate in the appropriate clinical context, supporting a clinical diagnosis of neonatal sepsis when blood cultures are often negative and the infant remains unwell. Ideally, future investigations with newer technologies will identify whether or not specific microbes in chorioamnionitis better correlate with clinical outcomes. Many cases of chronic villitis lacking a clinical history of infection, or characteristic viral cytopathic effects (villitis of unknown etiology), are attributed to a maternal alloimmune response to fetal/trophoblastic antigens 131 . While reasonable and probable, the surprising ubiquity of microbes, or at least, genetic traces of microbes, suggest it is still best to call these patterns, “villitis of unknown etiology” until our understanding evolves.


These developmental changes can be primary due to an intrinsic placental defect or secondary to intervillous hypoxemia. The patterns should not be confused with the clinical term placental insufficiency, which indicates a complication of pregnancy in which the placenta cannot carry enough oxygen and/or nutrients to the growing fetus and usually implies the presence of FGR. 37

The placental response to hypoxia does not follow a single pathway, 21 the type of placental maturation and villous vascularity being the 2 key features used in classification of placental chronic-hypoxic patterns (Table 2). Features listed in Table 1 and discussed in the preceding section are also helpful. 21,31,38

Patterns of Chronic Hypoxic Placental Injury

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The preuterine pattern (PR) of chronic, hypoxic placental injury features, histologically, homogeneous placental maturation (ie, the chorionic villi are plump and of similar size from field to field) increased villous vascularity, including practically all cases of diffuse chorangiosis decreased extracellular matrix of chorionic villi and villous cytotrophoblasts increased Hofbauer cells and increased nonapoptotic syncytial knotting 21,31 (Figure 2, A). Because of villous enlargement, the placenta looks hypomature, except for the focally increased syncytial knots, which is not, however, a constant feature. Placentomegaly and more-advanced gestational age are frequently seen. 21,39 The pattern is evoked by maternal hypoxemia secondary to decreased oxygen pressure in the environment (pregnancies at high altitudes), decreased oxygen binding capacity of the maternal blood (maternal anemia), air pollution and maternal smoking, increased distension of the uterus (multiple pregnancy), and maternal diabetes mellitus (abnormal oxygen-hemoglobin dissociation curve). 21,38–44 Clusters of multinucleate giant cells in the decidua basalis and excessive numbers of extravillous trophoblasts are less commonly seen in this than in other patterns of diffuse hypoxic placental injury, 21 most likely because of the association of PR with deep trophoblastic, myometrial invasion, which was proven, at least in maternal anemia. 45 This pattern has a better prognosis than other types of chronic hypoxic injury, 21 probably because of hypoxic preconditioning and resistance to ischemia-reperfusion injury during labor, as was proven in pregnancies at high altitudes 46 and multiple pregnancies. 43

Basic patterns of hypoxic placental injury. A, Preuterine hypoxic pattern, 36-week pregnancy, maternal anemia homogeneous villous hypomaturity, chorangiosis, focally increased syncytial knotting, decreased extracellular matrix of chorionic villi, increased Hofbauer cells and villous cytotrophoblasts. B, Uterine hypoxic pattern, 32 weeks, severe preeclampsia, focal villous hypermaturity, focally increased villous cytotrophoblasts (left side of microphotograph), decreased extracellular matrix of chorionic villi, increased Hofbauer cells and nonapoptotic syncytial knotting. C, Diffuse postuterine hypoxic pattern, 21 weeks, maternal substance abuse, retained stillbirth homogeneous maturation, increased extracellular matrix decreased vascularity. D, Diffuse postuterine hypoxic pattern, 41 weeks, fetal growth restriction homogeneous villous hypermaturity with terminal villous hypoplasia, increased extracellular matrix, decreased vascularity, villous cytotrophoblasts, increased apoptotic syncytial knotting. E, Focal postuterine pattern (fetal artery thrombosis), 34 weeks, gestational hypertension and fetal growth restriction a cluster of avascular chorionic villi with increased extracellular matrix. F, Dysmature/hypomature placenta (villous maturation defect), 39 weeks, unexpected stillbirth inset, CD34 immunostain highlighting poorly formed vasculosyncytial membranes (hematoxylin-eosin, original magnifications ×10 [A, B, and E], ×20 [C, D, and F], and ×40 [inset]).

