Perspectives on Hair Evolution Based on Some Comparative Studies on Vertebrate Cornification
Article first published online: 18 JUN 2012
© 2012 WILEY PERIODICALS, INC.
Journal of Experimental Zoology Part B: Molecular and Developmental Evolution
Volume 318, Issue 5, pages 325–343, July 2012
How to Cite
How to cite this article: 2012. Perspectives on hair evolution based on some comparative studies on vertebrate cornification. J. Exp. Zool. (Mol. Dev. Evol.) 318:325–343..
- Issue published online: 18 JUN 2012
- Article first published online: 18 JUN 2012
- Manuscript Accepted: 16 FEB 2012
- Manuscript Revised: 11 FEB 2012
- Manuscript Received: 6 DEC 2011
Hair evolution contributed to the biological success of mammals. Hair origin from synapsid scales is speculative and requires extensive modifications of the morphogenetic process transforming lens-shaped dermis of scales into small dermal papillae in hair. Hair evolution from glands is hypothetical but is supported from studies on the signaling control of hair vs. glandular morphogenesis. Based on immunocytochemical and comparative studies, it is hypothesized that the onion-like organization of hair derived from glandular pegs which central part produced lipids and some keratin. In a following stage, involucrin, trichohyalin, and keratins were produced in the central cells of the gland and formed a solid medulla surrounded by keratinocytes of the inner root sheath. The origin of this protohair was possibly related to increased concentration of beta-catenin and other signaling molecules in epithelial cells following the evolution of a dermal papilla. The latter activated the keratogenic genes, already utilized in cells of the claws, in concentric layers of cells of the glandular peg. Lipidogenic genes were depressed. As new genes evolved in the genome of synapsids, new circular layers of keratinocytes containing specialized hard keratins and keratin-associated proteins were formed around medullary cells. The new keratinocytes probably originated the cortex separating medulla from the external cells that became the inner root sheath. The hypothesis indicates that in a following stage, the medulla was obliterated or replaced by cortical cells while the external part of the cortex formed a cuticular surface due to the different growth rate with inner root sheath cells. J. Exp. Zool. (Mol. Dev. Evol.) 318B:325-343, 2012. © 2012 Wiley Periodicals, Inc.