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

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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 of 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 proposed 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.