A SEM study of cell shedding during the formation of the area pellucida in the chick embryo.

Intrauterine eggs at different stages of development were retrieved from hens and the blastodiscs were dissected out and prepared for scanning electron microscopy. Our observations trace the polarized shedding of cells from the central area of the lower layers of the blastodisc, which leads to the formation of the area pellucida. Prior to the shedding, single central blebs appear on the ventral side of the cells facing the sub-blastodermic cavity. We also noted the presence of lobopodia which establish contact with nearby cells. The possible function of the lobopodia is discussed. The cells of the blastodisc are at the same time (stages IV and V, E.-G. & K). also intimately interconnected via a profusion of threadlike cytoplasmic processes (filopodia). When shedding starts the blebs as well as the lobopodia disappear, and the filopodia undergo typical withdrawal steps such as showing a beaded stage and appearing drawn out or taut, before they finally disrupt to leave behind a smooth round cell which is detached from its neighbours. The beading phenomenon points to the possibility of a disassembly of the microtubular skeleton of the filopodia operated by some physical force influenced by gravity.