Presence of Na-K-ATPase in mitochondria-rich cells in the yolk-sac epithelium of larvae of the teleost Oreochromis mossambicus

The purpose of this study is to provide biochemical evidence for the functions of the mitochondria-rich cell (MR cell) in the yolk-sac epithelium of the developing larvae of tilapia Oreochromis mossambicus. Western blotting with the antibody (6F) raised against avian Na-K-ATPase alpha1 subunit demonstrated the presence of Na-K-ATPase in yolk-sac epithelium of tilapia larvae and about 1. 46-fold more of the enzyme in seawater larvae than in freshwater ones. The yolk-sac MR cells were immunoreacted to the antibody (alpha5) against the alpha subunit of avian Na-K-ATPase and were double-labeled with anthroylouabain and dimethylaminostyrylethyl-pyridiniumiodine, suggesting the existence and activity of Na-K-ATPase in these cells. Binding of 3H-ouabain in the yolk sac of seawater larvae was much higher than in that of freshwater larvae (4.183+/-0.143 pmol/mg protein versus 1.610+/-0. 060 pmol/mg protein or 0.0508+/-0.0053 pmol/yolk sac versus 0. 0188+/-0.0073 pmol/yolk sac). These biochemical results are further evidence that yolk-sac MR cells are responsible for a major role in the osmoregulatory mechanism of early developmental stages before the function of gills is fully developed.     

Yolk sac development in lizards (Lacertilia: Scincidae): New perspectives on the egg of amniotes

Embryos of oviparous reptiles develop on the surface of a large mass of yolk, which they metabolize to become relatively large hatchlings. Access to the yolk is provided by tissues growing outward from the embryo to cover the surface of the yolk. A key feature of yolk sac development is a dedicated blood vascular system to communicate with the embryo. The best known model for yolk sac development and function of oviparous amniotes is based on numerous studies of birds, primarily domestic chickens. In this model, the vascular yolk sac forms the perimeter of the large yolk mass and is lined by a specialized epithelium, which takes up, processes and transports yolk nutrients to the yolk sac blood vessels. Studies of lizard yolk sac development, dating to more than 100 years ago, report characteristics inconsistent with this model. We compared development of the yolk sac from oviposition to near hatching in embryonic series of three species of oviparous scincid lizards to consider congruence with the pattern described for birds. Our findings reinforce results of prior studies indicating that squamate reptiles mobilize and metabolize the large yolk reserves in their eggs through a process unknown in other amniotes. Development of the yolk sac of lizards differs from birds in four primary characteristics, migration of mesoderm, proliferation of endoderm, vascular development and cellular diversity within the yolk sac cavity. Notably, all of the yolk is incorporated into cells relatively early in development and endodermal cells within the yolk sac cavity align along blood vessels which course throughout the yolk sac cavity. The pattern of uptake of yolk by endodermal cells indicates that the mechanism of yolk metabolism differs between lizards and birds and that the evolution of a fundamental characteristic of embryonic nutrition diverged in these two lineages. Attributes of the yolk sac of squamates reveal the existence of phylogenetic diversity among amniote lineages and raise new questions concerning the evolution of the amniotic egg. J. Morphol. 278:574–591, 2017. © 2017 Wiley Periodicals, Inc.     

The follicular placenta of the viviparous fish,Heterandria formosa. I. Ultrastructure and development of the embryonic absorptive surface

