Scanning electron microscopy of the fetal membranes of an oviparous squamate,the corn snake Pituophis guttatus (Colubridae)

Although the fetal membranes of viviparous squamates have received much study, morphology of their homologues among oviparous reptiles is poorly understood. The scarcity of information about these membranes in egg‐laying reptiles hampers attempts to distinguish specializations for viviparity from ancestral oviparous features. We used scanning electron microscopy to examine fetal membranes of an oviparous snake (Pituophis guttatus) throughout the developmental period from oviposition to hatching. The external surface of the chorion contains broad, flattened cells that lack surface features; these cells form a continuous layer over the allantoic capillaries and offer a minimal barrier to respiratory exchange. In contrast, the surface epithelium of the omphalopleure bears elaborate surface ridges suggestive of absorptive capabilities. These ridges are prominent in the first few weeks after oviposition, but diminish thereafter. During development, the isolated yolk mass (IYM) of the omphalopleure becomes depleted, and the tissue becomes heavily vascularized by allantoic vessels. Surface features of the omphalopleure progressively take on the appearance of the chorioallantois, but the changes are not synchronous with loss of the IYM or membrane vascularization. Previous studies on viviparous snakes suggest that the chorioallantois and omphalopleure are respectively specialized for gas exchange and absorption in the intrauterine environment. Our studies of fetal membranes in P. guttatus offer evidence that cytological specializations for these functions originated under oviparous conditions, reflecting functional capacities that predate viviparity. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Histology of the extraembryonic membranes of an oviparous snake: towards a reconstruction of basal squamate patterns

An understanding of the evolutionary morphology of extraembryonic membranes in reptiles requires information about oviparous as well as viviparous species. We are studying histology and ultrastructure of the extraembryonic membranes of snakes to clarify the evolutionary history of reptilian fetal membranes, including determination of basal (ancestral) ophidian and squamate patterns. Microscopic anatomy of the membranes of oviparous corn snakes (Elaphe guttata) was examined using light and electron microscopy. At mid-development the inner surface of the eggshell is lined by two extraembryonic membranes, the chorioallantois and the omphalallantoic membrane. The chorioallantois consists of a bilayered cuboidal epithelium that overlies the allantoic blood vessels. During development, allantoic capillaries become more abundant, and the chorionic epithelium thins, decreasing the diffusion distance for respiratory gas exchange. The abembryonic pole of the egg is delimited by a bilaminar omphalopleure and isolated yolk mass, the latter of which is lined on its inner face by the allantois. The isolated yolk mass regresses developmentally, and patches of yolk droplets become isolated and surrounded by allantoic blood vessels. By late development, the abembryonic hemisphere has been fully vascularized by allantoic vessels, forming a "secondary chorioallantois." With regard to its extraembryonic membranes, Elaphe gutatta is similar to viviparous snakes. However, this species exhibits features that have not previously been reported among squamates, perhaps reflecting its oviparous reproductive habits. Morphological evidence for the uptake of eggshell material by epithelia of the chorion and omphalopleure suggests that the potential for absorption by extraembryonic membranes predates the origin of viviparity.     

Placentation in watersnakes II: Placental ultrastructure in Nerodia erythrogaster (Colubridae: Natricinae)

Ultrastructure of the placental tissues from redbelly watersnakes (Nerodia erythrogaster ) was analyzed during late pregnancy to provide insight into placental development and function. Examination of the chorioallantoic placenta with transmission electron microscopy reveals that chorionic and uterine epithelia are extremely attenuated but intact and that the eggshell membrane is vestigial and lacks a calcareous layer. These features minimize the interhemal diffusion distance across the placenta. Scanning electron microscopy reveals that fetal and maternal components of the placentas are richly vascularized by dense networks of capillaries. Although the yolk sac omphalopleure has largely been replaced by chorioallantois by late gestation, it retains patches of yolk droplets and regions of absorptive cells with microvilli and abundant mitochondria. Transmission electron microscopy reveals that yolk material is taken up for digestion by endodermal cells. As yolk is removed, allantoic capillaries invade to occupy positions just beneath the epithelium, forming regions of chorioallantoic placentation. Ultrastructural features indicate that the chorioallantoic placenta is specialized for gas exchange, while the omphalallantoic (“yolk sac”) placenta shows evidence of functions in yolk digestion and maternal‐fetal nutrient transfer. Placental features of this species are consistent with those of other thamnophines, and are evolutionarily convergent on snakes of other viviparous clades.     

