Abstract
The Hair Follicle (HF) is a vital component of mammalian skin and represents a unique, highly regenerative system that undergoes phases of rapid growth, regression, and resting periods. The hair cycling is of profound clinical relevance since majority of the hair growth disorders occur as a result of cycle changes. The influence of many molecules governing the formation of HF has been investigated and many of important cycle mediators have been identified. Cellular and molecular events during cycling are controlled by a network of sequential activation of autocrine, paracrine and endocrine signaling pathways. This implies variations in the expression or activity of the Wnt family molecules, Fibroblast Growth Factor (FGF), Transforming Growth Factor β (TGF-β), Hedgehog pathway, β-Catenin pathway, noggin, transcription factor Stat3, Epidermal Growth Factor (EGF), Insulin Growth Factor-1 (IGF- 1), Vascular Endothelial Growth Factor (VEGF), Thyrotropin Releasing Hormone (TRH), Polyamine, Spermidine, Neurotrophins (NT3, NT4), prolactin, retinoids, Bone Morphogenetic Protein 4 (BMP4), cathepsin L, 17-β estradiol, dihydrotestosterone and many others. Despite considerable progress in this area, the key elements of cycle control have not been identified. Therefore, for the most common hair disorders several agents are available, even none of these is curative or preventive. The one of the prime challenges of hair research is a better understanding of the molecular controls of hair cycling and developing drug which would effectively manipulate the cycle. Future therapy strategies will be based on new and better knowledge about the HF biology. Until than, alopecia areata, telogen effluvium and androgenetic alopecia, will remain unsolved medical problems.
Highlights
The Hair Follicle (HF) is a vital component of mammalian skin
Regarding the origin of its structures, the mature HF can be divided into the mesenchymal part, consisting of the Dermal Papilla (DP) with connective tissue sheath, and the epithelial part, including transient amplifying cells of the hair matrix that envelope the DP, hair shaft, inner root sheath and outer root sheath
Number of hairs in kenogen increases parallel with the number of vellus hair and reducing normal hair cycles, which is the main feature of Androgenetic Alopecia (AGA) deterioration [44]
Summary
The Hair Follicle (HF) is a vital component of mammalian skin. Thick scalp hair gives protection from actinic damage, while specialized nasal hairs, eyebrows and eyelashes have some environmental protective role. The HF is a uniquely dynamic system that undergoes continuous cycling throughout adult life during which elements of its own morphogenesis are recapitulated [9] This miniature organ during the normal human lifespan regenerates itself more than 8 to 10 times [2,10]. Regarding the origin of its structures, the mature HF can be divided into the mesenchymal part, consisting of the Dermal Papilla (DP) with connective tissue sheath, and the epithelial part, including transient amplifying cells of the hair matrix that envelope the DP, hair shaft, inner root sheath and outer root sheath. Bulge zone is the lowest part of the permanent HF segment and it is histological evident as unilateral thickening of the outer root sheath [17] It is inhabited by epithelial stem cells, precursors of melanocytes, mast cells and Langerhans cells. Isolated human HF can be maintained in organ culture, exhibiting emergent properties of great biological relevance: controlled cell proliferation, differentiation, apoptosis and organ regeneration [35,36]
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