Embryos of the poeciliid Heterandria formosa develop to term in the ovarian follicle in which they establish a placental association with the follicle wall (follicular placenta) and undergo a 3,900% increase in embryonic dry weight. This study does not confirm the belief that the embryonic component of the follicular placenta is formed only by the surfaces of the pericardial and yolk sacs; early in development the entire embryonic surface functions in absorption. The pericardial sac expands to form a hood-like structure that covers the head of the embryo and together with the yolk sac is extensively vascularized by a portal plexus derived from the vitelline circulation. The hood-like pericardial sac is considered to be a pericardial amnion-serosa. Scanning and transmission electron microscopy reveal that during the early and middle phases of development (Tavolga's stages 10–18 for Xiphophorus maculatus) the entire embryo is covered by a bilaminar epithelium whose apical surface is characterized by numerous, elongate microvilli and coated pits and vesicles. Electron-lucent vesicles in the apical cytoplasm appear to be endosomes while a heterogeneous group of dense-staining vesicles display many features characteristic of lysosomes. As in the larvae of other teleosts, cells resembling chloride cells are also present in the surface epithelium. Endothelial cells of the portal plexus lie directly beneath the surface epithelium of the pericardial and yolk sacs and possess numerous transcytotic vesicles. The microvillous surface epithelium becomes restricted to the pericardial and yolk sacs late in development when elsewhere on the embryo the non-absorptive epidermis differentiates. We postulate that before the definitive epidermis differentiates, the entire embryonic surface constitutes the embryonic component of the follicular placenta. The absorptive surface epithelium appears to be the principle embryonic adaptation for maternal-embryonic nutrient uptake in H. formosa, suggesting that a change in the normal differentiation of the surface epithelium was of primary importance to the acquisition of matrotrophy in this species. In other species of viviparous poeciliid fishes in which there is little or no transfer of maternal nutrients, the embryonic surface epithelium is of the non-absorptive type.     

Uterine epithelial changes during placentation in the viviparous skink Eulamprus tympanum

We used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to describe the complete ontogeny of simple placentation and the development of both the yolk sac placentae and chorioallantoic placentae from nonreproductive through postparturition phases in the maternal uterine epithelium of the Australian skink, Eulamprus tympanum. We chose E. tympanum, a species with a simple, noninvasive placenta, and which we know, has little net nutrient uptake during gestation to develop hypotheses about placental function and to identify any difference between the oviparous and viviparous conditions. Placental differentiation into the chorioallantoic placenta and yolk sac placenta occurs from embryonic Stage 29; both placentae are simple structures without specialized features for materno/fetal connection. The uterine epithelial cells are not squamous as previously described by Claire Weekes, but are columnar, becoming increasingly attenuated because of the pressure of the impinging underlying capillaries as gestation progresses. When the females are nonreproductive, the luminal uterine surface is flat and the microvillous cells that contain electron-dense vesicles partly obscure the ciliated cells. As vitellogenesis progresses, the microvillous cells are less hypertrophied than in nonreproductive females. After ovulation and fertilization, there is no regional differentiation of the uterine epithelium around the circumference of the egg. The first differentiation, associated with the chorioallantoic placentae and yolk sac placentae, occurs at embryonic Stage 29 and continues through to Stage 39. As gestation proceeds, the uterine chorioallantoic placenta forms ridges, the microvillous cells become less hypertrophied, ciliated cells are less abundant, the underlying blood vessels increase in size, and the gland openings at the uterine surface are more apparent. In contrast, the yolk sac placenta has no particular folding with cells having a random orientation and where the microvillous cells remain hypertrophied throughout gestation. However, the ciliated cells become less abundant as gestation proceeds, as also seen in the chorioallantoic placenta. Secretory vesicles are visible in the uterine lumen. All placental differentiation and cell detail is lost at Stage 40, and the uterine structure has returned to the nonreproductive condition within 2 weeks. Circulating progesterone concentrations begin to rise during late vitellogenesis, peak at embryonic Stages 28-30, and decline after Stage 35 in the later stages of gestation. The coincidence between the time of oviposition and placental differentiation demonstrates a similarity during gestation in the uterus between oviparous and simple placental viviparous squamates.     

