Loricrin-like immunoreactivity during keratinization in lizard epidermis

Two modalities of keratinization are present in lizard epidermis: alpha (soft-pliable corneous layers) and beta (hard and inflexible corneous layers). While beta-keratinization is probably due to the synthesis of a new (beta)-keratin gene product, alpha keratinization resembles in part that of mammalian epidermis. The goal of this study was to test whether a sulfur-rich molecule similar to the mammalian corneous cell envelope protein loricrin is also present in lizard epidermis. This was done using X-ray microanalysis and immunocytochemical and ultrastructural methods. In the epidermis of the lizard Podarcis muralis small (0.1-0.3 microm) to large (1-5 microm) keratohyalin-like granules (KHLGs) are produced in alpha-keratinizing cells, especially in the clear layer. Small KHLGs contain sulfur and show weak filaggrin-like and stronger loricrin-like immunoreactivities. The latter is also present in keratinizing alpha-layers but is absent in the beta layers. Large KHLGs in the clear layer derive from the aggregation of the small granules with other components, including lipid material. These large granules show some loricrin-like immunoreactivity and contain sulfur and phosphorous, histidine, but not filaggrin-like immunoreactivity. It is suggested here that phosphorous derives from their phospholipid component. The present study shows that the modality of alpha-keratinization of lizard epidermis resembles that of mammals and suggests that the basic molecular mechanisms of keratin aggregation and formation of the corneous cell envelope were already present in the therapsid line of reptiles from which mammals evolved.