Plantar epidermis of the bovine heel pad as well as human plantar and palmar epidermis contain large amounts of an acidic (type I) keratin polypeptide (No. 9) of Mr 64,000 which so far has not been found in epidermis of other sites of the body. We present evidence for the keratinous nature of this protein, including its ability to form cytokeratin complexes and intermediate-sized filaments in vitro. We have isolated RNA from plantar epidermis of both species and show, using translation in vitro, that these polypeptides are genuine products of distinct mRNAs. Using immunofluorescence microscopy with specific antibodies against this protein, we demonstrate its location in most cells of suprabasal layers of plantar epidermis as well as in sparse keratinocytes which occur, individually or in small clusters, in upper layers of epidermis of other body locations. We conclude that cytokeratin No. 9 is characteristic of a special program of keratinocyte differentiation which during morphogenesis is expressed in most epidermal keratinocytes of soles and palms but only in a few keratinocytes at other body sites. This example of cell type-specific expression of a member of a multigene family in relation to a body site-related program of tissue differentiation raises important biological questions concerning the regulation of keratinocyte differentiation and morphogenesis as well as the function of such topological heterogeneity within a given type of tissue.

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