Placentation and associated aspects of gestation in the bonnethead shark, Sphyrna tiburo

The left ovary of the bonnethead shark, Sphyrna tiburo, is rudimentary, and the right ovary supplies both oviducts which share a common ostium situated in the falciform ligament. Preceding ovulation the nidamental gland of each oviduct hypertrophies and the caudal two-thirds of each oviduct is modified to form a uterus. In the Florida-Caribbean area Sphyrna tiburo probably mates in March and 3–7 eggs are fertilized in the vicinity of the nidamental gland of each oviduct. The developing embryo is nourished during the first 3–4 months of gestation by yolk stored in its extensive yolk sac. Approximately three and one-half months after fertilization, the distal portion of the yolk sac becomes convoluted and interdigitates with deep folds in the uterine wall to form a yolk-sac placenta. As the placenta develops, the maternal uterine epithelium is reduced from columnar cells to squamous cells, and the foetal yolk-sac epithelium is reduced from columnar and cuboidal cells to squamous cells. Exchange between the maternal and foetal blood systems takes place through maternal endothelium, reduced maternal epithelium, egg-case membrane, reduced foetal epithelium, and foetal endothelium.     

Changes in chloride cell distribution during early larval stages of Clupea harengus

A non-uniform distribution of cutaneous chloride cells was found in the early, pre-feeding larval stages of herring Clupea harengus . Chloride cells on the head, yolk-sac and trunk regions were unevenly distributed, whereas more densely packed chloride cells were observed in the pericardial and prebranchial regions. The pattern of chloride cell distribution changed during development and two distinct changes are described. The density of choride cells on the ventral trunk increased substantially during the period of yolk absorption, presumably due to contraction of the yolk sac and selective retention of yolk-sac chloride cells. Also during this period the cells on the lateral body wall increased in number and became distributed in segmental bands overlying the myosepta. Most chloride cells were found in association with the haemocoel or primordial blood vessels. Superficial segmental blood vessels were not found in the early larva, but the segmental bands of chloride cells overlay nerve tracts in the myosepta which were tentatively identified as the focal innervation of myotomes. It is concluded that both the circulatory system and the peripheral nervous system may play a role in determining chloride cell distribution in early larvae.     

Gene expression of cytokeratin endo A and endo B during embryogenesis and in adult tissues of mouse.

We have examined the pattern of gene expression of mouse cytokeratin endo A and endo B during postimplantational development and in adult organs by Northern blot and in situ hybridization analyses. Both mRNAs localized in the ectoplacental cone, trophoblastic giant cells surrounding the parietal yolk sac, trophoblast cells in placenta, visceral yolk sac, and simple epithelium of the embryo during postimplantational development and in simple or transitional epithelial tissues in adult organs. These results indicate that endo A and endo B are coexpressed and may play some roles in these tissues.     

An examination of surface epithelium structures of the embryo across the genus Poeciliopsis (Poeciliidae)

In viviparous, teleost fish, with postfertilization maternal nutrient provisioning, embryonic structures that facilitate maternal‐fetal nutrient transfer are predicted to be present. For the family Poeciliidae, only a handful of morphological studies have explored these embryonic specializations. Here, we present a comparative morphological study in the viviparous poeciliid genus, Poeciliopsis . Using microscopy techniques, we examine the embryonic surface epidermis of Poeciliopsis species that vary in their level of postfertilization maternal nutrient provisioning and placentation across two phylogenetic clades and three independent evolutionary origins of placentation. We focus on surface features of the embryo that may facilitate maternal‐fetal nutrient transfer. Specifically, we studied cell apical‐surface morphology associated with the superficial epithelium that covers the body and sac (yolk and pericardial) of embryos at different developmental stages. Scanning electron microscopy revealed common surface epithelial cells across species, including pavement cells with apical‐surface microridges or microvilli and presumed ionocytes and/or mucus‐secreting cells. For three species, in the mid‐stage embryos, the surface of the body and sac were covered in microvillus epithelium. The remaining species did not display microvillus epithelium at any of the stages examined. Instead, their epithelium of the body and sac were composed of cells with apical‐surface microridges. For all species, in the late stage embryos, the surface of the body proper was composed of apical‐surface microridges in a “fingerprint‐like arrangement.” Despite the differences in the surface epithelium of embryos across Poeciliopsis species and embryonic developmental stages, this variation was not associated with the level of postfertilization maternal nutrient provisioning. We discuss these results in light of previous morphological studies of matrotrophic, teleost fish, phylogenetic relationships of Poeciliopsis species, and our earlier comparative microscopy work on the maternal tissue of the Poeciliopsis placenta.     

