Structural and Immunocytochemical Characterization of Keratinization in Vertebrate Epidermis and Epidermal Derivatives

The study examined the inner root sheath (IRS) of hairs in monotremes, marsupials, and placental mammals, focusing on the localization of trichohyalin and transglutaminase using electron microscopy and immunocytochemistry. It found that trichohyalin is recognized across these species, suggesting common epitopes. The IRS in monotremes was multistratified but lacked a Henle layer, indicating a simpler ancestral structure. The research proposed that the evolution of the IRS, with its stratified layers, facilitated the safe exit of hair through the epidermis, crucial for mammalian hair development. The study also speculated on the evolutionary transition from reptilian scales to mammalian hairs, highlighting the role of dermal papillae in this process.

The document reviewed the advancements in understanding hair follicle differentiation and regulation since Hardy's 1992 review. It covered the structure of hair and hair follicles, the characterization of genes for keratin and keratin-associated proteins, and the identification of their expression locations within different cell layers of the hair follicle. Additionally, it discussed the molecular signals that controlled keratin gene expression and the post-translational events occurring in the terminal stages of hair formation.

The study examined the distribution of keratin and associated proteins in the epidermis of monotreme and marsupial mammals, comparing them to eutherian mammals. It found significant interspecific variation in keratohyalin granules (KHGs) size and distribution, particularly in hairless or low-density hair regions. Monotreme and marsupial epidermis showed similarities to eutherians in the distribution of acidic and basic proteins, but differences in the cross-reactivity of antibodies against certain proteins like involucrin and trichohyalin, suggesting species-specific molecular structures. Loricrin distribution in monotremes resembled that in reptiles and birds, indicating a primitive pattern of amniote α-keratogenesis. The study suggested that the presence of loricrin-like proteins in amniote corneous tissues is a primitive feature, and the redistribution mechanism in monotreme keratinocytes might be less specialized than in therian mammals.

The study cataloged human type II hair keratins, identifying six members divided into two groups: group A (hHb1, hHb3, hHb6) and group C (hHb2, hHb4, hHb5). Using specific antisera, a two-dimensional catalog was established, revealing isoelectric variants and their roles in hair differentiation. hHb5 and hHb2 were linked to early hair differentiation stages, while hHb1, hHb3, and hHb6 were expressed in advanced stages. hHb4 was found in the tongue, not hair follicles. The study suggested random keratin-pairing during differentiation and integrated type II keratins with type I and epithelial keratins into a comprehensive human keratin catalog.