The degu Octodon degus is one of the very few members of caviomorph or hystricognath Rodentia that possesses a simply arranged chorioallantoic placenta without advanced lobulation. Therefore this species was used as a model to study regional development and growth processes of the placenta, based on the examination of 20 individuals by light and electron microscopy as well as by using markers for proliferation, trophoblast and endometrial stroma. The results were interpreted by comparison with other hystricognaths in the light of their evolutionary history. It was found that trophoblast derived from the trophospongium is essential for extension of the placenta including the labyrinth: extensive proliferation is restricted to trophoblast cells at the outer margin of the placenta and along internally directed, finger-tip like protrusions of fetal mesenchyme towards the labyrinth. This kind of placental development is regarded as part of the stem species pattern of hystricognaths, evolved more than 40 million years ago. It is indicated for the first time that the replenishment of the syncytiotrophoblast is similar to corresponding processes in the human placenta. In conclusion, the degu is a useful model for placental growth dynamics, particularly because of its simply arranged placental architecture, and may also serve as an animal model in comparison to human pregnancies. J. Exp. Zool.(Mol. Dev. Evol.) 308B, 2007.(c) 2007 Wiley-Liss, Inc.

Until now, defining characters of hystricognath rodents have been the subplacenta and the lobulation of the main chorioallantoic placenta. However, recent studies have revealed hystricognaths without marked lobulation, which is a plesiomorphic condition of the group. The question thus arises whether the subplacenta of these taxa is structurally homologous to that of other hystricognaths. Therefore, subplacental morphology and ontogeny were investigated in Octodon and Petromus by conventional light and electron microscopy, and the stem species pattern of Hystricognathi was reconstructed by applying MacClade(trade mark). The subplacentae of both species share important similarities with other hystricognaths. The organ develops early in gestation but degenerates towards term. It consists of folded layers of cellular and syncytial trophoblast, the latter enclosing maternal blood lacunae and electron-dense particles. Root-like syncytial outgrowths (syncytial streamers) and extraplacental trophoblast cells occur at the lateral and basal borders of the organ. Maternal vascularisation by blood lacunae within the subplacental syncytiotrophoblast is acquired early but lost during mid-gestation. Vascularisation by fetal vessels is established later. Fetomaternal exchange via blood circulation inside the subplacenta is unlikely to occur, since periods of maternal and fetal vascularisation show little overlap. In conclusion, the subplacentae of both species are regarded as homologous to other hystricognaths, comprising 18 character conditions that belong to their stem species pattern. Thus, the systematic unity of the group can be confirmed. J. Exp. Zool.(Mol. Dev. Evol.) 306B, 2006.(c) 2006 Wiley-Liss, Inc.

The placental vasculature of five hystricomorph rodents was examined by latex injection of the blood vessels, immunohistochemistry and scanning electron microscopy of vessel casts. The pattern of branching of the vessels is described at the level of fine structure. The placenta is divided into lobes separated by interlobular trophoblast. Fetal arteries course through the interlobular areas and give rise to capillaries from which blood drains into veins at the centre of the lobes. Maternal blood reaches the placenta through spiral arteries that pass around the perimeter of the subplacenta. They supply large maternal blood sinuses, lined by trophoblast, which run through the interlobular areas and into the centre of the lobes. Here they supply fine channels that run parallel to the fetal capillaries, so that maternal blood flows from the centre of the lobe to the periphery. This arrangement provides the morphological basis for countercurrent exchange. The maternal channels of the labyrinth drain into spaces formed by the latticework of the interlobular trophoblast and thence through venous lacunae to a basal venous lacunar ring. The subplacenta is supplied by a single fetal artery. The vessels within the subplacenta pursue a tortuous course with dilatations and constrictions as in an endocrine gland.