Placentation in the lizard Gerrhonotus coeruleus with a comparison to the extraembryonic membranes of the oviparous Gerrhonotus multicarinatus (Sauria,Anguidae)

Paraffin sections of an ontogenetic series of embryos of the viviparous lizard Gerrhonotus coeruleus and the oviparous congener G. multicarinatus reveal that although general features of the development of the chorioallantoic and yolk sac membranes are similar, differences are evident in the distribution of the chorioallantoic membrane in late stage embryos. An acellular shell membrane surrounds the egg throughout gestation in both species although the thickness of this structure is much reduced in G. coeruleus over that of G. multicarinatus. The initial vascular membrane to contact the shell membrane in both species is a trilaminar omphalopleure (choriovitelline membrane) composed of ectoderm, mesoderm of the area vasculosa, and endoderm. This transitory membrane is replaced by the vascularized chorioallantois as the allantois expands to contact the inner surface of the chorion. Prior to the establishment of the chorioallantois at the embryonic pole, a membrane begins to form within the yolk ventral to the sinus terminalis. This membrane, which becomes vascularized, extends across the entire width of the abembryonic region and isolates a mass of yolk ventral to the yolk mass proper. The outer membrane of the yolk pole is a nonvascular bilaminar omphalopleure (chorionic ectoderm and yolk endoderm). In G. multicarinatus the bilaminar omphalopleure is supported internally by the vascularized allantoic membrane, whereas in G. coeruleus the allantois does not extend beyond the margin of the isolated yolk mass and the bilaminar omphalopleure is supported by the vascularized intravitelline membrane. Both the chorioallantoic placenta (uterine epithelium, chorionic ectoderm and mesoderm, and allantoic mesoderm and endoderm) and the yolk sac placenta at the abembryonic pole (uterine epithelium, chorionic ectoderm, and yolk sac endoderm) persist to the end of gestation in G. coeruleus.     

Placentation in watersnakes I: Placental histology and development in North American Nerodia (Colubridae: Natricinae)

As part of a broad survey of placental structure, function, and evolution in reptilian sauropsids paraffin‐section histology was used to study microscopic anatomy of the uterus and fetal membranes of three species of North American watersnakes (Nerodia : Colubridae). The pre‐ovulatory uterus is poorly vascularized with inactive shell glands. These shell glands are activated during vitellogenesis but regress during pregnancy. Two placentas develop through apposition of the uterine lining to the chorioallantois and the yolk sac omphalopleure. Fetal and maternal components of the chorioallantoic placenta are progressively vascularized during development. Their epithelia are attenuated, but (contrary to a previous report), epithelia of neither the uterus nor the chorion are eroded. The fetal portion of the yolk sac placenta is an omphalallantois, formed of avascular omphalopleure, isolated yolk mass, and allantois. This placenta is progressively replaced by chorioallantoic placenta during mid‐ to late‐development through depletion of the isolated yolk mass. The chorioallantoic placenta is anatomically specialized for maternal–fetal gas exchange, and its expansion during development reflects the growing needs of the fetus for gas exchange. The yolk sac placenta is morphologically unsuited for gas exchange, but may serve other functions in maternal‐fetal exchange.     

Placentation in garter snakes: scanning EM of the placental membranes of Thamnophis ordinoides and T. sirtalis

Surface topography and cross-sections of the placental membranes were examined by scanning electron microscopy in two species of Thamnophis. The chorionic epithelium of the chorioallantoic placenta consists of broad, squamous cells that lack surface specializations. The apposed uterine epithelium contains ciliated cells and larger, nonciliated cells. Neither the epithelium of the chorion nor that of the uterus is eroded; thus, underlying capillaries are not exposed to the luminal surface. In both the omphaloplacenta and the omphalallantoic placenta, epithelium of the omphalopleure consists of brush-border cells bearing prominent microvilli, interspersed with cells bearing minuscule microvilli. These surface epithelial cells are joined at their apices and their lateral surfaces are extensively sculpted by intercellular channels, presenting the appearance of an epithelium specialized for absorption. Deep to the epithelium lie the yolk spheres of the isolated yolk mass, interspersed with endodermal cells. Surface topography of the uterine epithelia of the omphaloplacenta and omphalallantoic placenta is relatively unspecialized. The acellular shell membrane separates maternal and fetal tissues in each of the three placental types. Marked differences in surface features of the chorioallantois and omphalopleure probably reflect different roles of these membranes in gas exchange and transfer of water and nutrients.     

