Ultrastructural analysis of the yolk processing pattern in embryonic pond slider turtles (Trachemys scripta: Emydidae).

Evolution of the large-yolked, amniotic egg required mechanisms by which extracellular yolk could be made available for embryonic development. In birds, the endodermal lining of the yolk sac absorbs and digests the yolk. In contrast, recent studies on lizards and snakes (squamates) have revealed that yolk is processed by means of a proliferating mass of "spaghetti-like" strands formed by endodermal cells attached to anastomosing blood vessels. To clarify the method of yolk processing in chelonians, we applied electron microscopy to an extensive series of embryos of the pond slider turtle, Trachemys scripta. Our findings demonstrate that proliferating endodermal cells phagocytose yolk spheres. These cells remain attached to one another following mitosis, thereby forming clumps that progressively occupy the yolk sac cavity. Upon invasion of blood vessels, the cells become organized into elongated, vascularized "spaghetti-like" strands of cells like those found in squamates. Residual yolk found in the body cavity of new hatchlings chiefly consists of these vascularized strands. Such strands of cells also develop in the false map turtle, Graptemys pseudographica (Emydidae). We infer that the developmental pattern by which yolk is processed is ancestral for both Chelonia and Reptilia, and therefore must have been modified or abandoned in birds or their archosaur ancestors.