Evidence from several sources supports a close phylogenetic relationship between elephants and sirenians. To explore whether this was reflected in similar placentation, we examined eight delivered placentae from the Amazonian manatee using light microscopy and immunohistochemistry. In addition, the fetal placental circulation was described by scanning electron microscopy of vessel casts. The manatee placenta was zonary and endotheliochorial, like that of the elephant. The interhaemal barrier comprised maternal endothelium, cytotrophoblasts and fetal endothelium. We found columnar trophoblast beneath the chorionic plate and lining lacunae in this region, but there was no trace in the term placenta of haemophagous activity. The gross anatomy of the cord and fetal membranes was consistent with previous descriptions and included a four-chambered allantoic sac, as also found in the elephant and other afrotherians. Connective tissue septae descended from the chorionic plate and carried blood vessels to the labyrinth, where they gave rise to a dense capillary network. This appeared to drain into shorter vessels near the chorionic plate. The maternal vasculature could not be examined in the same detail, but maternal capillaries ran rather straight and roughly parallel to the fetal ones. Overall, there is a close resemblance in placentation between the manatee and the elephant.

Mammalian fetal survival and growth are dependent on a well-established and functional placenta. Although transient, the placenta is the first organ to be formed during pregnancy and is responsible for important functions during development, such as the control of metabolism and fetal nutrition, gas and metabolite exchange, and endocrine control. Epigenetic marks and gene expression patterns in early development play an essential role in embryo and fetal development. Specifically, the epigenetic phenomenon known as genomic imprinting, represented by the non-equivalence of the paternal and maternal genome, may be one of the most important regulatory pathways involved in the development and function of the placenta in eutherian mammals. A lack of pattern or an imprecise pattern of genomic imprinting can lead to either embryonic losses or a disruption in fetal and placental development. Genetically modified animals present a powerful approach for revealing the interplay between gene expression and placental function in vivo and allow a single gene disruption to be analyzed, particularly focusing on its role in placenta function. In this paper, we review the recent transgenic strategies that have been successfully created in order to provide a better understanding of the epigenetic patterns of the placenta, with a special focus on imprinted genes. We summarize a number of phenotypes derived from the genetic manipulation of imprinted genes and other epigenetic modulators in an attempt to demonstrate that gene-targeting studies have contributed considerably to the knowledge of placentation and conceptus development.

Abstract Objectives: In this study, we aimed at determining whether human immature dental pulp stem cells (hIDPSC) would be able to contribute to different cell types in mouse blastocysts without damaging them. Also, we analysed whether these blastocysts would progress further into embryogenesis when implanted to the uterus of foster mice, and develop human/mouse chimaera with retention of hIDPSC derivates and their differentiation. Materials and Methods: hIDPSC and mouse blastocysts were used in this study. Fluorescence staining of hIDPSC and injection into mouse blastocysts, was performed. Histology, immunohistochemistry, fluorescence in situ hybridization and confocal microscopy were carried out. Results and Conclusion: hIDPSC showed biological compatibility with the mouse host environment and could survive, proliferate and contribute to the inner cell mass as well as to the trophoblast cell layer after introduction into early mouse embryos (n = 28), which achieved the hatching stage following 24 and 48 h in culture. When transferred to foster mice (n = 5), these blastocysts with hIDPSC (n = 57) yielded embryos (n = 3) and foetuses (n = 6); demonstrating presence of human cells in various organs, such as brain, liver, intestine and hearts, of the human/mouse chimaeras. We verified whether hIDPSC would also be able to differentiate into specific cell types in the mouse environment. Contribution of hIDPSC in at least two types of tissues (muscles and epithelial), was confirmed. We showed that hIDPSC survived, proliferated and differentiated in mouse developing blastocysts and were capable of producing human/mouse chimaeras.

