Lateral interactions account for the pattern of the hair cell array in the chick basilar papilla.

It has been suggested that lateral interactions set up the array of hair cells and supporting cells in the chick basilar papilla. The presence of a hair cell would inhibit adjacent cells from becoming hair cells, and promote the formation of supporting cells. Models of cell specification were tested, starting with a closely packed array of multipotent progenitor cells. Lateral interactions, in which emerging hair cells promoted a supporting cell phenotype in adjacent cells, and in which emerging supporting cells promoted a hair cell phenotype in adjacent cells, produced an array of cells similar to that observed experimentally in the distal and central parts of the basilar papilla. In these areas, the ratio of supporting cells to hair cells is very close to 2:1, each hair cell on average being surrounded by six supporting cells, and each supporting cell being surrounded by three hair cells and three supporting cells. Identical patterns of hair and supporting cells could be produced by models in which either of the lateral inhibitory factors was replaced by a diffusive factor, i.e. a factor which acts on all cells in the model irrespective of position. The agreement of the model with observed cell ratios supports the view that the fate of both hair cells and supporting cells in the chick basilar papilla is a product of cell interactions within the papilla. It is possible that one factor, that provides contact lateral inhibition and promotes the formation of supporting cells, is the Notch/Delta system. It is possible that the other factor is retinoic acid, a diffusive factor that promotes the formation of hair cells.