The foot sole epidermis of the fore and hind feet of the adult mouse contains an acidic (type I) mRNA-encoded 73-kDa keratin polypeptide which cannot be detected in any other skin site of the mouse integument. Western blot analysis using an antibody specific for the 64-kDa keratin 9 of human and bovine callus-forming epidermis [A. C. Knapp et al. (1986) J. Cell Biol. 103, 657-667] demonstrates that the 73-kDa keratin represents the murine analog of keratin 9 of man and cow. Concomitant investigations in two related rodent species indicate that the size of this keratin varies more among species than that of any other orthologous keratin. Histological examination of adult mouse foot sole skin reveals an extremely thick and undulated epidermis covering the apical portion of the six footpads, whereas the epidermal-dermal junction of the lateral walls of these nodular protuberances as well as that of the remainder of the foot sole skin is essentially flat. If sections of adult foot sole skin are investigated by indirect immunofluorescence with the keratin 9-specific antibody, intense cytoplasmic staining is restricted to the apical rete pegs of the footpad epidermis in which virtually all suprabasal cells express keratin 9. However, we also observed keratin 9-negative cell columns ascending straight above the tips of the dermal papillae and separating the keratin 9-positive rete pegs from each other. At the transition from the strongly undulated apical epidermis to the flat epidermis of the lateral walls of the footpads, keratin 9-positive cells loose their coherence and gradually disappear toward the inter-footpad epidermis. This intimate relationship between the morphogenesis of epidermal ridges and inter-ridges and the expression of keratin 9 is also visible in foot sole epidermis of neonatal mice. Here we observed the appearance of keratin 9-positive suprabasal cells concomitant with the onset of pronounced folding of the apical footpad epidermis by about Day 3 after birth. Our findings confirm the view that the expression of keratin 9 is characteristic of a highly specialized pathway of epidermal differentiation. We propose a hypothesis for keratin expression in skin sites which are subject to pronounced mechanical wear and tear.