Guinea pig related rodents possess numerous derived placental characters. We attempt to identify diversity within the visceral yolk sac and its association with the chorioallantoic placenta in three species of caviids, two of them possessing a capsule formed by the decidua that covers the chorioallantoic placenta. The results verify that in early pregnancy all three species have an inverted yolk sac placenta. In advanced pregnancy the species differ: Galea spixii, as representative without a capsule, bear a yolk sac in apposition to the chorioallantoic placenta with signs of exchange activity until term. Galea is similar to other caviomorphs in this respect. In Dasyprocta leporina and Cuniculus paca, the representatives possessing a capsule, the yolk sac endoderm lacks signs of substance exchange. Evidently, the presence of a capsule prevents such an interaction. The variations established here must be considered if animal models for human placentation are required which have restricted access to the chorioallantoic placenta from the outside.

The morphological characteristics of the oviduct of 12 sexually mature rheas (Rhea americana) were studied. Only the left oviduct is developed as a long tube with a length of 122 +/- 23.1 cm, and is subdivided into infundibulum (15.2 +/- 4.0 cm), magnum (63.3 +/- 9.4 cm), isthmus (5.6 +/- 3.1 cm), uterus (16.0 +/- 4.2 cm) and vagina (11.5 +/- 1.4 cm). The mucous membrane of the oviduct, as a whole, possesses luminal folds covered by ciliated columnar epithelium with secretory cells. The infundibulum part presents a cranial opening with thin and long fimbriae with few tubular glands in caudal tubular portion. In the magnum, the largest portion of the oviduct, the folds are thicker and are filled with tubular glands. The isthmus is short and presents less bulky folds and a few tubular glands. A bag-shaped uterus in the cranial area shows thin folds, and in the caudal region (shell gland) more ramified folds with few tubular glands. The vagina has long luminal folds and a thick muscular tunic; no glands with sperm-storage characteristics have been observed. In conclusion, the oviduct in sexually mature rhea has morphological similarities with the other species of birds already described; however it presents its own characteristics to produce a big egg.

To elucidate the morphological differences between placentas from normal and cloned cattle pregnancies reaching term, the umbilical cord, placentomes and interplacentomal region of the fetal membranes were examined macroscopically as well as by light and scanning electron microscopy. In pregnancies established by somatic nucleus transfer (NT), the umbilical cord and fetal membranes were edematous. Placentomal fusion was common, resulting in increased size and a decreased number of placentomes. Extensive areas of the chorioallantoic membrane were devoid of placentomes. An increased number of functional or accessory microcotyledons (<1cm) were present at the maternally oriented surface of fetal membranes. Extensive areas of extravasated maternal blood were present within the placentomes and in the interplacentomal region. The crypts on the caruncular surface were dilated and accommodated complexes of more than one primary villus, as opposed to a single villus in non-cloned placentae. Scanning electron microscopy of blood vessel casts revealed that there was also more than one stem artery per villous tree and that the ramification of the vessels failed to form dense complexes of capillary loops and sinusoidal dilations as in normal pregnancies. At the materno-fetal interface, however, the trophoblast and uterine epithelium had normal histology. In conclusion, the NT placentas had a range of pathomorphological changes; this was likely associated with the poor clinical outcome of NT pregnancies.

The mostly binucleate trophoblast giant cells (TGC) found in bovine placentomes, in addition to synthesizing and releasing hormones play an important role in fetal development and maternal adaptation to pregnancy. Placentomes from early gestation were collected, and for isolation of mature TGC, three cellular disaggregation methods, mechanical (MECH), enzymatic by trypsin (TRYP) or collagenase (COLL) were compared to each other. Further on, the cell survival in culture medium (DMEM) supplemented with either 10% fetal calf serum (FCS) or 10% serum replacement (SR) on culture plates free of any substrate was evaluated over a period of 90 days by trypan blue exclusion. The cells were further characterized by HOECHST 33342 nuclear staining, and immunocytochemical staining with monoclonal antibodies against vimentin and cytokeratin. A mean total rate of TGC survival of 82.56% was recorded. Statistical analysis showed significantly higher survival rates after enzymatic disaggregation with COLL (86.23%) than following MECH (80.38%) or TRYP (80.91%) treatment. Supplementation of DMEM with FCS resulted in significantly higher cellular survival rates (87.13%) when compared to the addition of SR (77.73%). Analysis of the influence of both, disaggregation method and medium supplementation on TGC survival revealed statistically significant differences between the following groups: MECH-SR (71.09%) was significantly lower than all other groups; TRYP-SR (78.03%) was significantly different from all other groups; TRYP-FCS (83.43%) and COLL-SR (84.08%) were significantly lower than MECH-FCS (89.98%) which together with COLL-FCS (88.25%) showed the highest cellular survival rate. In summary, our results show that TGC isolated from early gestation placentomes may be viable for more than 90 days of culture. However, whether these TGC produce placental lactogen throughout this period has yet to be determined.

