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1.
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.  相似文献   

2.
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.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
《Journal of morphology》2017,278(5):665-674
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.  相似文献   

6.
《Journal of morphology》2017,278(5):675-688
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.  相似文献   

7.
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.  相似文献   

8.
Topological and histological analyses of Mabuya mabouya embryos at different developmental stages showed an extraembryonic membrane sequence as follows: a bilaminar omphalopleure and progressive mesodermal expansion around the whole yolk sac at gastrula stages; mesodermal split and formation of an exocoelom in the entire embryonic chamber at neurula stages; beginning of the expansion of the allantois into the exocoelom to form a chorioallantoic membrane at pharyngula stages; complete extension of the allantois into the exocoelom between limb-bud to preparturition stages. Thus, a placental sequence could be enumerated: bilaminar yolk sac placenta; chorioplacenta; allantoplacenta. All placentas are highly specialized for nutrient absorption from early developmental stages. The bistratified extraembryonic ectoderm possesses an external layer with cuboidal cells and a microvillar surface around the whole yolk sac, which absorbs uterine secretions during development of the bilaminar yolk sac placenta and chorioplacenta. During gastrulation, with mesodermal expansion a dorsal absorptive plaque forms above the embryo and several smaller absorptive plaques develop antimesometrially. Both structures are similar histologically and are active in histotrophic transfer from gastrula stages until the end of development. The dorsal absorptive plaque will constitute the placentome and paraplacentome during allantoplacental development. At late gastrula-early neurula stages some absorptive plaques form chorionic concavities or chorionic bags that are penetrated by a long uterine fold and seem to have a specialized histotrophic and/or metabolic role. The extraembryonic mesoderm does not ingress into the yolk sac and neither an isolated yolk mass nor a yolk cleft are formed. This derived pattern of development may be related to the drastic reduction of the egg size and obligatory placentotrophy from early developmental stages. Our results show new specialized placentotrophic structures and a novel arrangement of extraembryonic membrane morphogenesis for Squamata.  相似文献   

9.
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.  相似文献   

10.
In reptilian sauropsids, fetal (extraembryonic) membranes that line the eggshell sustain developing embryos by providing for gas exchange and uptake of water and eggshell calcium. However, a scarcity of morphological studies hinders an understanding of functional specializations and their evolution. In kingsnakes (Lampropeltis getula), scanning electron microscopy reveals two major fetal membranes: the chorioallantois and yolk sac omphalopleure. In early development, the chorioallantois contains tall chorionic epithelial cells, avascular connective tissue, and enlarged allantoic epithelial cells. During its maturation, the chorionic and allantoic epithelia thin dramatically and become underlain by a rich network of allantoic capillaries, yielding a membrane ideally suited for respiratory gas exchange. Yolk sac development initially is like that of typical lizards and snakes, forming an avascular omphalopleure, isolated yolk mass (IYM), and yolk cleft. However, unlike the situation in most squamates studied, the omphalopleure becomes transformed into a “secondary chorioallantois” via three asynchronous events: flattening of the epithelium, regression of the IYM, and vascularization by the allantois. Progressive expansion of chorioallantois parallels growing embryonic needs for gas exchange. In early through mid‐development, external surfaces of both the chorionic and omphalopleure epithelium show an abundance of irregular surface protrusions that possibly increase surface area for water absorption. We postulate that the hypertrophied allantoic epithelial cells produce allantoic fluid, a viscous substance that facilitates water uptake and storage. Our findings are consistent with a previous study on the corn snake Pantherophis guttatus, but include new observations and novel functional hypotheses relevant to a reconstruction of basal squamate patterns. J. Morphol. 276:1467–1481, 2015. © 2015 Wiley Periodicals, Inc.  相似文献   

11.
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.  相似文献   

12.
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.  相似文献   

13.
Virginia striatula is a viviparous snake with a complex pattern of embryonic nutrition. Nutrients for embryonic development are provided by large, yolked eggs, supplemented by placental transfer. Placentation in this species is surprisingly elaborate for a predominantly lecithotrophic squamate reptile. The embryonic-maternal interface consists of three structurally distinct areas, an omphalallantoic placenta and a regionally diversified chorioallantoic placenta. The chorioallantoic placenta over the embryonic hemisphere (paramesometrial region) of the egg, features close apposition of embryonic and uterine blood vessels because of the attenuate form of the interceding epithelial cells. The periphery of the chorioallantoic placenta, which is adjacent to the omphalallantoic placenta, is characterized by a simple cuboidal uterine epithelium apposed to a stratified cuboidal chorionic epithelium. There are no sites with attenuate epithelial cells and close vascular apposition. The morphology of the omphalallantoic placenta is similar to that of the peripheral chorioallantoic placenta, except that the height of uterine epithelial cells is greater and allantoic blood vessels are not associated with the embryonic epithelium. The functional capabilities of the three placental regions are not known, but structural characteristics suggest that the omphalallantoic placenta and peripheral zone of the chorioallantoic placenta are sites of nutritional provision via histotrophy. The paramesometrial region of the chorioallantoic placenta is also nutritive, in addition to functioning as the primary embryonic respiratory system. The structure of the chorioallantoic placenta of V. striatula is a new placental morphotype for squamate reptiles that is not represented by a classic model for the evolution of reptilian placentation.  相似文献   

