International Journal of Trichology

: 2014  |  Volume : 6  |  Issue : 3  |  Page : 88--94

Beyond goosebumps: Does the arrector pili muscle have a role in hair loss?

Niloufar Torkamani, Nicholas W Rufaut, Leslie Jones, Rodney D Sinclair 
 University of Melbourne; Epworth Hospital, Melbourne, Victoria, Australia

Correspondence Address:
Niloufar Torkamani
Epworth Hospital, Dermatology Lab, 185 187 Hoddle Street, Richmond, Victoria


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. The APM mediates thermoregulation by contracting to increase air-trapping, but was thought to be vestigial in humans. The APM attaches proximally to the hair follicle at the bulge, a known stem cell niche. Recent studies have been directed toward this muscle«SQ»s possible role in maintaining the follicular integrity and stability. This review summarizes APM anatomy and physiology and then discusses the relationship between the follicular unit and the APM. The potential role of the APM in hair loss disorders is also described, and a model explaining APM changes in hair loss is proposed.

How to cite this article:
Torkamani N, Rufaut NW, Jones L, Sinclair RD. Beyond goosebumps: Does the arrector pili muscle have a role in hair loss?.Int J Trichol 2014;6:88-94

How to cite this URL:
Torkamani N, Rufaut NW, Jones L, Sinclair RD. Beyond goosebumps: Does the arrector pili muscle have a role in hair loss?. Int J Trichol [serial online] 2014 [cited 2023 Jan 29 ];6:88-94
Available from:

Full Text


Hair loss or alopecia is a common and unpleasant disorder. It affects both genders and is seen in all age groups. This condition is a great psychological burden for the patient. Hair loss can be divided broadly into two subtypes; scarring and nonscarring. Scaring alopecia occurs when inflammation or injury damages the epithelial stem cell population in the bulge and leads to destruction of the entire follicular unit. Nonscarring alopecia can be focal patterned or generalized. The inflammation or injury is focused on the hair matrix and spares the bulge. The follicular unit retains the capacity to regenerate, and the hair loss is potentially reversible, at least in the early stages. [1]

The appendageal structures of the skin include the hair follicle, sebaceous gland, the apocrine gland, and the arrector pili muscle (APM), these structures are all located in the dermis and are functionally and anatomically related. The APM has a role in the development and maintenance of the pilosebaceous unit. [2],[3],[4],[5] This hypothesis paper will review the structure and known function of the APM and present a novel model proposing an integral role for the APM in the hair cycle, wound healing and the development of androgenetic alopecia, the most common cause of human hair loss.


Hair has a unique anatomical and physicochemical structure. The hair follicle can be divided into four parts; the bulb, suprabulbar area, isthmus and infundibulum. In the scalp, the lowest portions of the hair follicles (the bulbs) are located in the upper part of the sub-dermal fat. [6] The follicle epithelium consists of the following concentric layers, from medial to lateral: The medulla, the cortex and cuticle of the hair shaft, the inner root sheath, and the outer root sheath. The dermal papilla (DP) and dermal sheath are mesenchymal compartments. The DP and surrounding epithelial matrix cells, comprise bulb. [7]

The inner root sheath surrounds the hair in the suprabulbar and isthmus areas and ends at the insertion of the sebaceous duct at the lower end of the infundibulum.

The follicle bulge is a vestigial structure in the outer root sheath at the level of the insertion site of the APM. [8] The presence of epithelial stem cells in the human hair bulge has been confirmed through localization of label retaining cells just above the insertion site of the APM and below the sebaceous glands. [9],[10] Bulge cells have an undifferentiated structure, [11] are highly proliferative and multipotent. [12],[13],[14],[15],[16] Signals from the mesenchymal DP initiate stem cell activation in early anagen, resulting in the generation of transient amplifying cells. [11] These transient amplifying cells proliferate rapidly and differentiate to form a new bulb that subsequently gives rise to a new hair shaft. The bulge is also a source of migrating stem cells in wound healing. [17] The bulge, bulb and suprabulbar areas of the follicle all exhibit relative immune privilege. [18] In the dermis surrounding the bulge, there is an accumulation of mesenchymal cells that are in turn surrounded by an abundant nutritive and systemic circulatory apparatus. [19],[20],[21] The up-regulation of development, morphogenesis and organogenesis genes in the human bulge and the down-regulation of cell proliferation, cycle and mitosis genes suggest that the human bulge is responsible for the homeostasis and regeneration of the hair follicle. [2]


