Synthesis of serum proteins by cultures of chick embryo yolk sac endodermal cells

Cdx regulates gene expression through PRC2-mediated epigenetic mechanisms

The extra-embryonic yolk sac contains adjacent layers of mesoderm and visceral endoderm. The mesodermal layer serves as the first site of embryonic hematopoiesis, while the visceral endoderm provides a means of exchanging nutrients and waste until the development of the chorioallantoic placenta. While defects in chorioallantoic fusion and yolk sac hematopoiesis have been described in Cdx mutant mouse models, little is known about the gene targets and molecular mechanisms through which Cdx members regulate these processes. To this end, we used RNA-seq to examine Cdx-dependent gene expression changes in the yolk sac. We find that loss of Cdx function impacts the expression of genes involved in yolk sac hematopoiesis, as previously described, as well as novel Cdx2 target genes. In addition, we observed Cdx-dependent changes in PRC2 subunit expression accompanied by altered H3K27me3 deposition at a subset of Cdx target genes as early as E7.5 in the embryo proper. This study identifies additional Cdx target genes and provides further evidence for Cdx-dependent epigenetic regulation of gene expression in the early embryo, and that this regulation is required to maintain gene expression programs in the extra-embryonic yolk sac at later developmental stages.

Yolk sac endoderm is the major source of serum proteins and lipids and is involved in the regulation of vascular integrity in early chick development

An important function of the vascular system is nutrient delivery. In adult animals, this is mediated through a close contact of the mesoderm-derived vasculature with the endoderm-derived enterocytes and hepatocytes. During embryonic development, the yolk sac (YS) endoderm has been suggested to play a similar role. Physiological and molecular nature of the contact between the YS endoderm and the vasculature is not well-understood. To understand roles of the YS endoderm in early development, we used the avian model and carried out a gene expression profiling analysis of isolated area vasculosa YS endoderm tissues from embryonic day 2-4 chick embryos, covering the first 48 hr of postcirculation development. Genes involved in lipid metabolism are highly enriched, indicating an active modification of lipid components during their transfer from the yolk to the circulatory system. We also uncovered genes encoding major serum proteins and key regulators of vascular integrity. In particular, PTGDS, an enzyme controlling the last step of prostaglandin D2 production, shows high expression in the YS endoderm. Experimental introduction of prostaglandin D2 into embryonic circulation led to intraembryonic vessel rupture. These data suggest that the YS endoderm is the major, if not exclusive, source of lipid and protein constituents of the early embryonic serum and plays an important role in the regulation of vascular integrity in developing embryo.

Isolation and characterization of porcine visceral endoderm cell lines derived from in vivo 11-day blastocysts

Two porcine cell lines of yolk-sac visceral endoderm, designated as PE-1 and PE-2, were derived from in vivo 11-d porcine blastocysts that were either ovoid (PE-1) or at the early tubular stage of elongation (PE-2). Primary and secondary culture of the cell lines was done on STO feeder cells. The PE-1 and PE-2 cells morphologically resembled visceral endoderm previously cultured from in vivo-derived ovine and equine blastocysts and from in vitro-derived bovine blastocysts. Analysis of the PE-1- and PE-2-conditioned medium by 2D-gel electrophoresis and matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry demonstrated that they produced serum proteins. Reverse transcriptase polymerase chain reaction analysis showed that the cells expressed several genes typical for yolk-sac endoderm differentiation and function including GATA-6, DAB-2, REX-1, HNF-1, transthyretin, alpha-fetoprotein, and albumin. Unlike a porcine liver cell line, the PE-1 and PE-2 cell lines had relatively low inducible P-450 content and EROD activity, and, while they cleared ammonia from the cell culture medium, they did not produce urea. Transmission electron microscopy revealed that the cells were a polarized epithelium connected by complex junctions resembling tight junctions and by lateral desmosomes. Rough endoplasmic reticulum was prominent within the cells. Immunocytochemistry indicated that the PE-1 cells expressed cytokeratin 18 and had robust microtubule networks similar to those observed in in vivo porcine yolk-sac endoderm. Metaphase spreads prepared at passage 26 of the PE-1 cell line indicated a diploid porcine karyotype of 38 chromosomes. The cells have been grown for over 1 yr for multiple passages at 1:10 or 1:20 split ratios on STO feeder cells. The cell lines will be of interest as an in vitro model of the porcine preimplantation yolk-sac tissue.