This study examines middle and late gestational placentae from 13 Tayassu tajacu (collared peccary) and 3 Tayassu pecari (white-lipped peccary), which are Artiodactyla belonging to the Family Tayassuidae. The chorionic sac of Tayassu species is diffuse and chorioallantoic. These epitheliochorial placentae show no trophoblast invasion into the uterine epithelium and there is interdigitation between fetal and maternal microvilli. Two distinct regions of the fetomaternal interface can be identified: the interareolar and the areolar regions. The uterine epithelium has eosinophilic cytoplasm with dispersed, basophilic and electron-dense granules. Trophoblast cells are irregularly cuboidal on top of the fetal ridges and columnar on troughs, where cells have cytoplasmic vesicles and large basal vacuoles, surrounded by whorls of smooth membranes. Capillaries indent the trophoblast cells forming a placental barrier 3mum or less thick. The columnar uterine glandular epithelium has a subpopulation of granules staining with Perl's Prussian blue reaction, suggesting iron secretion. In areolar areas, the trophoblast cells show apical microvilli, a basophilic cytoplasm with electron-dense intracellular vacuoles and cisternae. The placenta can therefore be classified as non-deciduate. The ultrastructural aspects of this study reveal features that have not previously been described and extend our knowledge of functions relating to materno-fetal transport in these species.

A recent phylogenetic analysis achieved good resolution between the 5 suborders of rodent. As a novel finding it suggested a basal split that gave rise to a monophyletic group comprising Hystricomorpha and Sciuromorpha. We asked whether the new tree could cast light on the evolution of the interhaemal barrier in rodents where at least seven variants have been described. To supplement existing data we first examined the placenta of the common gundi, Ctenodactylus gundi. It was shown to be haemochorial with a single layer of syncytiotrophoblast in the interhaemal membrane but with nests of cytotrophoblast elsewhere. Next we used character mapping on the recent tree to determine the pattern of evolution of the placenta with respect to principal type (e.g. haemochorial) and the trophoblast found within the interhaemal barrier. This indicated that the common ancestor of living rodents had a haemochorial placenta and that there were two independent transformations to the endotheliochorial type. Moreover, the interhaemal barrier was found to have had a single layer of syncytial trophoblast in the common ancestor of rodents, a condition that was retained in the clade comprising Hystricomorpha and Sciuromorpha. In contrast the second clade shows multiple character transformations.

The placentation of the Hottentot golden mole (Amblysomus hottentotus) has been examined using light and electron microscopy and lectin histochemistry of nine specimens at both mid and late gestation. The placentae were lobulated towards the allantoic surface and the lobules contained roughly parallel arrays of labyrinthine structures converging on a central spongy zone. At mid gestation, the arrays were composed of an inner cellular and outer syncytial trophoblast layer, the inner layer enclosing scant connective tissue and fetal capillaries. Maternal blood spaces coursed through the outer trophoblast and were lined by trophoblastic microvilli; the blood spaces were narrow in mid gestation but enlarged near term, while the inner trophoblast layer became thinner and seemed to be syncytial. These features were confirmed by transmission electron microscopy. The microvillous surfaces and dispersed cytoplasmic particles were heavily glycosylated, as shown by lectin histochemistry, and exhibited changes with maturation, particularly a loss in N-acetyl glucosamine oligomers bound by Phytolacca americana lectin on the microvilli lining the maternal blood spaces and outer trophoblast particles. A substantial yolk sac was present both in mid and late gestation stages. It was clearly unattached to the uterus in the later stages. These morphological features are discussed in relation to the phylogenetic position of Amblysomus with respect to other members of Afrosoricida and Afrotheria.