The arrector pili are a smooth muscle. The APM has a less complicated innervation pattern compared to other appendageal structures. The muscle has a dense network of noradrenergic fibers and a less prominent cholinergic system. These fibers run parallel to the muscle. Nerve fibers are more abundant at the APM insertion site to the follicle with a more prominent distribution of the cholinergic network. The density of the nervous system decreases in the distal ends of the APM. This muscle is derived from the paraxial mesoderm. [22] APM cells are characteristically fusiform and demonstrate "cigar shaped" centralized nuclei in histological sections. These cells have no cytoplasmic striations. [3] The APM is usually seen as a metachromatic structure located on the side of the follicle forming an obtuse angle with the skin surface. [4] The proximal end of the APM surrounds the whole follicle in the bulge region of both terminal [23] and vellus hairs. [4]

Each hair follicle was once believed to be attached to a separate APM, [24] but this has been refuted by more recent studies. Poblet et al. proposed that the APM associated with follicles in one follicular unit converge into one muscular unit and hence that all follicles within a unit share a single muscle. [25] In a study conducted by Song et al., the structure of the APM was evaluated by three-dimensional reconstructions. [26] The authors reported a variation in which two follicular units can share a single APM. They also suggested that only one muscular structure is involved in the follicular unit, which inserts tightly to the furthest follicle. [27] Song et al. also reported that the APM forms a concave support for the sebaceous gland lobules. These lobules are located between the follicles and APM, forming an angular area. Additional sebaceous lobules are also localized in the counter angular area. [28]


Follicle bulge cells interact closely with the APM. [8] Tiede et al. introduced an outer root sheath (ORS) protrusion "the trochanter" at the APM insertion site of anagen VI hair follicles. [29] Narisawa et al. [4] have demonstrated the presence of knob-like swellings and villous projections in human terminal and vellus hair follicles. The authors also demonstrated the presence of skirt-like projections in small vellus hair but not in large vellus hairs. The APM-bulge connection persists throughout the hair growth cycle and has been suggested to play an important role in morphogenesis and renewal of hair follicles. [30],[31],[32],[33]

APM differentiation and anchorage to the bulge is thought to be mediated by components of the follicle basement membrane. [34] Tendon genes such as Scx (scleraxis), [35] Mitf, Igfbp5, Fbln1 (fibulin-1), Postn (periostin), Tnc (tenascin-C), Sparc, Igfbp6, and Fgf18 [36] are expressed by bulge stem cells. These cells are known to highly express periostin, which is a major component of the tendon extracellular matrix (ECM). Accordingly it has been concluded that bulge stem cells serve as tendon cells for the APM. [7]

Nephronectin is an ECM protein expressed by stem cells in the bulge. [7] Nephronectin expression in bulge and hair germ cells results in epidermal basement membrane heterogeneity. Nephronectin is seen in telogen and anagen hair, in the basement membranes of the APM proximal attachment site in the bulge, of the APM itself, and of the hair bulb. α8β1 integrin is the nephronectin receptor. α8 is expressed in the APM and the DP, where it is co-localized with nephronectin at the junction of the telogen hair germ. During the development, fibroblasts positive for α8 integrin are thought to be progenitors of the APM. These cells appear to attach to nephronectin in the bulge and then differentiate into α-smooth muscle actin (SMA)-expressing APM cells. In an in vitro assay, α8-positive cells isolated from skin bound to nephronectin and up-regulated expression of α-SMA and smooth muscle protein 22-α (Sm22a). Accordingly, nephronectin provoked expression of APM markers in these cells. [7] α8 integrin also has a role in the differentiation of vascular and intestinal smooth muscles. [37],[38]

Major changes in the APM have been observed in the skin of nephronectin knockout mice (Npnt-/-). [7] While in normal mice the APM attaches to the bulge (88.3% of follicles), in Npnt-/- mice it is displaced to an EGFL6-positive zone distal to the bulge (76.3% of follicles). EGFL6 is also a α8β1 ligand. The number of hair follicles with APM, and APM attachments to the follicle also decrease in Npnt-/- skin. Interestingly, piloerection is maintained despite the fact that the APM attachment site is displaced. In α8 integrin knockout mice, the APM attaches to both the nephronectin and EGFL6-positive regions. These mice have normal nephronectin expression in the bulge, but the APM lacks nephronectin. [7] Overall, the phenotypes of both knockout mice indicate that nephronectin and α8β1 integrin play a significant role in determining the proximal attachment site of the APM.


