58-5 Friday, Jan. 5 11:15 - 11:30 Yolk Cellularization and Amniote Egg Evolution BLACKBURN, DG; Trinity College, Hartford CT firstname.lastname@example.org http://internet2.trincoll.edu/FacProfiles/default.aspx?fid=1000597&tid=0
Evolution of the amniotic (terrestrial) egg required a new form of cleavage and a new mechanism for processing of yolk for development. Reptiles are widely assumed to process yolk as do birds – implying a developmental mechanism that dates back ~340 million years. However, recent work in our laboratory reveals that snakes, lizards, and turtles cellularize yolk very differently than birds. In the avian pattern, a vascularized yolk sac surrounds the liquid yolk, and cells that line it progressively phagocytose and digest the yolk material. In contrast, in the reptiles that we have studied, endodermal cells proliferate and take up yolk droplets, forming large clumps that protrude into the yolk sac cavity. Upon vascularization, these yolk-filled cells become arranged in a monolayer around each blood vessel. As a result, the yolk sac cavity becomes filled with a compact mass of spaghetti-like strands of yolk- coated blood vessels. Yolk nutrients in non-avian reptiles are thereby digested and delivered efficiently into the blood stream for embryonic use. If the developmental pattern of squamates and turtles is ancestral for amniotes, the clade leading to birds must have abandoned it in favor of a very different pattern. Consequently, the history of the amniote egg is more complicated than is commonly assumed, and must be revised to recognize dual mechanisms for yolk processing among sauropsids.