Adhesion molecules and homeoproteins in the phenotypic determination of skin appendages.

We examined the roles of adhesion molecules and homeoproteins in the morphogenesis of skin appendages using feather as a model. The expression pattern of these molecules in different stages of feather development were very dynamic. For example, neural cell adhesion molecules are present first in the dermal condensations, then in distal bud epithelium, then in the dermal papilla, and finally in the marginal and axial plates. Tenascin is present first in the placode, then in the anterior bud epithelium and mesoderm, and then in the dermal papilla. The expression patterns suggest that the adhesion molecules are involved in forming the boundary of cell groups that interact to form skin appendages. Antibody perturbation of embryonic skin-explant cultures showed that liver cell adhesion molecules are involved in establishing the hexagonal pattern, neural cell adhesion molecules are involved in the formation of dermal condensations, tenascin appears to be involved in the growth of feather buds, and integrin is essential for epithelial-mesenchymal interactions. Using antibodies to XlHbox 1 (similar to Hox 3.3 or C6) and Hox 4.2 (or D4), we showed that there is a homeoprotein gradient within the feather buds, and that the expression pattern is position-specific. It is hypothesized that Hox codes, derived from the combined expression pattern of homeoproteins, determine the phenotypes and orientation of skin appendages. Experiments using retinoids in the media or retinoid-soaked beads to create a local retinoid gradient are consistent with this hypothesis. As demonstrated here, feather development provides an excellent opportunity to analyze the molecular cascade of skin-appendage morphogenesis.