The structural details and components of the distal attachment site of the APM have remained largely unclear. Narisawa et al. studied the distal end of the APM in the scalp skin of a 12-month-old infant and 30 and 36-week-old fetuses. [39] In the infant's scalp skin, 91% of the APM demonstrated an ending in the upper dermis. In vertical sections of the fetal scalp skin, the desmin-reactive APM were closely juxtaposed with the Ks20-8 reactive bulge, and with epidermal, dermal and Merkel cells. Clifton et al. demonstrated that the APM had several branches in the distal region. [40] They also showed that interaction between α5β1 integrin and fibronectin marks the APM distal attachment to the dermal ECM. [40] Co-localization of α5β1 and fibronectin at every connecting point between the APM and the dermal ECM or epidermal basement membrane suggested that the muscle actually interacts with its surroundings via many points in the papillary and reticular dermis and also at the dermal-epidermal junction. Furthermore, α1β1 integrin was shown to be involved in muscle cell-to-cell adhesion by Mendelson et al. [41]


Piloerection and sebum secretion

It is well-known that a major function of the APM is to raise the hair from the skin surface (piloerection) [42],[43] resulting in goose bumps. Piloerection can be a response to cold or emotional state.

Sebum is an oily substance secreted by the sebaceous glands that help prevent hair and skin from drying out. It has been speculated that the APM might have a role in sebum secretion. [28],[42] As the APM shortens during contraction, the concave portion abutting the sebaceous gland would flatten and tighten. The sebaceous lobules would be squeezed between the follicle and APM, thereby actively causing sebum secretion. [28]

Follicle cycling

The stem cells necessary for hair regeneration reside in the follicle bulge. [11],[44] It has been shown that coordinated signaling between epithelial stem cells in the bulge and the underlying mesenchymal DP is present during hair development, and also in adult follicle cycling. [45] This signaling induces stem cell proliferation and provokes the cell differentiation cascade, eventually leading to regeneration of a new lower follicle that replaces the regressed catagen follicle. [44],[46] Although never directly investigated, the co-expression of nephronectin in the bulge, APM and DP [7] raises the possibility of common signaling pathways in these structures.


Follicular unit integrity

The APM's role in follicular unit integrity has not been thoroughly studied. Together with the sebaceous glands and the hair follicle, the APM forms the pilosebaceous unit. Poblet et al. suggested that the APM plays a crucial role in maintaining the follicular integrity by holding together each of the hair follicles in the follicular unit at the isthmus level. The splaying of hair follicles in a follicular unit only happens above the insertion site of the APM. Hence Poblet et al. described the APM as "the ribbon on a bunch of flowers," holding together the hair follicles in a follicular unit. In piloerection, hair shafts follow the same orientation. This strongly suggests that the APM is a muscular unit rather than multiple single muscle fibers. [42] It has been demonstrated that the APM expresses actin and vimentin which are essential for the hair follicle to cope with the movement of the hair shaft during development. [47]

The APM is considered one of the most important appendageal structures in hair transplant studies. Sato et al. studied single follicular unit transplants in androgenic alopecia (AGA) patients. They demonstrated that restoration of the APM and adjacent nervous system induces the regeneration of the neurofollicular and neuromuscular junctions in the follicle bulge. [8],[48]

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. The APM structure in normal, telogen effluvium (TE), alopecia areata (AA) and AGA affected scalp skin has been studied. [49],[50] It has been demonstrated that the APM remains connected to the bulge of miniaturized follicles in AA and TE, which are reversible, but not in irreversible male and female pattern AGA. [50] Although APM attachment to vellus hairs is lost in AGA, attachment to terminal hairs remains preserved. It has not been defined whether APM regression in irreversible hair loss was a cause or effect of the disease.

