BEHAVIOR OF DISSOCIATED HYPOBLAST CELLS ON THE BASAL LAMINA AND ON EXTRACELLULAR FIBRILS TN THE GASTRULATING CHICKEN-EMBRYO

The spreading behaviour of dissociated hypoblast cells on and besides a band of aligned fibrils associated with the basal lamina of the epib last was investigated by the use of scanning electron microscopy. A ho rse-shaped band of aligned fibrils, first demonstrated by Wakely and E ngland (1979), is present during the gastrulation stages of chicken em bryos on the ventral side of the epiblast at the cranial and lateral b orders of the area pellucida. The basal lamina of the area pellucida s ituated inside the fibrillar band enables the spreading and probably t he locomotion of dissociated cells, which appeared as polarized cells. Numerous cells were also found on the fibrillar band, and these cells lacked distinct lamellae and a polarized shape. Extensions of the cel ls contacted the extracellular fibrils and, at these sites of contact, the pattern of the fibrils was frequently deformed. From these observ ations and from previous results emerged the concept that spreading an d locomotion of dissociated hypoblast cells, as well as single mesobla st cells and healing hypoblast epithelium, are inhibited by the band o f extracellular fibrils, which acts as a physical barrier. The cell bi ological basis of the mechanism by which extracellular fibrils associa ted with the basal lamina arrest the migration of hypoblast and mesobl ast cells, but guide the migration of primordial germ cells, is discus sed.