Placental specializations of the mountain spiny lizard Sceloporus jarrovi

The lizard Sceloporus jarrovi (Phrynosomatidae) is one of the most widely studied viviparous reptiles of North America. Past research has assumed that placentation in this species is relatively simple and functions mainly in gas exchange. Our examination of the late stage placenta via transmission electron microscopy reveals that S. jarrovi has a unique combination of placental characteristics, with unusual specializations for secretion and absorption. In the chorioallantoic placenta, chorionic and uterine tissues are directly apposed through eggshell loss, and their epithelia are greatly attenuated, enhancing gas exchange; this placenta shows evidence of both nutrient transfer and endocrine function. Contrary to past inferences, a yolk sac placenta forms from the avascular omphalopleure and persists through the end of gestation. The uterine epithelium is enlarged and secretory, and the fetal omphalopleure shows branching absorptive channels and other specializations for uptake. Elsewhere, the omphalopleure develops elongated folds that protrude into a coagulum of degenerating shell membrane and other organic material. Uterine tissue in this region shows specializations for absorption. Placental features in S. jarrovi have unexpected functional implications, and challenge assumptions that specializations for nutrient transfer are confined to matrotrophic species. J. Morphol. 271:1153–1175, 2010. © 2010 Wiley‐Liss, Inc.     

Scanning electron microscopy of the placental interface in the viviparous lizard Sceloporus jarrovi (Squamata: Phrynosomatidae)

Placental membranes mediate maternal‐fetal exchange in all viviparous reptilian sauropsids. We used scanning electron microscopy to examine the placental interface in the mountain spiny lizard, Sceloporus jarrovi (Phrynosomatidae). From the late limb bud stage until birth, the conceptus is surrounded by placental membranes formed from the chorioallantois and yolk sac omphalopleure. The chorioallantois lies directly apposed to the uterine lining with no intervening shell membrane. Both fetal and maternal sides of the chorioallantoic placenta are lined by continuous layers of flattened epithelial cells that overlie dense capillary networks. The chorioallantoic placenta shows specializations that enhance respiratory exchange, as well as ultrastructural evidence of maternal secretion and fetal absorption. The yolk sac placenta contains enlarged fetal and maternal epithelia with specializations for histotrophic nutrient transfer. This placenta lacks intrinsic vascularity, although the vascular allantois lies against its inner face, contributing to an omphallantoic placenta. In a specialized region at the abembryonic pole, uterine and fetal tissues are separated by a compact mass of shed shell membrane, yolk droplets, and cellular debris. The omphalopleure in this region develops elongate folds that may contribute to sequestration and absorption of this material. Fetal membrane morphogenesis and composition in S. jarrovi are consistent with those of typical squamates. However, this species exhibits unusual placental specializations characteristic of highly placentotrophic lizards. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Ontogeny of the extraembryonic membranes of the oviparous lizard, Eumeces fasciatus (Squamata: scincidae)

