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

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'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.

INTRODUCTION

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.[2345] 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.

THE HAIR FOLLICLE

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.[910] Bulge cells have an undifferentiated structure,[11] are highly proliferative and multipotent.[1213141516] 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.[192021] 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]

ARRECTOR PILI MUSCLE ANATOMY

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]

PROXIMAL ATTACHMENT OF THE ARRECTOR PILI MUSCLE

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.[30313233]

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.[3738]

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.

DISTAL ATTACHMENT OF THE ARRECTOR PILI MUSCLE

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]

KNOWN ARRECTOR PILI MUSCLE FUNCTIONS

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)[4243] 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.[2842] 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.[1144] 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.[4446] 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.

HYPOTHESIZED ARRECTOR PILI MUSCLE FUNCTIONS

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.[848]

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.[4950] 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.[4950] 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 1a 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 1c 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 1e]. 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 1f and g].

Figure 1

Progressive miniaturization within follicular units. (a and b) Arrector pili muscle (APM) loses attachment to secondary follicles in herald units but remains attached to the primary follicles. (c and d) Miniaturization of secondary follicles and detachment of the APM from these follicles is then extended to the remaining follicular units. (e) Primary follicles in herald units are affected by miniaturization, and eventually muscle attachment is totally lost. (f and g) Baldness occurs when the entire follicular unit is miniaturized. The same pattern of miniaturization and muscle loss continues until all follicular units are affected

POSSIBLE MECHANISMS OF ARRECTOR PILI MUSCLE DEGENERATION IN HAIR LOSS

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.[5152] However, there is little inflammation associated with the fat infiltration in AGA.

Figure 2

Fat infiltration and arrector pili muscle (APM) muscle degeneration in androgenic alopecia (AGA). (a) Three-dimensional reconstruction of follicular unit in AGA. Green: Fat, red: APM and blue: Hair follicle. (b) Transverse section of scalp biopsy in AGA, yellow arrow: APM, green arrow: Fat (Mason's trichrome stain)

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.[55565758]

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.[596061] 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.[646566] Follicle cycling is associated with the movement of cells between the DP and dermal sheath.[6768] It is thought that this process is disrupted in AGA to cause the loss of cells from the DP and consequent follicle miniaturization.[296970] Cells from the DP and dermal sheath are capable of undergoing both smooth muscle[2933] and adipose[322] 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.

CONCLUSION

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.