Isolation and characterization of a bovine visceral endoderm cell line derived from a parthenogenetic blastocyst

A cell line, BPE-1, was derived from a parthenogenetic 8-d in vitro-produced bovine blastocyst that produced a cell outgrowth on STO feeder cells. The BPE-1 cells resembled visceral endoderm previously cultured from blastocysts produced by in vitro fertilization (IVF). Analysis of the BPE-1 cells demonstrated that they produced serum proteins and were negative for interferon-tau production (a marker of trophectoderm). Transmission electron microscopy revealed that the cells were a polarized epithelium connected by complex junctions resembling tight junctions in conjunction with desmosomes. Rough endoplasmic reticulum was prominent within the cells as were lipid vacuoles. Immunocytochemistry indicated the BPE-1 cells had robust microtubule networks. These cells have been grown for over 2 yr for multiple passages at 1:10 or 1:20 split ratios on STO feeder cells. The BPE-1 cell line presumably arose from embryonic cells that became diploid soon after parthenogenetic activation and development of the early embryo. However, metaphase spreads prepared at passage 41 indicated that the cell population had a hypodiploid (2n = 60) unimodal chromosome content with a mode of 53 and a median and mean of 52. The cell line will be of interest for functional comparisons with bovine endoderm cell lines derived from IVF and nuclear transfer embryos.

Lipoprotein receptors in extraembryonic tissues of the chicken

Yolk is the major source of nutrients for the developing chicken embryo, but molecular details of the delivery mechanisms are largely unknown. During oogenesis in the chicken, the main yolk components vitellogenin and very low density lipoprotein (VLDL) are taken up into the oocytes via a member of the low density lipoprotein receptor gene family termed LR8 (Bujo, H., Hermann, M., Kaderli, M. O., Jacobsen, L., Sugawara, S., Nimpf, J., Yamamoto, T., and Schneider, W. J. (1994) EMBO J. 13, 5165-5175). This endocytosis is accompanied by partial degradation of the yolk precursor protein moieties; however, fragmentation does not abolish binding of VLDL to LR8. The receptor exists in two isoforms that differ by a so-called O-linked sugar domain; the shorter form (LR8-) is the major form in oocytes, and the longer protein (LR8+) predominates in somatic cells. Here we show that both LR8 isoforms are expressed at ratios that vary with embryonic age in the extraembryonic yolk sac, which mobilizes yolk for utilization by the embryo, and in the allantois, the embryo's catabolic sink. Stored yolk VLDL interacts with LR8 localized on the surface of the yolk sac endodermal endothelial cells (EEC), is internalized, and degraded, as demonstrated by the catabolism of fluorescently labeled VLDL in cultured EEC. Addition to the incubation medium of the 39-kDa receptor-associated protein, which inhibits all known LR8/ligand interactions, blocks the uptake of VLDL by EEC. The levels of endogenous receptor-associated protein correspond to those of LR8+ but not LR8-, suggesting that it may play a role in the modulation of surface presentation of LR8+. Importantly, EEC express significant levels of microsomal triglyceride transfer protein and protein disulfide isomerase, key components required for lipoprotein synthesis. Because the apolipoprotein pattern of VLDL isolated from the yolk sac-efferent omphalomesenteric vein is very different from that of yolk VLDL, these data strongly suggest that embryo plasma VLDL is resynthesized in the EEC. LR8 is a key mediator of a two-step pathway, which affects the uptake of VLDL from the yolk sac and the subsequent delivery of its components to the growing embryo.

Trace mineral metabolism in the avian embryo

Trace mineral metabolism in the developing avian embryo begins with the formation of the egg and the trace mineral stores contained within it. Vitellogenin, the yolk precursor protein, serves as a trace mineral transporting protein that mediates the transfer of these essential nutrients from stores within the liver of the hen to the ovary and developing oocyte, and hence, to the yolk of the egg. Lipovitellin and phosvitin, derived from intraoocytic proteolytic processing of vitellogenin, are also trace mineral binding proteins that form important storage sites within the granule subfraction of yolk. The mobilization and uptake of egg trace mineral stores is mediated by the extra-embryonic membranes, principally the yolk sac membrane. The yolk sac also serves as a short-term storage site for trace minerals. Because it is an important site of plasma protein synthesis, the yolk sac has the ability to regulate the export of trace minerals to the embryo during development. Within the embryo, specific metaloproteins function in the interorgan transport cellular uptake, and intracellular storage of trace minerals. Thus, embryonic trace mineral homeostasis is established through the coordinated actions of the yolk sac, which mobilizes and exports trace minerals derived from egg stores; the vitelline circulation, which transports them to the embryo; and the liver, which accumulates trace minerals and distributes them to the rest of the tissues of the embryo via the embryonic circulation.