ABSTRACT: BACKGROUND: The guinea pig is an attractive model for human pregnancy and placentation, mainly because of its haemomonochorial placental type, but is rather small in size. Therefore, to better understand the impact of body mass, we studied placental development in the capybara which has a gestation period of around 150 days. We paid attention to the development of the lobulated arrangement of the placenta, the growth of the labyrinth in the course of gestation, the differentiation of the subplacenta, and the pattern of invasion by extraplacental trophoblast. METHODS: Material was collected from six animals at pregnancy stages ranging from the late limb bud stage to midgestation. Methods included latex casts, standard histology, immunohistochemistry for cytokeratin, vimentin, alpha-smooth muscle actin, and proliferating cell nuclear antigen as well as transmission electron microscopy. RESULTS: At the limb bud stage, the placenta was a pad of trophoblast covered by a layer of mesoderm from which fetal vessels were beginning to penetrate at folds in the surface. By 70 days, the placenta comprised areas of labyrinth (lobes) separated by interlobular areas. Placental growth resulted predominantly from proliferation of cellular trophoblast situated in nests at the fetal side of the placenta and along internally directed projections on fetal mesenchyme. Additional proliferation was demonstrated for cellular trophoblast within the labyrinth. Already at the limb bud stage, there was a prominent subplacenta comprising cellular and syncytial trophoblast with mesenchyme and associated blood vessels. At mid gestation differentiation was complete and similar to that seen in other hystricognath rodents. Overlap of fetal vessels and maternal blood lacunae was confirmed by latex injection of the vessels. At all stages extraplacental trophoblast was associated with the maternal arterial supply and consisted of cellular trophoblast and syncytial streamers derived from the subplacenta. CONCLUSIONS: All important characteristics of placental development and organization in the capybara resembled those found in smaller hystricognath rodents including the guinea pig. There were no features dependent on body size. Clearly, placentation in hystricognaths adheres to an extraordinarily stable pattern suggesting they can be used interchangeably as models of human placenta.

Pathological immune complexes (PIC) are revealed in high percent (100%) of cases by placental immunomorphological studies in insulin-dependent tissue. This leads to the development of an immunopathological process in the placental tissue. Parallel electron microscopy shows that some placental vessels are in close contact with the basal membrane of syncytiotrophoblast. PIC deposition in the area of confluence of the basal membranes of a syncytiotrophoblast and the vascular endothelium, which act as filters, results in the destruction of the basal membranes to rupture formation, causing the transplacental transport of substances together with immune complexes in the form of endocytotic vacuoles from the intervillous lacuna directly to the vessel. Active proliferation of vascular endotheliocyes leads to typical angiopathy as the body's defense reaction.

The aim of the present study was to investigate structural pattern of human placental barrier elements using light and electron microscopy and immunocytochemistry. Some important peculiarities of organization of the placental barrier were detected: difference in structure and amount of collagen IV in the basal lamina of endothelium and trophoblast, occurrence of smooth muscle actin in the capillary wall forming syncytiocapillary membranes. In the intercapillary stroma of terminal villi, both fibroblasts and macrophages but not myofibroblasts were found. Since smooth muscle cells and myofibroblasts are absent, pericytes are most likely cells to contain smooth muscle actin in the area of syncytiocapillary membranes.

