Perspectives on Hair Evolution Based on Comparative Studies on Vertebrate Cornification

The document hypothesized that hair evolution in mammals originated from glandular structures rather than synapsid scales, supported by immunocytochemical and comparative studies. It proposed that hair's onion-like organization derived from glandular pegs, with central cells producing lipids and keratin, eventually forming a solid medulla surrounded by keratinocytes. Increased concentrations of beta-catenin and other signaling molecules in epithelial cells led to the activation of keratogenic genes, forming concentric layers of keratinocytes. As new genes evolved, specialized keratinocytes formed the cortex, separating the medulla from the inner root sheath. The medulla was later replaced by cortical cells, and the cortex's external part formed a cuticular surface.