Expression of the albumin gene in the yolk sac and liver during chick embryogenesis

1. Albumin mRNA is first detectable in vascular yolk sac on the third day of egg incubation, increases to peak level on day 14 and declines to zero by day 19. 2. Vascular yolk sac RNA contains a 6-10-fold higher level of albumin transcripts compared to non-vascularized yolk sac, suggesting a role for vascularization in promoting albumin gene expression. 3. Embryonic liver albumin transcripts are first detectable at day 6.5, increase 6-fold by day 8, continue to rise at a lower rate until day 14 and remain constant thereafter. 4. Albumin protein synthesis in liver cubes also exhibits a large increase over days 7-10. In contrast, another liver-specific constitutive protein, apolipoprotein B, shows a different biosynthetic pattern. 5. The data suggest development of hepatic albumin gene-specific regulatory factors over days 7-10.

Flow of egg white ovalbumin into the yolk sac during embryogenesis

Western blot analyses of yolk proteins of the White Leghorn hens showed that an ovalbumin-like molecule was included in day 16 eggs but not in the ovarian follicles, and very little in newly deposited eggs. Northern hybridization, as well as in vitro translation, of poly(A)+ RNAs prepared from the yolk sac membranes of developing embryos gave no signal for ovalbumin messages. These results imply that the present ovalbumin-like protein of yolk has its origin in egg white, not being a de novo synthesis product in the yolk sac membranes.

Is extraembryonic angiogenesis in the chick embryo controlled by the endoderm? A morphology study

The area vasculosa of the chick embryo is subdivided into two concentric zones: the inner transparent area pellucida vasculosa (AVP) and the less transparent surrounding area opaca vasculosa (AOV). The different optical properties of these zones are caused by the different morphology of the endoderm, which consists of flat cells in the APV and of high-prismatic cells containing large yolk vacuoles in the AOV. The present study describes how this endodermal subdivision of the area vasculosa is related to the development of the extraembryonic vascular pattern. By injection of ink into the vascular system of chick embryos at stages 12 to 20 (Hamburger and Hamilton 1951 "HH"), it has been demonstrated that the vascular net of the area vasculosa from stage 14 (HH) onwards develops into different patterns in APV and AOV. The small loops of uniform capillary vessels of stage 13 (HH) are widened due to the rapid expansion of the extraembryonic mesoderm. In the AOV from stage 14 (HH) onwards numerous small blood vessels sprout into the enlarged intervascular spaces. This process is maximal at stage 17 (HH). In contrast, the blood vessels of the APV remain largely unbranched. These findings suggest that the development of the extraembryonic vascular pattern is controlled by the endodermal pattern. To test this hypothesis, both zones (APV and AOV) were examined by light microscopy, transmission and scanning electron microscopy, in vivo observations and by treatment with bromodeoxyuridine (BrdU). TEM examinations show that the ultrastructural organization of the APV mesoderm is different from that of the AOV: The splanchnopleuric cells of the APV form a continuous cover around the endothelial cells connected by numerous desmosomes, whereas the splanchnopleuric cells of the AOV are frequently separated by gaps. The largest gaps are seen in the small blood vessels at stage 17 (HH). These results should be considered in relation to the dynamic changes in the vascular pattern of the AOV. The endodermal cells of APV and AOV are two different populations. In vivo observation of the endodermal transition from APV to AOV detected no transformations of APV cells into AOV cells or vice versa. The borderline between the zones is stable. The AOV endoderm, having been overgrown by the expanding mesoderm, stops proliferating almost completely, whereas the proliferation of the APV endoderm is unaffected by contact with the mesoderm. The rate of its proliferation is approximately as high as that of the AOV prior to contact with the expanding mesoderm (results after treatment with BrdU).(ABSTRACT TRUNCATED AT 400 WORDS)

