Immunofluorescent microscopic investigation of the distal arrector pili: a demonstration of the spatial relationship between alpha5beta1 integrin and fibronectin

The arrector pili muscle, the bridge between the follicular stem cell niche and the interfollicular epidermis

Proximally, the arrector pili muscle (APM) attaches to the follicular stem cell niche in the bulge, but its distal properties are comparatively unclear. In this work, a novel method employing an F-actin probe, phalloidin, was employed to visualize the APM anatomy. Phalloidin staining of the APM was validated by comparison with conventional antibodies/stains and by generating three-dimensional reconstructions. The proximal attachment of the APM to the bulge in 8 patients with androgenic alopecia was studied using Masson's trichrome stain. Phalloidin visualized extensive branching of the APM. The distal end of the human APM exhibits a unique "C"-shaped structure connecting to the dermal-epidermal junction. The proximal APM attachment was observed to be lost or extremely miniaturized in androgenic alopecia. The unique shape, location, and attachment sites of the APM suggest a significant role for this muscle in maintaining follicular integrity. Proximally, the APM encircles the follicular unit and only attaches to the primary hair follicle in the bulge; this attachment is lost in irreversible hair loss. The APM exhibits an arborized morphology as it ascends toward the epidermis, and anchors to the basement membrane.

Epidermal Cells Expressing Putative Cell Markers in Nonglabrous Skin Existing in Direct Proximity with the Distal End of the Arrector Pili Muscle

Inconsistent with the view that epidermal stem cells reside randomly spread along the basal layer of the epidermal rete ridges, we found that epidermal cells expressing stem cell markers in nonglabrous skin exist in direct connection with the distal end of the arrector pili muscle. The epidermal cells that express stem cell markers consist of a subpopulation of basal keratinocytes located in a niche at the lowermost portion of the rete ridges at the distal arrector pili muscle attachment site. Keratinocytes in the epidermal stem cell niche express K15, MCSP, and α6 integrin. α5 integrin marks the distal end of the APM colocalized with basal keratinocytes expressing stem cell markers located in a well-protected and nourished environment at the lowermost point of the epidermis; these cells are hypothesized to participate directly in epidermal renewal and homeostasis and also indirectly in wound healing through communication with the hair follicle bulge epithelial stem cell population through the APM. Our findings, plus a reevaluation of the literature, support the hierarchical model of interfollicular epidermal stem cell units of Fitzpatrick. This new view provides insights into epidermal control and the possible involvement of epidermal stem cells in nonmelanoma skin carcinogenesis.

Destruction of the arrector pili muscle and fat infiltration in androgenic alopecia

BACKGROUND

Androgenic alopecia (AGA) is the most common hair loss condition in men and women. Hair loss is caused by follicle miniaturization, which is largely irreversible beyond a certain degree of follicular regression. In contrast, hair loss in telogen effluvium (TE) is readily reversible. The arrector pili muscle (APM) connects the follicle to the surrounding skin.

OBJECTIVES

To compare histopathological features of the APM in AGA and TE.

METHODS

Archival blocks of 4-mm scalp punch biopsies from eight patients with AGA and five with TE were obtained. New 4-mm biopsies from five normal cases were used as controls. Serial 7-μm sections were stained with a modified Masson's trichrome stain. 'Reconstruct' software was used to construct and evaluate three-dimensional images of the follicle and APM.

RESULTS

The APM degenerated and was replaced by adipose tissue in all AGA specimens. Remnants of the APM remained attached to the hair follicle. There was no fat in the normal skin specimens. Fat was seen in two of five TE specimens but could be attributed to these patients also showing evidence of AGA. Quantitative analysis showed that muscle volume decreased and fat volume increased significantly (P < 0·05) in AGA compared with controls.

CONCLUSIONS

APM degeneration and replacement with fat in AGA has not previously been described. The underlying mechanism remains to be determined. However, we speculate that this phenomenon might be related to depletion of stem or progenitor cells from the follicle mesenchyme, explaining why AGA is treatment resistant.

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.

Steady and temporary expressions of smooth muscle actin in hair, vibrissa, arrector pili muscle, and other hair appendages of developing rats

The hair erection muscle, arrector pili, is a kind of smooth muscle located in the mammalian dermis. The immunohistochemical study using an antibody against smooth muscle alpha actin (SMA) showed that the arrector pili muscle develops approximately 1–2 weeks after birth in dorsal and ventral skin, but thereafter they degenerate. The arrector pili muscle was not detected in the mystacial pad during any stage of development, even in the neighboring pelage-type hair follicle. A strong signal of SMA in the skin was located in the dermal sheath as well as in some outer root sheath cells in the hair and vibrissal follicles. Positive areas in the dermal and outer root sheaths were restricted to a lower moiety, particularly areas of similar height, where keratinization of the hair shaft occurs. This rule is valid for both pelage hair follicles and vibrissal follicles. At medium heights of the follicle, SMA staining in the dermal sheath was patchy and distant from the boundary between dermis and epidermis. In contrast to SMA, vimentin was expressed over the entire height of the dermal sheath. Unlike the arrector pili muscle, the expression of SMA in the dermal sheath was observed during fetal, neonatal, and adult stages. The presence of actin-myosin and vimentin fibers in supporting cells is thought to be beneficial for the hair follicle to cope with the movement of the hair shaft, which may be caused by physical contacts with outside materials or by the contraction of internal muscles.

