Organization of the chick blastoderm edge.

A band of cells forming the edge of the chick blastoderm, and attached to the vitelline membrane, causes the expansion of the blastoderm in the first few days of incubation by active migration across the vitelline membrane. The structure and organization of these cells was examined by light microscopy (both on whole mounts and sections) and transmission electron microscopy. The account presented differs markedly from previous descriptions of these cells. The band of cells at the blastoderm edge is an association, between 90 and 130/tm wide, of flattened, non-dividing cells forming a multilayer; some of these cells, and no other cells of the blastoderm, are attached to the vitelline membrane. Each attached cell has a thin flattened lamella, centrifugally oriented and underlapping the next cell distally, except (1) the most distal cell, whose lamella is thick and long, though tapering, and is not overlain by other cells; and (2) the most proximal attached cell which has a short centripetally oriented lamella, as well as a centrifugal underlapping one. The cells of the edge band not attached to the vitelline membrane also have flattened lamellae attached to the cells below; these lamellae are, however, unoriented. The cells of the edge band all have plentiful cortical filaments and cytoplasmic microtubules. Specialized plaques are involved in the attachment of edge band cells to the vitelline membrane. The form of this edge structure is compared with the outgrowth edge of a chick yolk sac epiblast explant cultured on vitelline membrane. It seems likely that the way the blastoderm edge cells are organized may explain their prodigious migratory activity.