We have proposed the following model for APM degeneration in an irreversible alopecia. [49],[50] Initially, multiple compound follicular units comprising a primary follicle and several secondary follicles are found across the scalp. In the early stages of hair loss, patients usually complain of hair thinning and a decrease in their pony tail volume, but there is little visible baldness. Miniaturization occurs first in the secondary follicles. As illustrated in [Figure 1]a and b, the APM initially loses its attachment to regressing secondary follicles in only some follicular units. We have named these follicular units "herald units." The muscle remains attached to the primary follicle in the herald units at this stage. Miniaturization of secondary follicles and detachment of the APM from these follicles is then extended to the remaining follicular units [Figure 1]c and d]. As the disease progresses, miniaturization continues, and the muscle completely loses attachment to the secondary follicles in herald units. Furthermore, primary follicles in herald units are affected by miniaturization, and eventually muscle attachment is lost [Figure 1]e]. Baldness occurs when the entire follicular unit is miniaturized. The same pattern of miniaturization and muscle loss continues until all follicular units are affected [Figure 1]f and g].{Figure 1}


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]. [49] It remains unclear how APM degeneration and fat infiltration are related to follicle miniaturization and hair loss. 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. [51],[52] However, there is little inflammation associated with the fat infiltration in AGA.{Figure 2}

Torkamani et al. have proposed that the APM undergoes fat degeneration in irreversible hair loss conditions such as advanced AGA. [49] Fat degeneration has mainly been described in skeletal muscles. However Cockerham et al. demonstrated fat infiltration and fibrosis of the superior tarsal muscle, an ocular smooth muscle, in patients with the thyroid associated ptosis. [53] Fat degeneration in skeletal muscles is a prominent feature in several pathological conditions which involve loss of muscular integrity. This loss is most evident in Duchenne muscular dystrophy where in some cases the entire muscle may be replaced by adipocytes. [54] Other conditions such as severe neurogenic atrophy, type II diabetes, obesity or age-related sarcopenia may also exhibit muscular fat degeneration. [55],[56],[57],[58]

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. [59],[60],[61] Uezumi et al. identified mesenchymal progenitor cells in muscular tissue with ectopic fat deposition and suggested that these cells are responsible for ectopic fat cell formation in pathological conditions such as Duchenne muscular dystrophy, denervation, obesity and ageing-related sarcopenia. 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. [62] It has been suggested that fatty infiltration in damaged muscles, such as after tendon tear, is not a degenerative process, but a necessary rearrangement of the tissue after macroarchitectural changes caused by musculo-tendinous retraction. [63]

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. [64],[65],[66] Follicle cycling is associated with the movement of cells between the DP and dermal sheath. [67],[68] It is thought that this process is disrupted in AGA to cause the loss of cells from the DP and consequent follicle miniaturization. [29],[69],[70] Cells from the DP and dermal sheath are capable of undergoing both smooth muscle [29],[33] and adipose [3],[22] differentiation in vitro. 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.


