Meeting Abstract
Development of oviparous amniotes is dependent on functional attributes of tissues that metabolize and transport nutrients from a large mass of yolk. Studies of birds reveal that yolk is overgrown by extraembryonic tissue forming a “yolk sac” which surrounds the yolk. Large endodermal cells on the inner aspect of the yolk sac take up yolk nutrients continuously throughout embryonic development and transport molecules to yolk sac blood vessels. A yolk sac similar to that of birds also develops in lizards and snakes, but in contrast to birds, endodermal cells proliferate within the yolk sac cavity and align along a developing vascular system to form an organ system. We analyzed a developmental series of several species of scincid lizards using light microscopy to characterize the “yolk organ.” In these species, all of the yolk is incorporated into large endodermal cells, which align along a vascular network within the yolk sac cavity prior to the height of the embryonic growth phase. Yolk metabolism is intracellular and the endodermal cells transfer nutrients to both arterial and venous circuitry. In addition to the large endodermal cells, the interstitial spaces of the yolk sac cavity contain aggregations of granulocyte-like cells which increase in density in later stages of incubation. Our findings are consistent with recent studies of snakes which reveal unappreciated diversity in structure and function of the egg of amniotes and support an hypothesis that specializations for yolk metabolism diverged in lineages leading to modern squamates and birds.
