CHARLESWORTH, T.D.; LEHMAN, A.H.*; BLACKBURN, D.G.: Cytology and ultrastructure of the extraembryonic membranes of the quail, Coturnix coturnix.
In avian eggs, the yolk sac and chorioallantoic membrane (CAM) provide nutrients and gas exchange for the developing embryo. Despite avian diversity, most of what is known about anatomy and physiology of these extraembryonic membranes in birds comes from studies on the domestic chicken. Quail tissues were harvested at Days 7 and 14 following oviposition, and studied by means of transmission EM and light microscopy of resin-embedded tissues. Chick tissues were sampled for comparison. At Day 7, the yolk sac is trilaminar, consisting of a thin monolayer of squamous epithelial cells, the vitelline capillaries, and the underlying yolk endoderm. The endodermal cells are rich in mitochondria and phagocytosed yolk droplets. Between the first and second week of incubation, the CAM undergoes progressive expansion at the expense of the yolk sac, and lines much of the inner surface of the eggshell with allantoic vessels. A thin squamous epithelium overlies the allantoic capillaries, providing a thin barrier to respiratory exchange. Tall intercalated cells with microvilli and abundant mitochondria span the CAM, extending towards the shell membrane. These cells have been implicated in calcium uptake in the chick by previous workers. Given that qualitative cytological differences between the quail and chick were not observed, our ongoing work is focusing on quantitative comparisons in abundance and ontogeny of cell populations.