1Peereboom-Wynia JD, van der Willigen AH, van Joost T, Stolz E. The effect of cyproterone acetate on hair roots and hair shaft diameter in androgenetic alopecia in females. Acta Derm Venereol 1989;69:395-8.
2Ohyama M, Vogel JC, Amagai M. Gene ontology analysis of human hair follicle bulge molecular signature. J Dermatol Sci 2007;45:147-50.
3Barwick K. Atlas of Diagnostic Immunohistopatholog. Philadelphia: J.B. Lippincott Co.; 1990. p. 4-15.
4Narisawa Y, Kohda H. Arrector pili muscles surround human facial vellus hair follicles. Br J Dermatol 1993;129:138-9.
5Skalli O, Ropraz P, Trzeciak A, Benzonana G, Gillessen D, Gabbiani G. A monoclonal antibody against alpha-smooth muscle actin: A new probe for smooth muscle differentiation. J Cell Biol 1986;103:2787-96.
6Harkey MR. Anatomy and physiology of hair. Forensic Sci Int 1993;63:9-18.
7Fujiwara H, Ferreira M, Donati G, Marciano DK, Linton JM, Sato Y, et al. The basement membrane of hair follicle stem cells is a muscle cell niche. Cell 2011;144:577-89.
8Gabella G. Development of visceral smooth muscle. Results Probl Cell Differ 2002;38:1-37.
9Ohyama M, William J. Cunliffe Scientific Awards. Advances in the study of stem-cell-enriched hair follicle bulge cells: A review featuring characterization and isolation of human bulge cells. Dermatology 2007;214:342-51.
10Ohyama M, Terunuma A, Tock CL, Radonovich MF, Pise-Masison CA, Hopping SB, et al. Characterization and isolation of stem cell-enriched human hair follicle bulge cells. J Clin Invest 2006;116:249-60.
11Cotsarelis G, Sun TT, Lavker RM. Label-retaining cells reside in the bulge area of pilosebaceous unit: Implications for follicular stem cells, hair cycle, and skin carcinogenesis. Cell 1990;61:1329-37.
12Morris RJ, Liu Y, Marles L, Yang Z, Trempus C, Li S, et al. Capturing and profiling adult hair follicle stem cells. Nat Biotechnol 2004;22:411-7.
13Ivanova NB, Dimos JT, Schaniel C, Hackney JA, Moore KA, Lemischka IR. A stem cell molecular signature. Science 2002;298:601-4.
14Taylor G, Lehrer MS, Jensen PJ, Sun TT, Lavker RM. Involvement of follicular stem cells in forming not only the follicle but also the epidermis. Cell 2000;102:451-61.
15Oshima H, Rochat A, Kedzia C, Kobayashi K, Barrandon Y. Morphogenesis and renewal of hair follicles from adult multipotent stem cells. Cell 2001;104:233-45.
16Müller-Röver S, Handjiski B, van der Veen C, Eichmüller S, Foitzik K, McKay IA, et al. A comprehensive guide for the accurate classification of murine hair follicles in distinct hair cycle stages. J Invest Dermatol 2001;117:3-15.
17Chaponnier C, Gabbiani G. Pathological situations characterized by altered actin isoform expression. J Pathol 2004;204:386-95.
18Duband JL, Gimona M, Scatena M, Sartore S, Small JV. Calponin and SM 22 as differentiation markers of smooth muscle: Spatiotemporal distribution during avian embryonic development. Differentiation 1993;55:1-11.
19Gown AM, Vogel AM, Gordon D, Lu PL. A smooth muscle-specific monoclonal antibody recognizes smooth muscle actin isozymes. J Cell Biol 1985;100:807-13.
20Miano JM, Cserjesi P, Ligon KL, Periasamy M, Olson EN. Smooth muscle myosin heavy chain exclusively marks the smooth muscle lineage during mouse embryogenesis. Circ Res 1994;75:803-12.
21Glukhova MA, Frid MG, Shekhonin BV, Balabanov YV, Koteliansky VE. Expression of fibronectin variants in vascular and visceral smooth muscle cells in development. Dev Biol 1990;141:193-202.
22Amoh Y, Li L, Katsuoka K, Hoffman RM. Embryonic development of hair follicle pluripotent stem (hfPS) cells. Med Mol Morphol 2010;43:123-7.
23Barcaui CB, Piñeiro-Maceira J, de Avelar Alchorne MM. Arrector pili muscle: Evidence of proximal attachment variant in terminal follicles of the scalp. Br J Dermatol 2002;146:657-8.
24Lesson TS, Lesson CR, Paparo AA. Text/Atlas of Histology. Philadelphia: W.B. Saunders; 1988. p. 125-35.
25Poblet E, Ortega F, Jiménez F. The arrector pili muscle and the follicular unit of the scalp: A microscopic anatomy study. Dermatol Surg 2002;28:800-3.
