INVOLUCRIN MESSENGER-RNA IS MORE ABUNDANT IN HUMAN HAIR-FOLLICLES THAN IN NORMAL EPIDERMIS

Involucrin is a precursor protein of the cornified cell envelope in ep idermal keratinocytes, where it has been located by immunohistochemist ry in the upper spinous and granular layers of human epidermis. In the hair follicle, involucrin has been found in the inner root sheath and in the upper layers of the infundibulum and the isthmus (upper outer root sheath), whereas its presence in the lower outer root sheath and the cortex has been controversial. Therefore, we analyzed the distribu tion of involucrin mRNA in adult scalp by Northern blotting and in sit u hybridization. Northern blots showed more abundant involucrin mRNA i n the follicular fraction than in the epidermal fraction of dissected scalp. In situ hybridization matched the immunohistologic results; tra nscripts of involucrin were expressed not only in the infundibulum and isthmus, but also in the hair cortex and medulla, in all layers of th e inner root sheath, and in the inner cells of the lower outer root sh eath (all of which lack a cell envelope at the ultrastructural level). However, involucrin was absent in the hair cuticle, which is the only compartment of the follicle possessing a morphologically distinct cel l envelope. Our results suggest, first, that involucrin does not serve as a precursor protein of the cornified cell envelope in adult hair f ollicles, and second, that it is perhaps not necessary for the formati on of the cell envelope in keratinocytes of the hair cuticle, as we di d not find this precursor protein with highly sensitive methodology.