Transport across endodermal cells of the chick yolk sac during early stages of development

Abstract

The endoderm of the chick yolk sac mediates the transfer of materials from the yolk mass to the embryonic circulation. There is little evidence of endocytotic activity in the area pellucida, but the endodermal cells of the area vasculosa possess many microvilli and bristle-coated pits and vesicles, as well as a canalicular system and vacuoles in the apical cytoplasm. Three tracers, horseradish peroxidase, ferritin, and latex spheres, were injected beneath the endoderm of both cultured embryos and embryos in ovo to study the pathway of uptake of extracellular materials. All tracers were sequestered in bristle-coated pits (200-500 nm in diameter) of the endodermal cells of the area vasculosa, but not those in the area pellucida. Both horseradish peroxidase and latex spheres (and probably ferritin) were incorporated into pleomorphic intracellular yolk drops through bristle-coated pits and vesicles, and then into apical vacuoles, which fuse with the intracellular yolk drops. Horseradish peroxidase and ferritin were also found within apical canaliculi. The apical junctions between endodermal cells prevented the intercellular passage of the tracers. A "topping-up" hypothesis is opposed whereby endodermal cells of the area vasculosa continually sequester extracellular yolk material, which is incorporated into a digesting intracellular yolk drop while, at the same time, digested yolk products are being transported across the vascular pole of the endoderm to the extraembryonic circulation and thence to the embryo.

The Developing Chicken Yolk Sac Acquires Nutrient Transport Competence by an Orchestrated Differentiation Process of Its Endodermal Epithelial Cells

“…7C), apoB levels were very low to undetectable in all lipoprotein fractions secreted by the area vitellina and negligible in comparison with the levels in area vasculosa-derived lipoproteins ( Fig. 8; note that the autoradiography exposure time for lanes 1-4 was ϳ30-fold longer than for lanes [5][6][7][8]. Interestingly, secreted apoA-V was associated mainly with lipoproteins of d Ͼ 1.063 produced in the area vitellina (Fig.…”

“…Immunoprecipitation with anti-apoB (Fig. 6, lanes [5][6][7][8] showed the vast majority of apoB in the two top fractions (lanes 5 and 6) and a minor amount in the fraction with 1.063 Ͻ d Ͻ 1.210 (lane 7). ApoA-I was absent from all immunoprecipitates obtained with anti-apoB (lanes 5-8), and thus apoA-I was, if at all, associated only with a minor population of particles also containing apoB (i.e.…”

“…Ultrastructural studies have provided information on the dynamics of the morphological transformations of the YS, particularly on the phenotypic changes of the EECs upon transition of the area vitellina to the area vasculosa (5,8). During this transition, the apical aspect of EECs acquires the typical characteristics of polarized epithelial cells with numerous villi, invaginations, and coated pits (5,6,8,9). Taken together, these and other observations are in agreement with the EECs in the area vasculosa being the most active yolk-absorbing cells in the entire YS (5,8,10).…”

“…Progression of avian YS formation is a highly coordinated sequence of events, in which rapidly dividing ectodermal cells start to spread from the embryo to ultimately fully cover the yolk compartment (5). Closely behind the migrating front of the ectodermal layer and between the yolk surface and the ectoderm, the proliferating endodermal epithelial cells (EECs) of the YS follow the ectodermal cells and form a tight epithelial layer in close contact with the yolk (5,6). Finally, from the so-called splanchnic mesoderm, which is tightly associated with the basal aspect of the EECs, cells migrate into the space between the ectodermal and endodermal layers and form a network of capillaries in tight contact with the other two cell layers (7).…”

Rapid clearance of circulating protein by early chicken embryo blood cells

“…The intracellular yolk droplets in the chick embryo are consumed by about 48 h of incubation, at which time the embryonic circulation has become established and the intracellular yolk of the area opaca has become available (Bellairs and Osmond, 1998). After the extra-embryonic circulation is established, the term yolk sac is usually substituted for area opaca; the nutrition contained in the yolk sac endodermal cells is obtained mainly by their unequal cleavage from the yolk cell and their uptake of material from the yolk (Mobbs and McMillan, 1981;Yoshizaki et al, 2004). The yolk sac develops quickly, and it encloses the whole yolk at E5.…”