Basic patterns of hypoxic placental injury. A, Preuterine hypoxic pattern, 36-week pregnancy, maternal anemia homogeneous villous hypomaturity, chorangiosis, focally increased syncytial knotting, decreased extracellular matrix of chorionic villi, increased Hofbauer cells and villous cytotrophoblasts. B, Uterine hypoxic pattern, 32 weeks, severe preeclampsia, focal villous hypermaturity, focally increased villous cytotrophoblasts (left side of microphotograph), decreased extracellular matrix of chorionic villi, increased Hofbauer cells and nonapoptotic syncytial knotting. C, Diffuse postuterine hypoxic pattern, 21 weeks, maternal substance abuse, retained stillbirth homogeneous maturation, increased extracellular matrix decreased vascularity. D, Diffuse postuterine hypoxic pattern, 41 weeks, fetal growth restriction homogeneous villous hypermaturity with terminal villous hypoplasia, increased extracellular matrix, decreased vascularity, villous cytotrophoblasts, increased apoptotic syncytial knotting. E, Focal postuterine pattern (fetal artery thrombosis), 34 weeks, gestational hypertension and fetal growth restriction a cluster of avascular chorionic villi with increased extracellular matrix. F, Dysmature/hypomature placenta (villous maturation defect), 39 weeks, unexpected stillbirth inset, CD34 immunostain highlighting poorly formed vasculosyncytial membranes (hematoxylin-eosin, original magnifications ×10 [A, B, and E], ×20 [C, D, and F], and ×40 [inset]).

The uterine (UH) pattern of chronic hypoxic placental injury (Figure 2, B) is typically seen in preeclampsia and late-onset FGR. 31,38 The term uterine is used here to refer to the maternal portions of the uteroplacental circulation, including its myometrial and decidual segments, which are the cause of pathologic placental changes. It is favored over uteroplacental to avoid confusion as to whether the maternal or the fetal side of the placenta is primarily involved. 47 Heterogeneous placental hypermaturity, with only focally increased villous vascularity, villous cytotrophoblast density, Hofbauer cells, and nonapoptotic syncytial knotting, and a decreased extracellular matrix of chorionic villi are the characteristic features. As with PR, the focal hypervascularity is an adaptive mechanism, reaching the level of chorangiosis in some cases, whereas, in other cases, the villous capillary profiles remain between 7 and 9 per chorionic villus (incipient or emerging chorangiosis). 22 The UH pattern is frequently associated with other placental features of uteroplacental malperfusion related to shallow, trophoblastic invasion, such as an accumulation of extravillous trophoblasts in the placental membranes (Figure 3, A), characteristically with membrane chorionic microcyst formation (Figure 3, B), and in the chorionic disc (Figure 3, C), again, typically, with microcyst formations (Figure 3, D), decidual clusters of multinucleate trophoblasts (Figure 3, E) and basal-plate myometrial fibers, and occult placenta accreta (Figure 3, F). 27,38,48–54 Excessive amounts of extravillous trophoblasts must be distinguished from massive perivillous fibrin deposition/maternal floor infarction, 49 which is associated with FGR, impaired neurologic development, recurrent fetal loss, and stillbirth, 9,12,55 but not necessarily with UH. Pathogenesis of the massive, perivillous fibrin deposition is different, 9 but it can evoke fetal hypoxia by virtue of eliminating a substantial amount of functional placental parenchyma. I regard the above-presented extravillous trophoblasts lesions, along with the decidual arteriolopathy (both hypertrophic and atherosis), as the complementary criteria for this type of chronic placental hypoxia in histologically borderline cases. 21 The UH and associated histologic findings cluster with severe preeclampsia, but not with mild preeclampsia gestational hypertension hemolysis, elevated liver enzymes, and low platelets (HELLP) or eclampsia 56 There is growing evidence that there are basic differences in the pathogenesis of mild and severe preeclampsia, the former usually occurring as a late onset, and the latter, usually having an early onset, 57–60 to which we recently contributed the molecular basis. 61

Extravillous trophoblastic hypoxic lesions. A, Increased amount of extravillous trophoblasts in placental membranes, double immunostain E-cadherin (red)/Ki-67 (brown), 31 weeks, severe preeclampsia. B, Membrane microscopic chorionic pseudocysts, 36 weeks, mild preeclampsia and fetal growth restriction, abnormal cardiotocography. C, Increased extravillous trophoblasts, stillbirth at 17 weeks 4 cell islands are seen only in this field. D, Microscopic, chorionic pseudocysts of chorionic disc nonmacerated stillbirth at 31 weeks. E, Multinucleate trophoblastic cells in decidua basalis, termination of pregnancy for double outlet right ventricle at 22 weeks. F, Occult placenta accreta, twin pregnancy, severe preeclampsia, 27 weeks, neonatal death on day 6 of life. No decidua is seen between myometrial fibers (left) and maternal flood extravillous trophoblasts (right). This lesion is associated with increased extravillous trophoblasts at the maternal floor 57 (original magnification ×40 [A] hematoxylin-eosin, original magnifications ×10 [B], ×4 [C through E], and ×40 [F]).

