Changing patterns of cell adhesion molecules during mouse pelage hair follicle development. 2. Follicle morphogenesis in the hair mutants, Tabby and downy.

Wild-type mice have three main types of hair in their pelage: tylotrichs, awls and zigzags. Tabby mice have a yellowish coat consisting of awls only, whereas downy mice have a sparse grayish coat consisting of unusually fine hairs. The spatial and temporal distribution of cell adhesion molecules (CAMs) during hair follicle morphogenesis was investigated in the mutants and compared with that in nonmutant mice. In Tabby embryos, awl follicles developed normally and showed normal immunostaining patterns for E-cadherin, P-cadherin and N-CAM. Prior to follicle initiation, however, some deviations from normal skin morphology and staining patterns indicated a delay in the development of the basal epidermal layer. On the other hand, the stratum corneum was formed prematurely. Therefore, the lack of tylotrich and zigzag follicles in Tabby mice might be explained by a general defect in epidermal development rather than by abnormal CAM expression. In downy embryos, tylotrich and awl follicles were initiated within the normal time periods, but elongation and differentiation of most follicles were abnormal. At birth, most follicles were small and/or severely deformed but showed normal CAM expression patterns. Extreme distortion and disorientation of follicles seemed to be associated with disintegration of the dermal papilla and abnormal mesenchymal cell condensations between the follicles. This suggests that abnormal hair development in downy mice might result from a defect in dermal rather than epidermal components of the skin.