Oviposited eggs of Eumeces fasciatus contain embryos in the limb bud stage. Amniogenesis is complete and two yolk sac membranes, vascular trilaminar omphalopleure (choriovitelline membrane) and bilaminar omphalopleure, enclose the yolk vesicle. A small allantoic vesicle contacts the chorion. The choriovitelline membrane is the primary vascular system. Blood islands, sites of hematopoiesis, are associated with omphalomesenteric vessels of the choriovitelline membrane. The bilaminar omphalopleure, which contacts the eggshell over the abembryonic hemisphere of the egg, lies external to an isolated yolk mass and yolk cleft and is not vascularized. The definitive yolk sac (splanchnopleure) is formed when the extraembryonic coelom and allantoic vesicle intrude into the choriovitelline membrane. Omphalomesenteric vessels are retained with the yolk sac splanchnopleure and the associated hematopoietic sites are present throughout incubation. The chorioallantoic membrane reaches the equator of the egg, entirely supplanting the choriovitelline membrane, after 25% of incubation is completed. Further growth of the allantois is stalled until 65% of incubation is completed when rapid expansion of the allantoic vesicle, in conjunction with resorption of the isolated yolk mass, supplants the bilaminar omphalopleure. As a result, the chorioallantoic membrane completely envelopes the egg for the final 35% of incubation. This developmental event is coincident with published reports for the timing of increased growth and metabolism of embryos. As the isolated yolk mass regresses, intravitelline cells associated with the yolk cleft invade and resorb the yolk to form a large cavity. The wall of this cavity is a germinal epithelium that produces cells that fill the cavity. This structure appears to be a site of hematopoiesis previously undescribed in vertebrates.     

Extraordinary case of matrotrophy in the African skink Eumecia anchietae

The viviparous African skink, Eumecia anchietae, exhibits a matrotrophic fetal nutritional pattern. Until well after the limb bud stage, extravitelline nutritional provision is in the form of holocrine secretion originating from the stratified uterine epithelium of the uterine incubation chambers. Uterine secretions are absorbed by a specialized yolk sac ectoderm and chorioallantois through histotrophy. The yolk sac is not in close contact with the uterine lining from the limb bud stage onwards. The yolk sac ectoderm forms invaginations filled with uterine secretion and consists of a single layer of vacuolated hypertrophied cells bearing microvilli. The chorioallantois at the limb bud stage is extensive, well-vascularized, and not intimately associated with the uterine epithelium. Where the uterus is folded, the chorioallantois may interdigitate loosely. Chorionic cells are low to high columnar, clearly vacuolated, and bear microvilli. The allantoic layer consists primarily of squamous cells exhibiting villous projections. By the time embryos have well-defined digits, the specialized yolk sac ectoderm has regressed and the yolk sac lumen has been invaded by vitelline cells. The chorioallantois is very extensive and in areas greatly folded. Where it contacts the uterine epithelium, a proper chorioallantoic placenta is formed. Cell layers of the chorioallantois and uterine epithelium are thin and cuboidal to squamous in appearance. The chorioallantoic placenta is simple in structure, occurs throughout the incubation chamber, and is epitheliochorial in arrangement. It is unknown whether the placentome observed in other highly matrotrophic scincids is formed in late stage embryos of this species.     

Histology and ultrastructure of the placental membranes of the viviparous brown snake,Storeria dekayi (Colubridae: Natricinae)

The placental membranes of the viviparous brown snake Storeria dekayi were examined following mid‐gestation by means of light microscopy, scanning electron microscopy, and transmission electron microscopy to reveal their structural organization and cytological composition. By Zehr stage 32, the chorioallantoic placenta (allantoplacenta) is established around much of the egg, and a well‐developed omphalallantoic placenta occurs in the abembryonic hemisphere. The allantoplacenta exhibits multiple features that enhance interhemal exchange: the uterus and allantois are well vascularized, the chorionic and uterine epithelia are attenuated, and the shell membrane is vestigial and has begun to degenerate. In the omphalallantoic placenta, the uterine epithelium is enlarged and appears to be secretory. The omphalopleure contains two distinct populations of cells, and shows cytological evidence for absorption. In intermediate areas, regions of omphalallantoic placenta are being transformed into allantoplacenta, through depletion of the isolated yolk mass and reduction in epithelial height of both uterus and omphalopleure. Morphological evidence suggests that the allantoplacenta is specialized for gas exchange, and the omphalallantoic placenta, for maternal secretion and fetal absorption. On the basis of the available evidence, we postulate that this pattern is characteristic of the thamnophine radiation of snakes. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Morphogenesis of the fetal membranes of an American mole, Scalopus aquaticus.