Extravillous trophoblastic hypoxic lesions. A, Increased amount of extravillous trophoblasts in placental membranes, double immunostain E-cadherin (red)/Ki-67 (brown), 31 weeks, severe preeclampsia. B, Membrane microscopic chorionic pseudocysts, 36 weeks, mild preeclampsia and fetal growth restriction, abnormal cardiotocography. C, Increased extravillous trophoblasts, stillbirth at 17 weeks 4 cell islands are seen only in this field. D, Microscopic, chorionic pseudocysts of chorionic disc nonmacerated stillbirth at 31 weeks. E, Multinucleate trophoblastic cells in decidua basalis, termination of pregnancy for double outlet right ventricle at 22 weeks. F, Occult placenta accreta, twin pregnancy, severe preeclampsia, 27 weeks, neonatal death on day 6 of life. No decidua is seen between myometrial fibers (left) and maternal flood extravillous trophoblasts (right). This lesion is associated with increased extravillous trophoblasts at the maternal floor 57 (original magnification ×40 [A] hematoxylin-eosin, original magnifications ×10 [B], ×4 [C through E], and ×40 [F]).

The postuterine (PU) (postplacental) pattern of chronic hypoxic placental injury is due to primary villous changes resulting in decreased intake of oxygen from the intervillous space, as in retained stillbirth (Figure 2, C), subsets of FGR (Figure 2, D) and preeclampsia, and fetal thrombotic vasculopathy (only focally) 13,38 (Figure 2, E). Clinically, abnormal Doppler flow velocity waveforms obtained from umbilical arteries that reflect the downstream blood flow impedance may give an indirect evidence of vascular tree abnormalities. 62 The fetoplacental blood flow is compromised to a far greater extent in FGR with absent end-diastolic blood flow, such that maternal blood leaving the placenta has higher oxygen content than it does under normal circumstances. 63 The PU features homogeneous placental hypermaturity and hypovascularity, with slender, pencillike chorionic villi so-called terminal villous hypoplasia 13 increased extracellular matrix of chorionic villi and apoptotic syncytial knots and decreased villous Hofbauer cells and villous cytotrophoblasts. 31 The clinical, umbilical cord compromise with focal placental lesions of decreased fetal blood flow, which is frequently a random pregnancy accident, did not correlate with diffuse PU, although they can produce the focal PU, so-called stasis-induced thrombotic vasculopathy, with clusters of avascular, fibrotic, and occasionally hemosiderotic chorionic villi. 64,65 Also, severe, mass-forming, congenital anomalies that interfere with blood return from the placenta to the fetus can produce the placental stasis-induced thrombotic vasculopathy, but not the diffuse PU. 66

Hypomature placentas (placentas with maturation defect) are pale, normal sized or even larger than normal, with plumper terminal chorionic villi, superficially resembling intermediate villi, 67 and show defective formation of both terminal villous sinusoids (although numerically usually normal) and vasculosyncytial membranes (Figure 2, F), which can be better highlighted with CD34 immunostain (Figure 2, F inset). Although the vasculosyncytial membranes are normally poorly formed in early pregnancy, they are well formed in term pregnancies and are absent in only 5% of chorionic villi at term. 12 In this type of placental pathology in pregnancies of 35 weeks or longer, on average, 1 vasculosyncytial membrane per terminal villus was found as compared with, on average, 3 per chorionic villi normally seen. 68 Villous hypomaturity is responsible for 22.5% of intrauterine deaths 69 and has a 70-fold increased risk of associated fetal death, with a 10-fold risk for recurrence, compared with baseline. 70 The fetuses die because of hypoxia, 1 but they can be rescued by earlier delivery. 70 There is an association between maternal diabetes mellitus 71 and fetal anomalies 72 and maturation defect placentas, but vasculosyncytial membranes are also decreased in umbilical cord hypercoiling. 73 Diabetes mellitus is a state of chronic oxidative stress, 74 and glycemia appears to have an affect on the capillary, but not the stromal component, of chorionic villi. 75 Placental angiogenesis is stimulated by insulin via ephrin-B2 expression, a signaling molecule implicated in sprouting. 76 Therefore, this subtype of PR chronic hypoxic pattern appears to include the PU component because of fetal hyperinsulinemia.