14.
The reptilian placenta is a composite structure formed by a functional interaction between extraembryonic membranes and the maternal uterus. Study of placental structure of squamate reptiles over the past century has established that each of the multiple independent origins of placentation, which characterize the reproductive diversity of squamates, has resulted from the evolutionary transformation of these homologous structures. Because each evolutionary transformation is an independent novel relationship between maternal and embryonic tissues, the resulting placentae are not homologous, even though the individual components may be. The evolution of reptilian placentation should reveal much about evolutionary patterns and mechanisms because similar structural-functional systems have been transformed along parallel trajectories on multiple occasions. We compared extraembryonic membrane and placental development and pattern of embryonic nutrition in thamnophiine snakes and Pseudemoia lizards in the context of recent hypotheses of phylogenetic relationships. Two primary types of placentation, chorioallantoic and yolk sac, evolved in each lineage. Smooth, highly vascular regions of chorioallantoic placentation are indistinguishable homoplasies that evolved in parallel, likely to facilitate respiratory exchange. The yolk sac placenta of each lineage is specialized for histotrophic nutrient transfer, yet composition of these structures differs because of variation in the ancestral snakes and lizards. In addition, the omphalopleure that contributes to yolk sac placentation persists to later embryonic stages compared to oviparous outgroups, but the two lineages have evolved different structures that prevent replacement of the omphalopleure by the allantois. Each lineage has also evolved unique structural specializations of the chorioallantoic placenta.  相似文献   

15.
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.  相似文献   

16.
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.  相似文献   

17.
In contrast to the few species of viviparous reptiles that developelaborate chorioallantoic placentae and ovulate eggs with relativelylow yolk content, most viviparous speciesovulate large yolkedeggs and have chorioallantoic placentae that are structurallyconservative. However, the placentae of the isolated yolk mass,the omphaloplacenta and omphalallantoic placenta, are sitesof structural elaboration in these species. Vitellogenesis providesthe primary source of nutrients for development, yet supplementalnourishment is contributed by the uterus. The embryo is dependenton the placentae for some materials, for example, gas and waterexchange, whereas other aspects of placental function are facultative,i.e., the provision of some inorganic and organic nutrients,and supplement yolk resources. Embryonic nutrition in thesepredominantly lecithotrophic species is characterized by featuresshared with oviparous ancestorscombined with supplemental advantagesto uterine gestation.  相似文献   

18.
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.  相似文献   

19.
We used light microscopy to study placental structure of the lizard Sceloporus mucronatus throughout 6 months of embryonic development. Three stages of placental development could be assigned to embryos based on the arrangement of the extraembryonic membranes. A highly vascular choriovitelline placenta was present in the embryonic hemisphere and a nonvascular bilaminar omphalopleure covered most of the abembryonic hemisphere of the egg during embryonic Stages 10-28. A chorioallantoic placenta replaced the choriovitelline placenta by embryonic Stage 29 and an omphaloplacenta covered the abembryonic hemisphere at this stage. The combination of these two placental types occurred in Stage 29-36 embryos. The final stage of placentation, embryonic Stages 37-40, was characterized by an omphalallantoic placenta in the abembryonic hemisphere and a chorioallantoic placenta in the embryonic hemisphere of the egg. The choriovitelline and chorioallantoic placentae are well vascularized, with closely apposed maternal and embryonic blood vessels. These structures are the most likely sites of respiratory exchange. In contrast, the omphaloplacenta and omphalallantoic placentae contain cuboidal or columnar epithelia and these structures may function in histotrophic exchange. Placentation of S. mucronatus is similar to that of predominantly lecithotrophic species in other squamate lineages suggesting that the evolution of this placental morphology is a response to similar factors and is independent of phylogeny.  相似文献   

20.
In the scincid lizard Chalcides chalcides, females ovulate small ova and supply most of the nutrients for development by placental means. The yolk is enveloped precocially by extraembryonic ectoderm and endoderm during the gastrula stage, establishing a simple bilaminar yolk sac placenta. The shell membrane begins to degenerate at this time, resulting in apposition of extraembryonic and maternal tissues. A true chorioplacenta has developed by the early pharyngula stage, as has a choriovitelline placenta and the first stages of an omphaloplacenta. Although the choriovitelline membrane disappears rapidly, the omphaloplacenta spreads to occupy the entire abembryonic pole. The yolk cleft is not confluent with the exocoelom, and no omphalallantoic placenta develops. By the limb-bud stage, an allantoplacenta has been established, with a mesometrial placentome composed of interdigitating ridges of chorioallantois and uterine mucosa. The discovery of five distinct placental arrangements in this species, three of which are transitory and two of which have not previously been recorded in reptiles, emphasizes the need for accounts that specify ontogenetic stages and the precise identity and composition of squamate placental membranes. Contrary to previous interpretations, the pattern of extraembryonic membrane development in C. chalcides is evolutionarily conservative, despite the presence of a reduced yolk mass and cytological specializations for nutrient transfer. Our observations indicate that substantial placentotrophy can evolve in squamates without major modifications of morphogenetic patterns. J Morphol 232:35–55, 1997. © 1997 Wiley-Liss, Inc.  相似文献   

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