Scalopus membranes are characterized by: Superficial nidation; antimesometrial orientation of the embryonic disc; amniogenesis by folding; an extensive but transitory choriovitelline placenta; a large yolk sac with late and incomplete inversion; large persistent allantoic vesicle; a very broad, thin, villous, epitheliochorial chorioallantoic placenta of annular shape interrupted mesometrially, dotted with numerous areolae, and bordered by a nonvillous sparsely vascular chorioallantoic membrane connected with the persistent bilaminar omphalopleure by a very narrow rim of chorion. There is no decidua. Electron microscopy shows that at 8 mm, CR, (limb bud embryo) the uterine epithelium of the interhemal membrane may be 0.5 micron or less in thickness, but that it shows no signs of degeneration. Trophoblastic microvilli often penetrate the epithelium to within 0.2 micron of its base. At this time there is active secretion by the uterine glands, and cellular hypertrophy and cytolysis of the epithelium at the gland mouths, with active phagocytosis by the areolar cytotrophoblast. The occurrence of absorptive areolae in an insectivore emphasizes the probable primitiveness of this widely distributed placental mechanism. In spite of similarities of the yolk sac to that of rabbits and rodents, the bilaminar omphalopleure produces no invasive trophoblastic giant cells. The definitive membranes of Parascalops breweri and Scapanus latimanus are like those of Scalopus. The placentae of Talpa europaea, Condylura cristata, and Neurotrichus gibbsii are discoid and relatively much smaller, thicker and more complex in internal structure. There is some reason to believe that the fetal membrane systems of moles and shrews (Soricoidea) are more like those of the ancestral mammalian stock than are those of any other recent eutherians.     

Placentation in garter snakes. II. Transmission EM of the chorioallantoic placenta of Thamnophis radix and T. sirtalis

Transmission electron microscopy was used to examine the ultrastructure of the allantoplacenta of garter snakes during the last half of gestation. This placenta occupies the dorsal hemisphere of the egg and is formed through apposition of the chorioallantois to the inner lining of the uterus. The uterine epithelium consists of flattened cells with short, irregular microvilli and others that bear cilia. The lamina propria is vascularized and its capillaries lie at the base of the uterine epithelial cells. The chorionic epithelium consists of a bilayer of squamous cells that are particularly thin superficial to the allantoic capillaries. Neither the chorionic epithelium nor the uterine epithelium undergoes erosion during development. Although a thin remnant of the shell membrane intervenes between fetal and maternal tissue at mid-gestation, it undergoes fragmentation by the end of gestation. Thus, uterine and chorionic epithelial are directly apposed in some regions of the allantoplacenta, forming continuous cellular boundaries at the placental interface. During development, capillaries proliferate in both the uterine and chorioallantoic tissues. By late gestation, the interhemal diffusion distance has thinned in some areas to less than 2 microm through attenuation of the uterine and chorionic epithelia. Morphologically, the allantoplacenta is well adapted for its function in gas exchange. However, the presence of cytoplasmic vesicles, ribosomal ER, and mitochondria in the chorionic and uterine epithelial cells are consistent with the possibility of additional forms of placental exchange.     

Multifunctional morphology of the chick chorioallantoic membrane

The chicken chorioallantoic membrane is assessed between days 7 and 20 of incubation using light and electron microscopy with morphometric analysis. The completion of its formation by the middle of embryogenesis is observed, after that a rapid increase in the embryo body mass continues. The bloodstream combines chorionic epithelium, mesoderm, and allantoic epithelium into the single multifunctional organ. The ordered antiparallel arrangement of afferent and efferent vessels within the mesoderm as well as their specialized endothelium assist in partial blood fluid filtration into the interstitium. The unique structure of the capillary network within and over the chorionic epithelium that provide for the effective gas exchange is depicted. It is revealed that the ultrastuctural specialization of the chorionic epithelial cells through which an active calcium transport from the eggshell takes place. The ultrastructure peculiarities of the allantoic epithelium concerning the water-salt balance maintaining in the growing embryo, are discussed.     

