Yolk sac endoderm: exclusive site of serum protein synthesis in the early chick embryo

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.

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.

Ontogenesis of transthyretin gene expression in chicken choroid plexus and liver

1. Chicken liver transthyretin cDNA hybridizes strongly with choroid plexus transthyretin mRNA from chickens, pigeons, quails and ducks. 2. In the chicken at hatching the choroid plexus has reached 70%, total brain 30%, and liver 5.8% of their organ masses in adults. 3. The proportion of transthyretin mRNA in total RNA is 0.45-times the adult value in the choroid plexus of the chicken at hatching. 4. In the liver at hatching, the proportion of transthyretin mRNA in total RNA is 1.1-times the value in adult chickens. 5. The pattern of maturation of transthyretin gene expression in chicken liver is comparable to that in precocial, but differs from that in altricial mammals.

Mineral metabolism in the developing turkey embryo--II. The role of the yolk sac

1. Turkey embryos were incubated in ovo or in long-term shell-less culture (ex ovo) for 14, 18, 22 or 26 days, at which time the concentrations of zinc, copper, iron, manganese and calcium in yolk and yolk sac membrane were determined. 2. Yolk manganese and calcium concentrations increased during incubation in ovo while the concentrations of zinc, copper and iron declined. The concentrations of zinc, copper and iron in yolk from ex ovo embryos did not decline. Yolk calcium concentration increased during incubation ex ovo, although to a much lesser extent than that observed in ovo. 3. The concentration of zinc, copper and iron declined in yolk sac tissue during incubation in ovo whereas no decline was observed for yolk sac tissue from ex ovo embryos. Yolk sac calcium and manganese concentrations increased during incubation in ovo and ex ovo, although the increase in calcium concentration for ex ovo yolk sac was much smaller than that observed in ovo. 4. A peak corresponding to metallothionein (MT) which bound both zinc and copper was isolated from yolk sac cytosol on day 14 of incubation in ovo using gel-permeation column chromatography. 5. Further fractionation of the MT peak by anion exchange chromatography revealed three metal-binding peaks designated MT-1, MT-2a and MT-2b. The majority of the zinc was bound to MT-2a and MT-2b whereas most of the copper was bound to a single peak (MT-2b). 6. The concentrations of zinc and copper in yolk sac cytosol reached a maximum on day 14 of incubation in ovo and declined through to day 28 (hatching).(ABSTRACT TRUNCATED AT 250 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.

Effects of momorcharins on the mouse embryo at the early organogenesis stage

Previous studies showed that the two forms of momorcharins which are isolated from seeds of Momordica charantia L. are effective in inducing early and midterm abortions in the mouse. Momorcharins were found to be teratogenic to the cultured mouse embryos at the early organogenesis stage. Morphological abnormalities were seen in the head, trunk and limbs. Ultrastructural studies on the visceral yolk sac of the momorcharin-treated embryos showed that the endodermal layer was deranged, membrane invaginations at the apical surface were decreased and intercellular space became distended. It is likely that the teratogenic action of momorcharins on mouse embryos in vitro is mediated through the deleterious effects on the visceral yolk sac, which functions as a vital transport organ for the conceptus at the immediate post-implantation period.

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.

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.

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

Endodermal cells were isolated from yolk sacs of 3-day chick embryos and cultured for 6 days in Eagle's minimal essential media plus 10% fetal calf serum. During this period cells rapidly lost their ability to synthesize DNA as judged by [3H]thymidine incorporation into DNA. In spite of this loss of DNA synthesis serum protein synthesis and secretion remained at a constant 45% of total protein synthesis and secretion. This was determined by immunoprecipitation of culture media using antibodies directed against embryonic chick serum proteins. Media were also analyzed for the synthesis and secretion of specific serum proteins using polyacrylamide gel electrophoresis. The relative synthesis and secretion of the individual serum proteins followed that previously observed in ovo with the exception of alpha-globulin-a which became undetectable. When culture media were supplemented with ovalbumin or insulin the relative synthesis and secretion of certin specific serum proteins were altered. However, analysis of these same media samples showed that the total amounts of serum protein synthesis and secretion were unaffected.