| Abstract: |
Background: The mature anagen hair follicle is composed of a multicylindric stem that contains the hair shaft in its center and originates as an oval hair bulb proximally. Holden by the hair bulb lies an onion-like structure, called the dermal papilla (DP). Apart from serving as hair shaft factory, the anagen hair bulb also provides the hair shaft’s trichocytes with melanin granules. Within the hair bulb is a population of cells with the highest proliferation rate in the human body: the keratinocytes of the hair matrix. Obviously, it is only guaranteed in the presence of sufficient and balanced keratin synthesis. For instance, deficiency of a single protein causes severe defects despite the co-expression of numerous keratins. Mutations of human hard keratin HB6 in monilethrix as well as genetic ablation of K17 increase hair fragility. The hair cycle represents a remarkable model for studies of the regulation of stem cell quiescence and activation, as well as transit-amplifying cell proliferation, cell-fate choice, differentiation and apoptosis in a regenerative adult epithelial tissue. The hair follicle cycle include three main stages: anagen, telogen and catagen. Anagen includes 6 stages. It is the stage of hair growth. Catagen is the dynamic transition between anagen and telogen. It includes 8 stages. During catagen, the lower ‘cycling’ portion of each hair follicle regresses entirely in a process that includes apoptosis of epithelial cells in the bulb and outer root sheath (ORS), the outermost epithelial layer. HS differentiation ceases, and the bottom of the HS seals off into a rounded structure called a club, which moves upward until it reaches the permanent, non-cycling upper follicle, where it remains anchored during telogen. Following catagen, follicles lie dormant in a resting phase (telogen). While it is undisputed that the biological ”clock” that drives hair follicle cycling resides in the hair follicle itself, the molecular nature of the underlying oscillator system remains to be clarified. Pigmentation in the adult hair follicle, results from precise sequential interactions between follicular melanocytes, matrix keratinocytes, and dermal papilla (DP) fibroblasts. It involves the melanogenic activity of follicular melanocytes, the transfer of their product, melanin granules, into cortical and medullary keratinocytes, and the formation of pigmented hair shafts. Hair is actively pigmented only during the anagen stage of the hair cycle, to which the melanogenic activity of follicular melanocytes is stringently coupled; melanin formation is switched-off in catagen remaining absent through telogen. In melanocytes, melanin synthesis is restricted to melanosomes that are structurally assembled via a process resembling lysosome biogenesis. The ‘‘follicular-melanin unit’’ resides in the immune privileged proximal hair bulb and consists of hair matrix melanocytes and keratinocytes, partly regulated by DP fibroblasts. Hair aging comprises weathering of the hair shaft and aging of the hair follicle. The former involves progressive degeneration of the hair fiber from the root to the tip, while the latter manifests as decrease of melanocyte function or graying and decrease in hair production in androgenetic and senescent alopecia. The scalp is also subject to intrinsic or physiologic aging and extrinsic or premature aging caused by external factors. Hair show variations of the cell number ,variations of functional characteristics, variations in shape, variations with race, variations of hair shaft length and thickness, variations in the physical properties, variations of Hair strength and variations with Age. Hair follicle stands out as one of the best model systems for studying adult stem cells.Objectives: This essay aims to discuss the various aspects of hair biology with emphasis on the clinical implications. Special attention is paid to the recent discoveries and future direction in this important topic.
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