Hair Follicle Pigmentation

Abstract

Hair shaft melanin components (eu- or/and pheomelanin) are a long-lived record of precise interactions in the hair follicle pigmentary unit, e.g., between follicular melanocytes, keratinocytes, and dermal papilla fibroblasts. Follicular melanogenesis (FM) involves sequentially the melanogenic activity of follicular melanocytes, the transfer of melanin granules into cortical and medulla keratinocytes, and the formation of pigmented hair shafts. This activity is in turn regulated by an array of enzymes, structural and regulatory proteins, transporters, and receptors and their ligands, acting on the developmental stages, cellular, and hair follicle levels. FM is stringently coupled to the anagen stage of the hair cycle, being switched-off in catagen to remain absent through telogen. At the organ level FM is precisely coupled to the life cycle of melanocytes with changes in their compartmental distribution and accelerated melanoblast/melanocyte differentiation with enhanced secretory activity. The melanocyte compartments in the upper hair follicle also provides a reservoir for the repigmentation of epidermis and, for the cyclic formation of new anagen hair bulbs. Melanin synthesis and pigment transfer to bulb keratinocytes are dependent on the availability of melanin precursors, and regulation by signal transduction pathways intrinsic to skin and hair follicle, which are both receptor dependent and independent, act through auto-, para- or intracrine mechanisms and can be modified by hormonal signals. The important regulators are MC1 receptor its and adrenocorticotropic hormone, melanocyte stimulating hormone, agouti protein ligands (in rodents), c-Kit, and the endothelin receptors with their ligands. Melanin itself has a wide range of bioactivities that extend far beyond its determination of hair color.

Differential expression of HPA axis homolog in the skin

“…In human skin, the process can be upregulated by UVR and factors increasing cAMP levels, while it is inhibited by several growth factors (Luger et al, 1999;Slominski and Pawelek, 1998;Slominski et al, 2004b;Slominski et al, 2000d). In mouse skin, POMC expression and the production of functional POMC-derived peptides is inhibited by glucocorticoids, and coupled to hair follicle cycling (Ermak and Slominski, 1997;Paus et al, 1999;Slominski et al, 1998a;Slominski et al, 2000d;Slominski et al, 2005d).…”

(Neuro-)endocrinology of epithelial hair follicle stem cells

“…In the past, almost all interest in the field has focused on thewell-recognized and long-studied, but still insufficiently characterized -impact of these hormones, neuropeptides, neurotransmitters and neurotrophins on the more differentiated progeny of HFeSC, namely on hair matrix keratinocytes and HF outer root sheath (ORS), on the melanocytes of the HF pigmentary unit, and on the specialized, inductive mesenchyme of the HF (connective tissue sheath and follicular dermal papilla) (Alonso and Fuchs, 2006;Gilhar et al, 2007;Slominski et al, 2005;Stenn and Paus, 2001). Now, however, there is growing awareness that HFeSCs themselves may underlie hormonal controls, and that the latter may well impact on hair normal follicle development, growth, cycling, hair shaft formation, and hair pigmentation.…”

Oncostatic effects of the indole melatonin and expression of its cytosolic and nuclear receptors in cultured human melanoma cell lines

“…In the epidermal keratinocytes, a constitutive and UV-induced melatonin metabolism has recently been described (23). Under physiologic conditions, melatonin participates in the regulation of keratinocyte proliferation (24)(25)(26), hair growth (26)(27)(28)(29)(30)(31) and melanogenesis (32,33). In conditions of oxidative stress, it acts as an antioxidant (18,(34)(35)(36) and UV-protective substance (18,22,37,38).…”