Histology of the late-stage placentae in the matrotrophic skink Chalcides chalcides (Lacertilia; Scincidae)

Examination of late-stage placental material of the lizard Chalcides chalcides from the Hubrecht Laboratorium (Utrecht, The Netherlands) reveals several cytological and histological specializations that appear to have been superimposed over a morphological pattern that is typical for squamates. The chorioallantoic placenta is highly vascularized and consists of a single mesometrial placentome and a generalized paraplacentomal region, both of which are epitheliochorial. The placentome is deciduate, and contains deeply interdigitating folds of hypertrophied uterine and chorioallantoic tissue. Chorionic epithelium lining the placentome comprises enlarged, microvilliated cells, a small proportion of which are diplokaryocytes. The placentomal uterine epithelium is not syncytial and consists of enlarged cells bearing microvilli. The yolk sac placenta is a true omphaloplacenta (sensu stricto), being formed by juxtaposition of uterine tissues to an avascular, bilaminar omphalopleure. Epithelium of the omphalopleure is stratified and is hypertrophied into papillae that project into detritus of the uterine lumen. The omphalopleure is separated from the yolk sac proper by a yolk cleft that is not confluent with the exocoelom and is not invaded by the allantois. Neither an omphalallantoic placenta nor a true choriovitelline placenta is present in late gestation. Morphologically, the mature placentae of C. chalcides are among the most specialized to have been described in reptiles, reflecting the substantial maternal-fetal nutrient transfer that occurs in this species. © 1993 Wiley-Liss, Inc.     

Development of yolk sac and chorioallantoic membranes in the Lord Howe Island skink,Oligosoma lichenigerum

Development of the yolk sac of squamate reptiles (lizards and snakes) differs from other amniote lineages in the pattern of growth of extraembryonic mesoderm, which produces a cavity, the yolk cleft, within the yolk. The structure of the yolk cleft and the accompanying isolated yolk mass influence development of the allantois and chorioallantoic membrane. The yolk cleft of viviparous species of the Eugongylus group of scincid lizards is the foundation for an elaborate yolk sac placenta; development of the yolk cleft of oviparous species has not been studied. We used light microscopy to describe the yolk sac and chorioallantoic membrane in a developmental series of an oviparous member of this species group, Oligosoma lichenigerum. Topology of the extraembryonic membranes of late stage embryos differs from viviparous species as a result of differences in development of the yolk sac. The chorioallantoic membrane encircles the egg of O. lichenigerum but is confined to the embryonic hemisphere of the egg in viviparous species. Early development of the yolk cleft is similar for both modes of parity, but in contrast to viviparous species, the yolk cleft of O. lichenigerum is transformed into a tube‐like structure, which fills with cells. The yolk cleft originates as extraembryonic mesoderm is diverted from the periphery of the egg into the yolk sac cavity. As a result, a bilaminar omphalopleure persists over the abembryonic surface of the yolk. The bilaminar omphalopleure is ultimately displaced by intrusion of allantoic mesoderm between ectodermal and endodermal layers. The resulting chorioallantoic membrane has a similar structure but different developmental history to the chorioallantoic membrane of the embryonic hemisphere of the egg. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.     

Embryo implantation and proteinase activities in a marsupial (Macropus eugenii)