26Song WC, Hwang WJ, Shin C, Koh KS. A new model for the morphology of the arrector pili muscle in the follicular unit based on three-dimensional reconstruction. J Anat 2006;208:643-8.
27Song WC, Hu KS, Koh KS. Multiunit arrector pili muscular structure as a variation observed by using computer-based three-dimensional reconstruction. Cell Tissue Res 2005;322:335-7.
28Song WC, Hu KS, Kim HJ, Koh KS. A study of the secretion mechanism of the sebaceous gland using three-dimensional reconstruction to examine the morphological relationship between the sebaceous gland and the arrector pili muscle in the follicular unit. Br J Dermatol 2007;157:325-30.
29Tiede S, Kloepper JE, Whiting DA, Paus R. The 'follicular trochanter': An epithelial compartment of the human hair follicle bulge region in need of further characterization. Br J Dermatol 2007;157:1013-6.
30Kobayashi K, Rochat A, Barrandon Y. Segregation of keratinocyte colony-forming cells in the bulge of the rat vibrissa. Proc Natl Acad Sci U S A 1993;90:7391-5.
31Morris RJ, Potten CS. Highly persistent label-retaining cells in the hair follicles of mice and their fate following induction of anagen. J Invest Dermatol 1999;112:470-5.
32Wilson C, Cotsarelis G, Wei ZG, Fryer E, Margolis-Fryer J, Ostead M, et al. Cells within the bulge region of mouse hair follicle transiently proliferate during early anagen: Heterogeneity and functional differences of various hair cycles. Differentiation 1994;55:127-36.
33Jiang S, Zhao L, Purandare B, Hantash BM. Differential expression of stem cell markers in human follicular bulge and interfollicular epidermal compartments. Histochem Cell Biol 2010;133:455-65.
34Blanpain C, Lowry WE, Geoghegan A, Polak L, Fuchs E. Self-renewal, multipotency, and the existence of two cell populations within an epithelial stem cell niche. Cell 2004;118:635-48.
35Brent AE, Schweitzer R, Tabin CJ. A somitic compartment of tendon progenitors. Cell 2003;113:235-48.
36Jelinsky SA, Archambault J, Li L, Seeherman H. Tendon-selective genes identified from rat and human musculoskeletal tissues. J Orthop Res 2010;28:289-97.
37Zargham R, Touyz RM, Thibault G. Alpha 8 Integrin overexpression in de-differentiated vascular smooth muscle cells attenuates migratory activity and restores the characteristics of the differentiated phenotype. Atherosclerosis 2007;195:303-12.
38Bolcato-Bellemin AL, Lefebvre O, Arnold C, Sorokin L, Miner JH, Kedinger M, et al. Laminin alpha5 chain is required for intestinal smooth muscle development. Dev Biol 2003;260:376-90.
39Narisawa Y, Hashimoto K, Kohda H. Merkel cells participate in the induction and alignment of epidermal ends of arrector pili muscles of human fetal skin. Br J Dermatol 1996;134:494-8.
40Clifton MM, Mendelson JK, Mendelson B, Montague D, Carter C, Smoller BR, et al. Immunofluorescent microscopic investigation of the distal arrector pili: A demonstration of the spatial relationship between alpha5beta1 integrin and fibronectin. J Am Acad Dermatol 2000;43:19-23.
41Mendelson JK, Smoller BR, Mendelson B, Horn TD. The microanatomy of the distal arrector pili: Possible role for alpha1beta1 and alpha5beta1 integrins in mediating cell-cell adhesion and anchorage to the extracellular matrix. J Cutan Pathol 2000;27:61-6.
42Poblet E, Jiménez F, Ortega F. The contribution of the arrector pili muscle and sebaceous glands to the follicular unit structure. J Am Acad Dermatol 2004;51:217-22.
43Nolano M, Provitera V, Caporaso G, Stancanelli A, Vitale DF, Santoro L. Quantification of pilomotor nerves: A new tool to evaluate autonomic involvement in diabetes. Neurology 2010;75:1089-97.
44Roh C, Tao Q, Lyle S. Dermal papilla-induced hair differentiation of adult epithelial stem cells from human skin. Physiol Genomics 2004;19:207-17.
45Chuong CM. Molecular Basis of Epithelial Appendage Morphogenesis. Georgetown, TX: Landes; 1998. p. 326-36.
46Bickenbach JR, Mackenzie IC. Identification and localization of label-retaining cells in hamster epithelia. J Invest Dermatol 1984;82:618-22.
47Morioka K, Arai M, Ihara S. Steady and temporary expressions of smooth muscle actin in hair, vibrissa, arrector pili muscle, and other hair appendages of developing rats. Acta Histochem Cytochem 2011;44:141-53.
