Suppression of feather morphogenesis in co‐aggregates of skin cells from embryos of different ages

These experiments test the capacity of younger skin cells to promote feather differentiation in aggregates of older skin cells; the results indicate that the older cells suppressed the normal differentiation of the younger cells. Skin sheets (epidermis and dermis) from the dorsum and from the featherless (apterous) area of 6‐ to 14‐day chick embryos were dissociated into suspensions of single cells and commingled in the following heterochronous combinations: (a) 6‐day dorsal skin cells with 14‐day dorsal skin cells, (b) 8‐day dorsal skin cells with 14‐day dorsal skin cells, (c) 6‐day dorsal skin cells with 10‐day mid‐dorsal skin cells, (d) 6‐day dorsal skin cells with 10‐day apterous skin cells. Single‐age suspensions served as controls. The cells were co‐aggregated in vitro for 24 hours and the resulting aggregates grafted to the chorioallantoic membrane for further development for seven days. In grafts of control aggregates of 6‐ or 8‐day skin cells there was excellent development of feathers; in aggregates of 14‐day cells no feathers developed; aggregates of 10‐day mid‐dorsal cells gave rise to atypical, stunted feathers; aggregates of 10‐day cells from apterous areas produced no feathers. In grafts of 6‐ or 8‐day skin cells co‐aggregated with 14‐day skin cells, there was complete suppression of feather development; the predominant structures were stratified and heavily keratinized skin sheets, characteristic of aggregates of 14‐day skin cells. Grafts of co‐aggregates of 6‐day skin cells with 10‐day mid‐dorsal skin cells gave rise to feathers and skin of patterns characteristic of the 10‐day cells. Six‐day skin cells co‐aggregated with 10‐day apterous skin cells reconstructed feathers and skin of the 6‐day type as well as skin of the 10‐day type. Simple dilution of feather‐forming by non‐feather‐forming cells cannot by itself explain feather suppression in co‐aggregates with cells from older skin and “assimilation” of the younger cells into the morphogenetic pattern of the older cells. It is assumed that the suppression results from interference by constitutents in the older cell population with interactions between specific cell groupings essential for feather development or, with the cellular sorting‐out process by which these groupings arise.