Duck hepatitis B virus replicates in the yolk sac of developing embryos

Duck hepatitis B virus (DHBV) is the only member of the hepadnavirus family in which nearly 100% vertical transmission from carrier mother to embryo has been reported. Large quantities of maternally transmitted virus particles are present in the yolk prior to incubation of the eggs, and replicative forms of DHBV DNA are detectable in the liver at 6 days of incubation. Since the yolk sac is similar to the liver in its production of serum proteins, we examined the yolk sacs of developing embryos for signs of viral replication. We detected the supercoiled form of DHBV DNA, DHBV RNA transcripts similar to those in the virus-replicating liver, and DNA polymerase activity and viral DNA in corelike particles in extracts of yolk sac tissue of naturally infected eggs. DHBV core antigen was strongly stained in only the endodermal layer of the yolk sac by immunofluorescence. DHBV RNA was detectable in the yolk sac from 4 days of incubation until hatching, and a larger quantity of DHBV RNA was present in the yolk sac than in the liver during all the stages of embryogenesis. Our data indicate that DHBV replicates actively in the yolk sac from an earlier stage than that previously reported in studies of embryonic liver and that replication is limited to the endodermal cell layer, which is ontogenetically and functionally related to the liver. The yolk sac may support the vertical transmission of DHBV.

Release of beta-D-galactoside-binding lectins into the cavities of aggregates of chick extraembryonic endoderm cells

Dissociated cells from the extraembryonic endoderm of gastrulating chick embryos form aggregates when cultured in rotating flasks. The large cellular aggregates are initially solid but subsequently cavitate to form hollow, thin-walled vesicles. These cells also contain an endogenous beta-D-galactoside-binding lectin. Previous work has shown that high extracellular concentrations of this lectin are associated with decreased cell-cell adhesion [Milos, N. and S.E. Zalik: Differentiation 21, 175-182 (1982)]. We have removed the fluid contents from aggregates cultured for 24 and 48 h and tested them for the presence of lectin activity. The results demonstrate that lectin activity is detectable in a higher number of aggregates cultured for 24 as opposed to 48 h (75% vs. 28%, respectively). The lectin activity per aggregate is also higher in aggregates cultured for 24 h (180 vs. 67 hemagglutinating units, respectively, for 24- and 48-h aggregates). Thus, at the time when cells are moving apart from one another during aggregate cavitation, detectable lectin activity is released into the vesicular contents of the aggregate.

Sequence of events in natural infection of Pekin duck embryos with duck hepatitis B virus

The major mode of natural infection of duck hepatitis B virus (DHBV) in Pekin ducks is vertical transmission, with 95 to 100% of the embryos from DHBV-infected dams eventually becoming infected. Maternally transmitted virus is present in large quantities in the yolk of unincubated eggs and is taken up by the embryo during early development. Synthesis of DHBV DNA in the embryo begins at about 6 days of incubation and coincides with the formation of the liver. DHBV DNA synthesis is incomplete, however, until 8 to 10 days of incubation, as shown by comparing the electrophoretic patterns of DHBV-specific nucleic acid species from embryonic livers at successive stages of development. From 8 days of incubation and continuing throughout embryonic development, subviral particles, which resemble viral replication intermediates isolated from infected livers of post-hatch ducklings, appear in the circulation. These particles contain a polymerase activity that utilizes an RNA template to synthesize viral DNA. Our results suggest that certain host functions, which appear during embryonic development, may be required for DHBV replication and assembly. It is possible that in mammals a similar developmental process occurs. The failure to find human hepatitis B virus in the circulation of most babies, born to hepatitis B virus carrier women, in the first few weeks after birth may reflect such a process.

Metabolic activation of cyclophosphamide by yolk sac endodermal cells of the early chick embryo

Endodermal cells isolated from the yolk sacs of day 3 chick embryos were able to activate metabolically cyclophosphamide. This was demonstrated by the cytotoxicity of cyclophosphamide to the endodermal cells themselves as well as by the ability of endodermal tissue to mediate a cytotoxic response in coculture with KB cells, a human tumor cell line unable to activate cyclophosphamide. Yolk sac endodermal cells from day 3 embryos were sensitive to cyclophosphamide when the drug was added immediately after the start of culture, but not when the drug was added after 24 hr of culture. The ability to metabolize cyclophosphamide by the day 3 embryo appeared limited to the endodermal cells of the yolk sac as cells derived from neither the embryo proper nor yolk sac mesoderm-ectoderm tissue were positive in these tests. Using whole blastoderms, cyclophosphamide activation was detected as early as 12 hr of egg incubation.