48Sato A, Toyoshima KE, Toki H, Ishibashi N, Asakawa K, Iwadate A, et al. Single follicular unit transplantation reconstructs arrector pili muscle and nerve connections and restores functional hair follicle piloerection. J Dermatol 2012;39:682-7.
49Torkamani N, Rufaut NW, Jones L, Sinclair R. Destruction of the arrector pili muscle and fat infiltration in androgenic alopecia. Br J Dermatol 2014;170:1291-8.
50Yazdabadi A, Whiting D, Rufaut NW, Sinclair R. Arrector pili muscle and alopecia. Int J Trichology 2012;4:154-7.
51Karnik P, Tekeste Z, McCormick TS, Gilliam AC, Price VH, Cooper KD, et al. Hair follicle stem cell-specific PPARgamma deletion causes scarring alopecia. J Invest Dermatol 2009;129:1243-57.
52Panicker SP, Ganguly T, Consolo M, Price V, Mirmirani P, Honda K, et al. Sterol intermediates of cholesterol biosynthesis inhibit hair growth and trigger an innate immune response in cicatricial alopecia. PLoS One 2012;7:e38449.
53Cockerham KP, Hidayat AA, Brown HG, Cockerham GC, Graner SR. Clinicopathologic evaluation of the Mueller muscle in thyroid-associated orbitopathy. Ophthal Plast Reconstr Surg 2002;18:11-7.
54Banker BQ, Engel AG. Myology. New York: McGraw-Hill; 2004. p. 691-747.
55Carpenter S, Karpati G. Pathology of Skeletal Muscle. New York: Oxford; 2001. p. 387-97.
56Goodpaster BH, Wolf D. Skeletal muscle lipid accumulation in obesity, insulin resistance, and type 2 diabetes. Pediatr Diabetes 2004;5:219-26.
57Greco AV, Mingrone G, Giancaterini A, Manco M, Morroni M, Cinti S, et al. Insulin resistance in morbid obesity: Reversal with intramyocellular fat depletion. Diabetes 2002;51:144-51.
58Visser M, Goodpaster BH, Kritchevsky SB, Newman AB, Nevitt M, Rubin SM, et al. Muscle mass, muscle strength, and muscle fat infiltration as predictors of incident mobility limitations in well-functioning older persons. J Gerontol A Biol Sci Med Sci 2005;60:324-33.
59Brack AS, Conboy MJ, Roy S, Lee M, Kuo CJ, Keller C, et al. Increased Wnt signaling during aging alters muscle stem cell fate and increases fibrosis. Science 2007;317:807-10.
60Shefer G, Wleklinski-Lee M, Yablonka-Reuveni Z. Skeletal muscle satellite cells can spontaneously enter an alternative mesenchymal pathway. J Cell Sci 2004;117:5393-404.
61Li Y, Huard J. Differentiation of muscle-derived cells into myofibroblasts in injured skeletal muscle. Am J Pathol 2002;161:895-907.
62Uezumi A, Fukada S, Yamamoto N, Takeda S, Tsuchida K. Mesenchymal progenitors distinct from satellite cells contribute to ectopic fat cell formation in skeletal muscle. Nat Cell Biol 2010;12:143-52.
63Meyer DC, Hoppeler H, von Rechenberg B, Gerber C. A pathomechanical concept explains muscle loss and fatty muscular changes following surgical tendon release. J Orthop Res 2004;22:1004-7.
64Lowell S, Jones P, Le Roux I, Dunne J, Watt FM. Stimulation of human epidermal differentiation by delta-notch signalling at the boundaries of stem-cell clusters. Curr Biol 2000;10:491-500.
65Jensen KB, Watt FM. Single-cell expression profiling of human epidermal stem and transit-amplifying cells: Lrig1 is a regulator of stem cell quiescence. Proc Natl Acad Sci U S A 2006;103:11958-63.
66Pellegrini G, Dellambra E, Golisano O, Martinelli E, Fantozzi I, Bondanza S, et al. p63 identifies keratinocyte stem cells. Proc Natl Acad Sci U S A 2001;98:3156-61.
67Lyle S, Christofidou-Solomidou M, Liu Y, Elder DE, Albelda S, Cotsarelis G. The C8/144B monoclonal antibody recognizes cytokeratin 15 and defines the location of human hair follicle stem cells. J Cell Sci 1998;111:3179-88.
68Liu Y, Lyle S, Yang Z, Cotsarelis G. Keratin 15 promoter targets putative epithelial stem cells in the hair follicle bulge. J Invest Dermatol 2003;121:963-8.
69Orringer JS, Hammerberg C, Lowe L, Kang S, Johnson TM, Hamilton T, et al. The effects of laser-mediated hair removal on immunohistochemical staining properties of hair follicles. J Am Acad Dermatol 2006;55:402-7.
70Nowak JA, Polak L, Pasolli HA, Fuchs E. Hair follicle stem cells are specified and function in early skin morphogenesis. Cell Stem Cell 2008;3:33-43.