Until now, defining characters of hystricognath rodents have been the subplacenta and the lobulation of the main chorioallantoic placenta. However, recent studies have revealed hystricognaths without marked lobulation, which is a plesiomorphic condition of the group. The question thus arises whether the subplacenta of these taxa is structurally homologous to that of other hystricognaths. Therefore, subplacental morphology and ontogeny were investigated in Octodon and Petromus by conventional light and electron microscopy, and the stem species pattern of Hystricognathi was reconstructed by applying MacClade(trade mark). The subplacentae of both species share important similarities with other hystricognaths. The organ develops early in gestation but degenerates towards term. It consists of folded layers of cellular and syncytial trophoblast, the latter enclosing maternal blood lacunae and electron-dense particles. Root-like syncytial outgrowths (syncytial streamers) and extraplacental trophoblast cells occur at the lateral and basal borders of the organ. Maternal vascularisation by blood lacunae within the subplacental syncytiotrophoblast is acquired early but lost during mid-gestation. Vascularisation by fetal vessels is established later. Fetomaternal exchange via blood circulation inside the subplacenta is unlikely to occur, since periods of maternal and fetal vascularisation show little overlap. In conclusion, the subplacentae of both species are regarded as homologous to other hystricognaths, comprising 18 character conditions that belong to their stem species pattern. Thus, the systematic unity of the group can be confirmed. J. Exp. Zool.(Mol. Dev. Evol.) 306B, 2006.(c) 2006 Wiley-Liss, Inc.

Rock cavies are rodents found in the semi-arid caatinga of Brazil. We studied the structure of the rock cavy placenta by light and transmission electron microscopy. The exchange area of the labyrinth was organized in lobes separated by interlobular areas. The interhaemal barrier was syncytial haemomonochorial. The syncytiotrophoblast had recesses in the basal membrane and some invaginations of the apical membrane, but transtrophoblastic channels could not be found. The interlobular regions comprised syncytiotrophoblast, enclosing maternal venous blood channels, and cytotrophoblast. There was a prominent subplacenta composed of cytotrophoblast and syncytiotrophoblast. Microvilli projected into spaces between the cytotrophoblasts and into lacunae within the syncytiotrophoblast. The yolk sac epithelium exhibited coated pits, endocytotic vesicles and larger vacuoles, consistent with a role in protein uptake from the uterine lumen. Tight junctions between these cells provided a barrier to diffusion by the intercellular route. The reproductive biology of the rock cavy differs from other members of the family, including the guinea pig, but the architecture of the placenta remains remarkably constant.

This combined light and scanning electron microscopic (SEM) analyses are the first investigation of the fine morphology of the fetal membranes of the placenta in the Asian elephant. We used two term placentas with gestation periods of 634 days (Zoological Garden Leipzig) and 657 days (Zoo Hagenbeck) and selected unequal sites as specimens, particularly - free membrane of the allantois,- lateral edge of the placenta band, the 'haemophagous zone',- allantochorion near the placental band, and allantochorion on the end of the chorioallantoic sac. Microscopically the free membrane of the allantois shows a simple, cuboidal epithelium with apical domains of microplicae and microvilli (fig. 1, inset). In the SEM analyses we documented a cobblestone-like architecture of the epithelial cells with various cell sizes, small, middle and giant (fig. 1.). Furthermore, we found pear-shaped cells with long pedicle, attached to the basement membrane and cell openings, presumably due to cell desquamation (fig. 2.). The haemophagous zone was characterized with a lamellate system between the long finger-like chorionic villi and the maternal blood lacunae. The simple, squamous and columnar trophoblast cells are here bathed in the maternal blood. In some chorionic villi brown-yellow material is deposited in the fetal connective tissue. A special result was the presence of strip-like microplicae in the middle of chorionic villi. The allantochorion near the placental band sometimes indicated simple and ramified chorionic villi with smooth, gyrus-like trophoblast cells and foamy cytoplasm. The attached allantois possesses a simple, columnar epithelium with microvilli. Furthermore, we observed villi-like projections rising up to the allantoic cavity. The white 'pustules' on the outer surface of the end of the chorioallantoic sac consist of stratified squamous epithelia. Numerous mitoses were documentedin the basal domain of the pustules. A surprisingly result was the presence of intra-epithelial capillaries. They invade the trophoblast cells to presume a nutrient exchange also outside of the placental band with its important functional metabolism. This study revealed unknown and interesting features of the epithelial layer of different locations in the placenta of the Asian elephant. Perhaps, with a detailed knowledge of the morphology of the epithelium we could convey a better understanding of functional cohesions of elephant placentation. Fig. 1: The free membranes of the allantois of the term placenta of the Asian elephant (Elephas maximus) shows a cobblestone-like architecture of the simple cuboidal epithelial cells with various sizes. The surface morphology was determined through microplicae and microvilli. Fig. 2: Numerous pear-shaped cells with long pedicle exist among the cuboidal epithelial cells, attached on the basement membrane. The cells possessed modifications of their surface, particularly microplicae. Some pear-shaped cells shows only the pedicle, perhaps that is a loss through preparation or its presumable due to cell desquamation. One erythrocyte is localized between these cells.

