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

Abstract

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.

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

“…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).…”

“…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.…”

“…The growing chicken YS consists of three morphologically and functionally different regions termed (i) area pellucida, the central area of the early blastoderm, which can be considered part of the embryo proper; (ii) area vasculosa, which forms the major, vascularized part of the nascent YS; and (iii) area vitellina, the most distal, non-vascularized, portion of the YS (7,8). 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).…”

“…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).…”

Hedgehog signaling is essential for endothelial tube formation during vasculogenesis

“…However, the first blood vessels within the embryo always form in mesoderm that is in close proximity to endoderm (Mato et al, 1964;Wilt, 1965;Gonzalez-Crussi, 1971;Mobbs and McMillan, 1979;Meier, 1980;Kessel and Fabian, 1985;Pardanaud et al, 1989). Furthermore, we have recently demonstrated that a signal originating from the endoderm is essential for the assembly of angioblasts into tubes (Vokes and Krieg, 2002a).…”