The above-discussed patterns of chronic hypoxic placental injury are of a developmental origin because they are programmed and initiated early in pregnancy, some of them probably due to fetal or maternal genetic causes, and are usually fully developed at the turn of the second and the third trimester or later. Serum biomarkers of preeclampsia have been shown to be altered at as early as 7 weeks gestation, indicating that the onset of placental abnormalities in preeclampsia occurs even earlier than the onset of maternal blood flow, when failure of transformation of spiral arteries are thought to occur. 77 Maldevelopment of the maternal spiral arteries in the first trimester predisposes to placental dysfunction and suboptimal pregnancy outcomes in the second half of pregnancy. 78 The UH is, therefore, a maternal, hypoxemia-induced, focal-adaptive villous change, whereas PU is associated with intervillous hyperoxemia. The patterns may predispose a patient toward a greater vulnerability for superimposed, acute hypoxic lesions (overlap patterns/lesions, see below) or may sometimes protect against acute hypoxia in labor because PR can be an adaptive placental response and, in fact, protect the fetus against birth-related hypoxia. 46

Although characteristic, histologic features were originally described for the above patterns, 31 I have proposed, based on the histologic picture, that one can predict what the pathogenesis of villous hypoxia is because the 3 patterns cluster with various clinical settings and associated placental findings. 56 The practical value of using the histologic terms PR, UH, and PU lies in keeping the pathologists aware that there is no single pattern of chronic hypoxic placental injury and, therefore, in expanding the pathologists' armamentarium in this respect.

In my experience, 21 UH is most commonly associated with severe preeclampsia and the HELLP syndrome PU, with abnormal Doppler results, induction of labor, clinical umbilical cord abnormalities, cesarean section rate, and FGR and PR, with multiple pregnancies. Overall, PU is the most ominous and PR the least ominous histologic subtype of chronic in utero hypoxia. Sensitivity of placental injury from diffuse, chronic hypoxic placental injury is less than 50% for the main clinical conditions known to be associated with in utero hypoxia, and most cases of preeclampsia, pregnancy-induced hypertension, diabetes mellitus, nonreassuring fetal heart rate tracing, abnormal Doppler results (absent or reversed end-diastolic umbilical artery flow), and FGR were found in patients without diffuse placental hypoxic patterns. On the other hand, the high (>90%) specificity of the chronic hypoxic patterns of placental injury means that, in the presence of those histologic patterns, the clinical manifestations of chronic in utero hypoxia are very likely therefore, the positive identification of one of the patterns is significant and unlikely to occur solely by chance.


Villi can also be classified by their relations:

  • Floating villi float freely in the intervillous space. They exhibit a bi-layered epithelium consisting of cytotrophoblasts with overlaying syncytium (syncytiotrophoblast).
  • Anchoring (stem) villi stabilize mechanical integrity of the placental-maternal interface.

Development Edit

The chorion undergoes rapid proliferation and forms numerous processes, the chorionic villi, which invade and destroy the uterine decidua and at the same time absorb from it nutritive materials for the growth of the embryo. They undergo several stages, depending on their composition.

Stage Description Period of gestation Contents
Primary The chorionic villi are at first small and non-vascular. 13–15 days trophoblast only [1]
Secondary The villi increase in size and ramify, while the mesoderm grows into them. 16–21 days trophoblast and mesoderm [1]
Tertiary Branches of the umbilical artery and umbilical vein grow into the mesoderm, and in this way the chorionic villi are vascularized. 17–22 days trophoblast, mesoderm, and blood vessels [1]

Until about the end of the second month of pregnancy, the villi cover the entire chorion, and are almost uniform in size—but after then, they develop unequally.

Microanatomy Edit

The bulk of the villi consist of connective tissues that contain blood vessels. Most of the cells in the connective tissue core of the villi are fibroblasts. Macrophages known as Hofbauer cells are also present.

Use for prenatal diagnosis Edit

In 1983, an Italian biologist named Giuseppe Simoni discovered a new method of prenatal diagnosis using chorionic villi.

Stem cell Edit

Chorionic villi are a rich source of stem cells. Biocell Center, a biotech company managed by Giuseppe Simoni, is studying and testing these types of stem cells. Chorionic stem cells, like amniotic stem cells, are uncontroversial multipotent stem cells. [2] [3] [4]

Infections Edit

Recent studies indicate that the chorionic villi may be susceptible to bacterial [5] and viral infections. Recents findings indicate that ureaplasma parvum can infect the chorionic villi tissues of pregnant women, thereby impacting pregnancy outcome. [6] DNA from JC polyomavirus and Merkel cell polyomavirus has been detected in chorionic villi from pregnant women and women affected by miscarriage. [7] [8] DNA from BK polyomavirus has also been detected in the same tissues but to a lesser extent. [7]

Early miscarriage Edit

In early miscarriage, the finding of chorionic villi in vaginal expulsions is often the only definite confirmation that there was an intrauterine pregnancy rather than an ectopic pregnancy.

Micrograph showing chorionic villi. Intermediate magnification. H&E stain.

Micrograph showing chorionic villi. Very high magnification. H&E stain.

Watch the video: The Placenta: Its Development and Function (June 2022).


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