Demonstration of a phagocytic cell system belonging to the hemopoietic lineage and originating from the yolk sac in the early avian embryo.

It is well established that hemopoietic cells arising from the yolk sac invade the avian embryo. To study the fate and role of these cells during the first 2.5-4.5 days of incubation, we constructed yolk sac chimeras (a chick embryo grafted on a quail yolk sac and vice versa) and immunostained them with antibodies specific to cells of quail hemangioblastic lineage (MB1 and QH1). This approach revealed that endothelial cells of the embryonic vessels are of intraembryonic origin. In contrast, numerous hemopoietic cells of yolk sac origin were seen in embryos ranging from 2.5 to 4.5 days of incubation. These cells were already present within the vessels and in the mesenchyme at the earliest developmental stages analyzed. Two hemopoietic cell types of yolk sac origin were distinguishable, undifferentiated cells and macrophage-like cells. The number of the latter cells increased progressively as development proceeded, and they showed marked acid phosphatase activity and phagocytic capacity, as revealed by the presence of numerous phagocytic inclusions in their cytoplasm. The macrophage-like cells were mostly distributed in the mesenchyme and also appeared within some organ primordia such as the neural tube, the liver anlage and the nephric rudiment. Comparison of the results in the two types of chimeras and the findings obtained with acid phosphatase/MB1 double labelling showed that some hemopoietic macrophage-like cells of intraembryonic origin were also present at the stages considered. These results support the existence in the early avian embryo of a phagocytic cell system of blood cell lineage, derived chiefly from the yolk sac. Cells belonging to this system perform phagocytosis in cell death and may also be involved in other morphogenetic processes.     

Neoplastic differentiation: characteristics of cell lines derived from a murine teratocarcinoma

Monolayer cultures of a mouse teratocarcinoma were established in vitro. These cultures contained embryonal carcinoma, the malignant stem cell, and its differentiated progeny: parietal yolk sac, neuroepithelial, and mesenchymal cells. Tissues such as squamous epithelium, cartilage, striated muscle, neuroepithelium, and glands were produced from embryonal carcinoma that was maintained under conditions of long term culture. Frequent subcultivation with pancreatin allowed the establishment of cell lines of embryonal carcinoma which have been maintained for more than 18 months in vitro and continue to produce differentiated cells under specific culture conditions. Chromosomally these lines of embryonal carcinoma have a stem line of 39 chromosomes. Two lines of parietal yolk sac cells have been established which produce basement membrane, are not tumorigenic, and chromosomally are hypotetraploid. This system may yield information concerning neoplastic differentiation and its possible use in therapy for cancer.     

Thyroid hormones are important for embryonic to larval transitory phase in zebrafish

In zebrafish, like many other teleost species, the development and differentiation of many major organs continue unabated into the yolk-sac larval stage before culminating in a free-swimming larva capable of exogenous feeding. We investigated the role of thyroid hormone (TH) in this important embryonic to larval transitory phase. Thyroid hormone receptor (TR) alpha and beta mRNAs are expressed during the early stages of zebrafish embryonic development. Beginning from the midblastula stage, the level of TR beta mRNAs increases dramatically and is maintained until the end of the transitory phase. Excessive exogenous thyroxine (T4; 30 nM) is toxic and causes severe developmental defects. Cotreatment of embryos with amiodarone, an antagonist of TR, and goitrogen methimazole (MMI) lead to severe retardation in the maturation of the gastrointestinal system, swim bladder, and the lower jaw cartilages and the resorption of the yolk sac. The developmental arrest is lethal, and treated larvae do not survive beyond 7 day postfertilization (dpf), but can be completely rescued by the presence of 10 nM T4. We propose that the embryonic to larval transitory phase in many teleost species is characterized by its dependency on the timely synthesis of TH and the concomitant autoinductive increase in TR beta mRNA levels.     