Summary Embryo implantation remains superficial (epithelio-chorial type) in most marsupials including the Macropodidae, but does involve formation of specialized contact zones of the trophoblast with the uterine epithelium. Since in eutherian mammals proteinases appear to play a central role in implantation-initiation mechanisms, a systematic histochemical investigation of proteinase patterns as related to implantation was performed in the tammar wallaby, Macropus eugenii (Macropodidae).Tammar uteri with embryos were collected at diapause and at days 7, 17, 18, 19, 20, 21 and 26 of the 27-day gestational period. Proteinase patterns were studied using a sensitive histochemical gelatin-substrate-film test previously optimized for the detection of trophoblast-dependent proteinase (blastolemmase) in the rabbit. Proteinase patterns were correlated with light-microscopical morphology of the processes of shedding of the extracellular embryo coverings (shell membrane) and attachment of the trophoblast to the uterine epithelium.At acid pH values an intracellular proteinase is detected in yolk sac endoderm and trophoblast as well as in endometrial glands and certain stromal cells. This enzyme is proposed to be a cathepsin indicating high catabolic activity connected particularly with protein transport from the endometrium into the yolk sac. Peak activity is found in the avascular (bilaminar) yolk sac at the phase when contact with the endometrium is being established.A particularly interesting proteinase active at alkaline pH values is detected in the trophoblast-endoderm complex. This enzyme appears to be extruded into the interface between trophoblast and uterine epithelium where it shows maximal activity for only approximately one day, around day (18-)19, exclusively in the bilaminar (avascular) yolk sac. The activity is correlated with the process of shedding of the extracellular embryo coverings (shell membrane) and of subsequent attachment of the trophoblast to the uterine epithelium, in the bilaminar but not the trilaminar (vascular) yolk-sac region. This is the first report on an extracellular (alkaline) proteinase activity possibly serving a specific function in embryo implantation in a marsupial.Abreviations BYS bilaminar (avascular) yolk sac membrane = bilaminar omphalopleure - dp.c. days post coitum - d RPY days after removal of pouch young - TYS trilaminar (vascular) yolk sac membrane = trilaminar omphalopleure Preliminary reports on portions of these investigations were presented at the 14th Annual Meeting of the Society for the Study of Reproduction 1981 (Biol Reprod 24 Suppl 1, p 78 A, 1981) and at the 3. Arbeitstagung der Anatomischen Gesellschaft 1982 (Anat Anz 153, 268, 1983)     

Placentation in garter snakes. III. Transmission EM of the omphalallantoic placenta of Thamnophis radix and T. sirtalis

The omphalallantoic placenta is a complex organ that is unique to viviparous squamates. Using transmission EM and light microscopy, we examined this placenta in garter snakes in order to understand its structural organization and functional capabilities. The omphalallantoic placenta is formed from the uterine lining and the bilaminar omphalopleure, the latter of which is associated with the isolated yolk mass and allantois. A thin shell membrane separates the fetal and maternal tissues throughout gestation. The uterine epithelium contains cuboidal cells with large droplets or granules and appears to be secretory. Epithelium of the omphalopleure is specialized for absorption and contains cells with prominent microvilli and others with large cytoplasmic droplets or granules. The brush-border cells are rich in mitochondria and Golgi bodies and interdigitate extensively with adjacent cells, forming elaborate intercellular canaliculi. Their morphology is consistent with their proposed role in sodium-coupled water movement. During development, the isolated yolk mass becomes depleted as yolk droplets are digested by cells of the omphalopleure and allantois. However, the allantois does not fuse to or vascularize the inner face of the omphalopleure. Consequently, the distance between fetal and maternal circulatory systems remains large (about 250-300 microm), precluding efficient gas exchange and hemotrophic transfer. The morphology of the omphalallantoic placenta strongly suggests that it functions in nutrient transfer through uterine secretion and fetal absorption.     

Morphology, development, and evolution of fetal membranes and placentation in squamate reptiles

Current studies on fetal membranes of reptiles are providing insight into three major historical transformations: evolution of the amniote egg, evolution of viviparity, and evolution of placentotrophy. Squamates (lizards and snakes) are ideal for such studies because their fetal membranes sustain embryos in oviparous species and contribute to placentas in viviparous species. Ultrastructure of the fetal membranes in oviparous corn snakes (Pituophis guttatus) shows that the chorioallantois is specialized for gas exchange and the omphalopleure, for water absorption. Transmission and scanning electron microscopic studies of viviparous thamnophine snakes (Thamnophis, Storeria) have revealed morphological specializations for gas exchange and absorption in the intra-uterine environment that represent modifications of features found in oviparous species. Thus, fetal membranes in oviparous species show morphological differentiation for distinct functions that have been recruited and enhanced under viviparous conditions. The ultimate in specialization of fetal membranes is found in viviparous skinks of South America (Mabuya) and Africa (Trachylepis, Eumecia), in which placentotrophy accounts for nearly all of the nutrients for development. Ongoing research on these lizards has revealed morphological specializations of the chorioallantoic placenta through which nutrient transfer is accomplished. In addition, African Trachylepis show an invasive form of implantation, in which uterine epithelium is replaced by invading chorionic cells. Ongoing analysis of these lizards shows how integration of multiple lines of evidence can provide insight into the evolution of developmental and reproductive specializations once thought to be confined to eutherian mammals.