How do Crocodylian embryos process yolk? Morphological evidence from the American alligator, Alligator mississippiensis.

Recent studies have demonstrated a mechanism of embryonic yolk processing in lizards, snakes and turtles that differs markedly from that of birds. In the avian pattern, cells that line the inside of the yolk sac take up products of yolk digestion and deliver nutrients into the vitelline circulation. In contrast, in squamates and turtles, proliferating endodermal cells invade and fill the yolk sac cavity, forming elongated strands of yolk-filled cells that surround small blood vessels. This arrangement provides a means by which yolk material becomes cellularized, digested, and transported for embryonic use. Ultrastructural observations on late-stage Alligator mississippiensis eggs reveal elongated, vascular strands of endodermal cells within the yolk sac cavity. The strands of cells are intermixed with free yolk spheres and clumps of yolk-filled endodermal cells, features that reflect early phases in the yolk-processing pattern. These observations indicate that yolk processing in Alligator is more like the pattern of other reptiles than that of birds.

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.

Morphological features of the yolk processing pattern in the eastern fence lizard, Sceloporus undulatus (Phrynosomatidae).

Features of embryonic development in birds traditionally have been assumed to be shared by sauropsids in general. Herein, we document a pattern of yolk processing and cellularization in the Eastern fence lizard (Sceloporus undulatus) that is fundamentally different from that of birds. In the avian pattern, cells of the yolk sac lining phagocytose, and digest yolk material. These cells release products of digestion into underlying blood vessels for transport back to the embryo. In contrast, microscopic examination of the developing eggs of S. undulatus reveals that the yolk mass is converted into vascularized, "spaghetti-like" strands that fill the yolk sac cavity. Three successive developmental stages are involved. First, the liquid yolk is invaded by proliferating endodermal cells, which phagocytose and digest the yolk material. These cells form clumps that progressively fill the yolk sac cavity. Second, small blood vessels derived from the yolk sac vasculature invade the yolk sac cavity. Third, the endodermal cells become organized in monolayers around these vessels. This arrangement provides a means by which large numbers of endodermal cells can digest yolk, with each cell being positioned to release products of digestion into an adjacent blood vessel for transport to the embryo. The mechanism of yolk processing in this lizard species is similar to that of recently studied snakes. From its phylogenetic distribution, we infer that this pattern probably is ancestral for squamate sauropsids.