Control of integumentary patterns in the chick

To elucidate some of the parameters of pattern formation, experiments were done using chick skin from feather, large-scale, and small-scale forming regions.Pieces of feather-forming thigh tract skin, isolated 2 days before feathers appear in ovo, develop normally when explanted to the chorioallantoic membrane. The first row forms near the edge of the explant, and the secondary primordia form sequentially, establishing the normal pattern of diagonally intersecting rows. Thus, both the sequential (temporal) appearance and the spatial arrangement of feathers are controlled within the skin itself.Normal development also occurs in each half of the tract when these are separated before explantation. Thus, no tract-specific “initiator row” (which would normally be contained only in the lateral half) is necessary for development.To determine whether a pattern forms early and remains latent until primordia appear, explants of equal size were cultured from young and old embryos. If a latent pattern exists, the explants from the younger embryos should produce a predictably larger number of primordia, which should also be spaced closer together. Neither prediction held, indicating that no latent pattern exists.To identify which tissue controls the temporal sequence of initiation, pieces were separated by trypsinization into dermis and epidermis. The tissues were then rotated 180° with respect to one another and recombined. On these explants, the first primordia appear in the region expected for dermal control.Finally, to determine which tissue controls spatial arrangements, dermal-epidermal recombinations were made between components from different regions and different ages of skin. In all cases the dermis controls spatial arrangement.