Progressive miniaturization within follicular units. The same pattern of miniaturization and muscle loss continues until all follicular units are affected. It has recently been demonstrated significant decreases in muscle volume and an extensive fat infiltration around the residual APM of hair follicles in AGA patients [ Figure 2 ].
Recent studies in mice have suggested that altered lipid metabolism underlies irreversible scarring alopecia. Disruptions to the cholesterol synthesis pathway lead to accumulation of proinflammatory lipids, which trigger an innate immune response and activate a lipid-mediated programmed cell death, resulting in follicle destruction. Torkamani et al. However Cockerham et al.
This loss is most evident in Duchenne muscular dystrophy where in some cases the entire muscle may be replaced by adipocytes.
The underlying mechanism of fat infiltration in such conditions remains unclear. Some studies suggest that fibrous and fat tissue can develop from myogenic cells through alternative lineage choice dictated by a pathological environment. They demonstrated that factors derived from myofibrils strongly inhibit the activation of adipocyte progenitor cells. Therefore, fat infiltration in degenerating muscles may result from the loss of muscle-derived inhibitory factors that regulate adipocyte progenitor cells.
The interaction between the APM and the follicle mesenchyme might be an essential part of the hair follicle cycle. The DP and dermal sheath include a population of mesenchymal stem cells that contribute to follicle the homeostasis. Thus, cells from the follicle mesenchyme may also contribute to the maintenance of the APM, and the muscle degeneration seen in AGA may be caused by the loss of progenitor cell population that maintains both the APM and the DP.
The arrector pili, is classically known to participate in the mammalian thermoregulation and to produce goose bumps in humans, It is observed to undergo changes during both cicatricle alopecia and potentially reversible hair loss. Further studies are needed to assess the possible role of the APM in the induction and maintenance of hair growth, the potential loss of the APM in hair transplantation and the impact of the APM in stimulating re-epithelialization from follicular epithelium.
The distal end of the APM remains poorly characterized, and the possibility of biochemical or biomechanical signaling between either end of this structure and adjoining tissues remains a question to be addressed.
Such studies will provide a clearer understanding of normal hair growth and also hair loss conditions. Conflict of Interest: None declared. National Center for Biotechnology Information , U. Journal List Int J Trichology v. Int J Trichology. Author information Copyright and License information Disclaimer. Address for correspondence: Dr. E-mail: moc. This is an open-access article distributed under the terms of the Creative Commons Attribution-Noncommercial-Share Alike 3.
This article has been cited by other articles in PMC. Abstract The arrector pili muscle APM consists of a small band of smooth muscle that connects the hair follicle to the connective tissue of the basement membrane. Keywords: Androgenic alopecia, arrector pili muscle, hair loss. Follicle cycling The stem cells necessary for hair regeneration reside in the follicle bulge.
The arrector pili muscle in hair loss The importance of the APM in hair follicle integrity can be best described by studying hair loss conditions in which the follicular unit is distorted.
Open in a separate window. Figure 1. Figure 2. The effect of cyproterone acetate on hair roots and hair shaft diameter in androgenetic alopecia in females. Acta Derm Venereol. Gene ontology analysis of human hair follicle bulge molecular signature.
J Dermatol Sci. Barwick K. Philadelphia: JB. Lippincott Co; Atlas of Diagnostic Immunohistopatholog; pp. Narisawa Y, Kohda H. Arrector pili muscles surround human facial vellus hair follicles. Br J Dermatol. A monoclonal antibody against alpha-smooth muscle actin: A new probe for smooth muscle differentiation. J Cell Biol. Harkey MR. Anatomy and physiology of hair. Forensic Sci Int. The basement membrane of hair follicle stem cells is a muscle cell niche.
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Capturing and profiling adult hair follicle stem cells. Nat Biotechnol. A stem cell molecular signature. Involvement of follicular stem cells in forming not only the follicle but also the epidermis.
Morphogenesis and renewal of hair follicles from adult multipotent stem cells. A comprehensive guide for the accurate classification of murine hair follicles in distinct hair cycle stages. J Invest Dermatol. Chaponnier C, Gabbiani G. Pathological situations characterized by altered actin isoform expression.
J Pathol. Calponin and SM 22 as differentiation markers of smooth muscle: Spatiotemporal distribution during avian embryonic development. A smooth muscle-specific monoclonal antibody recognizes smooth muscle actin isozymes. Smooth muscle myosin heavy chain exclusively marks the smooth muscle lineage during mouse embryogenesis. Circ Res. Expression of fibronectin variants in vascular and visceral smooth muscle cells in development.
Contraction of these muscles causes the hairs to stand on end - known colloquially as goose bumps. Each arrector pili is composed of a bundle of smooth muscle fibres which attach to several follicles a follicular unit , and is innervated by the sympathetic branch of the autonomic nervous system. The contraction of the muscle is therefore involuntary - stresses such as cold, fear etc.
Contraction of the muscles has a number of different purposes. Pressure exerted by the muscle may cause sebum to be forced along the hair follicle towards the surface, protecting the hair. Arrector pili muscle The arrector pili muscles are small muscles attached to hair follicles in mammals. Arrector pili muscle Base of pilosebaceous unit: An arrector pili muscle can be seen as the pink-staining band of tissue at the bottom center and bottom left of image.
Insertion of sebaceous glands into hair shaft. Cross-section of all skin layers. David H. Cormack 1 June Essential histology. ISBN Retrieved 15 May Skin and related structures. Stratum corneum Stratum lucidum Stratum granulosum Stratum spinosum Stratum basale.
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