Permeability of the primary decidual zone in the rat uterus: studies using fluorescein-labeled proteins and dextrans

The primary decidual zone (PDZ) is a transitory avascular region of transformed fibroblasts surrounding the luminal epithelium at the implantation site. Since this zone may restrict the passage of immunoglobulins, cells, nutrients, and other substances from maternal blood to the epithelium and embryo from Days 6 to 8 of pregnancy, it was of interest to study its permeability to blood-borne tracers. Fluorescein isothiocyanate (FITC)-labeled macromolecules were administered i.v. on Days 6 to 9 of pregnancy. The tracers included dextran (17 kDa), horseradish peroxidase (40 kDa), ovalbumin (45 kDa), dextran (66 kDa), bovine serum albumin (BSA: 66 kDa), dextran (156 kDa), bovine immunoglobulin G (IgG; 160 kDa), and apoferritin (450 kDa). Ten minutes after administration on Days 6 or 7, FITC-labeled tracers of molecular masses of 45 kDa or less were localized in the intercellular spaces of the PDZ and in the blastocyst in small amounts. Tracers with molecular masses of 66 kDa were not detected in these regions up to 1 h after administration but were present in small amounts at 5 h. The 156 kDa and 160 kDa tracers were absent or present only in very small amounts in the PDZ and blastocyst up to 7 h after injection and apoferritin was completely absent at this time. By Day 9 the PDZ had regressed and maternal blood spaces were present adjacent to Reichert's membrane. One hour after administration on Day 9, large quantities of labeled BSA, IgG, and apoferritin appeared in the yolk sac endoderm but not in the underlying embryonic cells. These observations indicate that the PDZ is selectively permeable to blood-borne tracers on Days 6 and 7 of pregnancy, with permeability decreasing with increasing molecular mass. By restricting the passage of high molecular weight substances such as immunoglobulins, microorganisms, and immunocompetent cells, the PDZ may serve a protective function for the embryo, which is no longer protected by the uterine epithelium and has not yet fully developed its own protective layers, especially the yolk sac and Reichert's membrane.     

Observations on the Oman Shark,Iago omanensis (Triakidae), with emphasis on the morphological and cytological changes of the oviduct and yolk sac during gestation

The shark Iago omanensis (Triakidae, Selachia) is encountered in large populations in the Gulf of Aqaba, Red Sea, at depths of 150–1,500 m. It is a placental viviparous species, reproductive all year round and giving birth to four (occasionally five) young of 170- to 180-mm total length (TL). Its distribution and morphometrics, as well as histological and cytological changes in the oviducts, were studied. The ratio of weight of the female genital organs to body weight changes from 0.7% in nongravid females to 19.8% in the final stages of pregnancy. The ripe, liberated eggs, which are 11–12 mm long and 5 mm wide, pass through the nidamental gland and settle in the uterus. The embryo attains 9- to 11-mm TL and settles on a protruding ridge of the submucosa, covered with a microvillar endometrium. At this site of attachment, a placenta is formed and the participating uterine endometrium and wall of the yolk sac undergo profound histocytological changes, forming two parts of this organ. Three forms of food provisioning occur in the growing embryos: (1) lecithotrophic, based on yolk transported from the egg to the embryonic gut via the umbilical cord; (2) mixed food provision, during which, in addition to nourishment provided via the umbilicus, food is transported across the placenta through transfer from the female blood vascular system to the embryonic yolk sac via the trophic villi of the yolk sac; and (3) histotrophic, when all yolk reserves have been used and nutrition is provided from the so-called “milk” within the yolk sac, metabolized by the trophic structures of the sac and transported by blood vessels. Despite the gradual utilization of yolk, the yolk sac mass initially increases from 0.5–1.0 cc to 2.0–2.2 cc with the addition of primary and secondary trophic villi until, during the final stages of embryogenesis, it decreases again to 1.4–1.6 cc. Neonate juveniles are 35–40 times heavier than the original eggs. J. Morphol. 236:151–165, 1998. © 1998 Wiley-Liss, Inc.     