OBJECTIVES: The epithelium of the human placenta comprises an inner cytotrophoblast (CT) which proliferates and fuses with the outer differentiated syncytiotrophoblast (ST). Turnover has been studied focussing on second and third trimester placentas but with a paucity of data describing the normal first trimester trophoblast. The aim of this study was to compare the nuclear CT:ST ratio in normal and pathological pregnancy and thus establish the relationship between cytotrophoblast and syncytiotrophoblast nuclear number during early gestation. METHODS: Archival first trimester material from placentas from healthy pregnancy and recurrent miscarriage (anti-phospholipid syndrome) was stained with H&E, cytokeratin-7 and Mib-1. The area of trophoblast as a fraction of total villous area was calculated and the number of sectioned cytotrophoblast and syncytiotrophoblast nuclei as well as the number of proliferating cytotrophoblast was evaluated. RESULTS: Normal features of trophoblast development during the first trimester (rise in trophoblast area, increase in number of syncytiotrophoblast nuclei, increase in number of proliferating cytotrophoblast, decrease in the nuclear CT:ST ratio) are absent/reversed in tissues from recurrent miscarriage (decreasing trophoblast area, constant number of syncytiotrophoblast nuclei, decreasing number of proliferating trophoblast, constant nuclear CT:ST ratio). CONCLUSIONS: Proliferation of cytotrophoblast in early gestation provides a pool of trophoblast stem cells critical for ongoing placental development. Premature cytotrophoblast differentiation in favour of syncytial fusion results in deficiencies of cytotrophoblast and rarification of villous trophoblast. Abnormal trophoblast differentiation in early gestation may be due to a premature onset of maternal perfusion of the placenta and may be a likely antecedent for conditions associated with failure of placentation such as recurrent miscarriage.

The blacknose shark, Carcharhinus acronotus, is a viviparous anamniote that develops an epitheliochorial yolk sac placenta. The fetal portion of the uteroplacental complex consists of a proximal portion that forms saccular evaginations. The cells are bilayered stratified squamous with surface microvilli and a high concentration of cytoplasmic filaments. The tertiary egg envelope intervenes between the distal portion of the placenta and uterus. It has delaminations on the uterine surface and is compacted on the placental surface. The uterine epithelium is cuboidal to columnar and is characterized by prominent RER, Golgi, and secretion vesicles. The capillary endothelium is continuous. Nutrient and respiratory exchange is effected between the uterus and distal portion of the yolk sac. The distal portion of the placenta is a bilayer. An elaborate array, of microvilli forms an interface with the egg envelope. Dense non-membrane bound granules occur in the interspace between the egg envelope and the distal placenta. This material, presumably of uterine origin, is endocytosed in smooth-walled vesicles of the placental cells. The endothelium of the capillaries is fenestrated.