Chapter category: Development
Late Events and the Regulation of Keratinocyte Differentiation in Hair and Feather Follicles
Molecular Basis of Epithelial Appendage Morphogenesis
Edited by: Cheng-Ming ChuongISBN: 1-57059-490-2
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Chapter authors:
George E. Rogers, Stephanie Dunn and Barry Powell
The late events of keratinocyte differentiation discussed here are mainly those that occur in the hair follicle. The reason for this emphasis is that more is known about the keratinization process of hair follicles than of feather follicles. The hair shaft keratinocyte is dedicated to the rapid synthesis of at least 70 different hair keratins. There are two groups of proteins, the keratin intermediate filament proteins and the keratin associated proteins, some of which combine to produce strikingly ordered arrays of filaments and matrix in the hair cortex whereas others produce amorphous structures in the hair cuticle. Near the end of hair differentiation the oxidation of the keratin proteins occurs to form a highly cross-linked, insoluble structure. Surrounding the hair shaft, the inner root sheath and outer root sheath layers provide biochemical and physical support to the growing hair. In the center of many hair fibers there is a column of medullary cells which provides stiffness to the fiber and promotes the insulating properties of hairs. The inner root sheath, adjacent to the growing hair, undergoes a hardening process involving the protein trichohyalin and two key enzymes, peptidylarginine deiminase and transglutaminase. Trichohyalin is also produced in great amounts in the medulla, where its fate is quite different. The inner root sheath does not normally emerge with the hair and is believed to be degraded by the action of the sebaceous gland.
Many of the genes for the structural proteins of hair have been characterized and their patterns of expression mapped. The keratin intermediate filament genes are the first to express in the upper region of the follicle bulb, followed by the families of keratin associated genes in various cell-specific and stage-specific patterns. Our understanding of the genetic control of hair keratin gene expression is just beginning, but one transcription factor, lymphoid enhancer factor 1, already appears to have a central role in the regulation of many hair genes, with the ability to coordinate the interactions of other regulatory factors. Other possible regulatory elements have been identified, but the majority of them remain to be examined for functionality.
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