This layer prevented the improperly keratinized hair shaft from penetrating the skin surface. The postnatal homozygous rat skin was characterized by abnormal keratinization of the hair shaft and formation of a thick and dense layer of corneocytes in the lower portion of the epidermal stratum corneum. We present a description of the histopathologic changes in heterozygous and homozygous CR hairless rat mutants during the first month of life. However, the designation ' hairless' has been used as an extension of the hairless mouse (hr) nomenclature on the basis of the clinical absence of hairs in both phenotypes. Despite its widespread use, the molecular basis of this monogenic mutation remains unknown, and the skin histologic features of this phenotype have never been described. The Charles River (CR) ' hairless' rat is one of the autosomal recessive hypotrichotic animal models actively studied in pharmacologic and dermatologic research. The Charles River ' hairless' rat mutation is distinct from the hairless mouse alleles.