Structure of the endodermal epithelium of the chick yolk sac during early stages of development.

The structure of the areas pellucida and vasculosa of the early chick embryo (stages 11-29) was examined by light, transmission and scanning electron microscopy. The most striking feature of the endodermal cells of these areas is the presence of large intracellular yolk drops which are characteristic of the regions in which they are found; lipid-like homogeneous drops in the area pellucida, heterogeneously composed pleomorphic drops in the mid-region of the area vasculosa and granular drops at the periphery of the area vasculosa in the region of the sinus terminalis. On morphological criteria it is postulated that granular drops may arise by direct engulfment of extracellular yolk, but this does not appear to be true for pleomorphic or homogeneous drops. Since the apical junctions between endodermal cells across the yolk sac are tight, they seal off the extraembryonic compartment from the vitelline circulation and presumably prevent intercellular passage of the yolk constituents. Thus the endodermal epithelium must mediate the transport of nutrients from the yolk mass to the developing embryo. Endodermal cells exhibit a variation across the yolk sac in the presence and number of structures associated with uptake of extracellular materials. The mid-region of the area vasculosa appears to be the most endocytotically active region with an abundance of microvilli, bristle-coated pits and vesicles and apical canaliculi and vacuoles. There is a close association between the endoderm and vitelline blood vessels and this association is maintained, as the yolk sac develops, by the formation of small vessels juxtaposed between the vascular surface of the endoderm and the walls of the large vitelline vessels.     

Ontogenesis and functional metamorphosis of the head-kidney in bottomspawner and mouthbrooder cichlid fishes (Cichlidae,Teleostei)

Histological and electron-microscopical studies of the head-kidney ontogenesis in a group of bottomspawner and mouthbrooder cichlid fishes reveal that this organ undergoes three functional and structural stages. At the initial stage, 2–12 days after fertilization, the head-kidney is a filtering organ of two glomeruli and adjacent renal tubules; the second stage, beginning 12–30 days after fertilization, is a functionally mixed one, with the nephron and developing hemopoietic and endocrine, chromaffin, and interrenal cells; during the third stage, which begins 30–60 days after fertilization, the nephron degenerates, and the head-kidney starts functioning solely as a lympho-adrenal organ. With age the interrenal tissue expands, forming fields, especially along the blood vessels. The number of melano-macrophage centers progressively increases within the hemopoietic parts. Data are given on the various cell types, the reticuloendothelial stroma, and the renal part of this organ in various species. The earlier ontogenesis and function of the head-kidney, as thymus and other organs, in bottomspawner larvae compared to the more protected mouthbrooders provide support for earlier free functioning in a dangerous environment. © 1996 Wiley-Liss, Inc.     

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.     

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.     

Target cells for avian myeloblastosis virus in embryonic yolk sac and relationship of cell differentiation to cell transformation

The yolk sac of the 12-day chicken embryo retains the blast stage progenitors to cells of the myeloid lineages with a very low level of contamination by more mature myeloid cells which have begun to express the characteristic myeloid cell markers. Both in vivo and in vitro experiments have supported the hypothesis that target cells for the BAI-A strain of avian myeloblastosis virus are contained within the myeloid lineages. An assay system for avian myeloblastosis virus was developed which utilizes this yolk sac cell system and which appears to be more sensitive than previous published assays. In addition, the kinetics of a liquid culture transformation system is presented in which at least 4% of the yolk sac cell population was transformed in a relatively synchronous fashion at 2 days after infection. The morphological transformation preceded an increased rate of cell proliferation. Cell separation procedures provided a 10- to 20-fold enrichment of target cells and demonstrated that the target cell population copurifies with macrophage colony-forming cells which are the committed progenitors to the macrophage lineage. In combination with earlier work, this work demonstrated that cells committed to the macrophage lineage at all stages of differentiation may serve as target cells for infection by avian myeloblastosis virus.