A new model for the morphology of the arrector pili muscle in the follicular unit based on three-dimensional reconstruction

In the classic model of the arrector pili (AP) muscle, each hair follicle is independently associated with a single AP muscle. Recently, new models have been proposed based on interpretations of serial histological cross-sectional images. The purpose of this study was to confirm the morphology of the AP muscle in relation to the follicular unit (FU) using quantitative three-dimensional (3D) reconstruction of serially sectioned images. Skin samples were obtained from the parietal region of the scalp, and processed using routine histological procedures. The serially sectioned slides were stained with Masson's trichrome. All the hairs and hair follicles of the FU exhibited some degree of slanting, with the single AP muscle slanted in the same direction. The distal portion of the AP muscle was divided into branches, as in a bronchial tree. There was branching in the single AP muscle present in the FU, and it was inserted into all the hair follicles of the FU either tightly or loosely. We suggest that the classic anatomy of the AP muscle must be modified with this new model, now confirmed by computer-based 3D reconstruction.

The contribution of the arrector pili muscle and sebaceous glands to the follicular unit structure

BACKGROUND

The evidence of hairs grouped into well-defined follicular units has given a new vision of hair anatomy and pathology. The sebaceous glands and the arrector pili muscle, as part of the pilosebaceous unit, should be viewed as important parts of this organized follicular unit structure.

OBJECTIVE

This study was performed to define the morphology and the relationships between the sebaceous glands, muscles, and follicles within the context of the follicular unit structure.

METHODS

This study analyzes horizontal, microscopic serial sections of large areas of normal human scalp skin stained with hematoxylin and eosin, Masson's trichrome, and desmin. The course of the arrector pili muscles from their superficial origins to their follicular attachments was followed in each section, which enabled us to match each muscle with its corresponding follicular unit.

RESULTS

Serial, horizontal sections show that, at the upper isthmus, the arrector pili muscle is arranged as a muscular unit structure at the periphery of each follicular unit. Then, at a lower level, the muscle divides and encircles the sebaceous gland, and penetrates between the sebaceous lobules towards the follicular attachment zone.

CONCLUSION

Microphotographic evaluation of large areas of scalp is important for assessing the anatomical relationships between the sebaceous glands, arrector pili muscles, and hair follicles as components of the follicular unit. We introduce the anatomical concept of a follicular unit served by a muscular unit, which can be identified in horizontal sections made at the upper level of the isthmus. This muscular unit results from the merging of the arrector pili muscles that originate from the hair follicles contained in that particular follicular unit. This anatomical disposition suggests that the arrector pili muscles could play an important role in the integrity of the follicular unit as well as in the secretion of the sebum contents.

The Arrector Pili Muscle and the Follicular Unit of the Scalp: A Microscopic Anatomy Study

BACKGROUND

The arrector pili muscle is a smooth muscle bundle that attaches to the bulge region of the follicle and extends to its superior attachment in the upper dermis. Hair follicles in the scalp are grouped, forming well-defined anatomic structures known as follicular units, which are composed of terminal and vellus hairs, the associated sebaceous glands, and the arrector pili muscles.

OBJECTIVE

To describe the microscopic anatomy of the arrector pili muscles in relation to the follicular units, using horizontal sections of scalp skin.

METHODS

Normal scalp skin was obtained from 20 adult cadavers. Tissue specimens were horizontally sectioned at all levels of depth, including the infundibulum, isthmus, and inferior segment. Sections were stained with hematoxylin and eosin, and Masson's trichrome.

RESULTS

The distribution of hair follicles in follicular units was only present above the insertions of the arrector pili muscles, at the infundibulum and isthmus. Masson's trichrome proved to be a very useful stain to delineate the follicular units. At the isthmus level, bulky cords of arrector pili muscle were located specifically at one pole of the follicular unit. These bulky cords appeared to be located at the same pole in different follicular units.

CONCLUSION

At the isthmus level, above their follicular attachments, the arrector pili muscles that originate from their respective follicles join together, forming a single muscular structure that extends upward to its superior attachment zone. An anatomic model is proposed in which a single arrector pili muscular structure is shared by all the follicles contained within the follicular unit.

Arrector pili muscle: evidence of proximal attachment variant in terminal follicles of the scalp

BACKGROUND

There is general agreement that the proximal attachment site of the arrector pili (AP) muscle, via elastic tendons, is the bulge region, where follicular stem cells are situated. A variant of this insertion, in which the AP muscle surrounds the follicle circumference, has been described in human facial vellus hair follicles.

OBJECTIVES

To investigate whether terminal hair follicles of the human scalp may also present this variant of proximal insertion of the AP muscle.

METHODS

Histological and immunohistochemical methods were used.

RESULTS

At the bulge level, bundles of the AP muscle were seen to encircle the entire hair follicle.

CONCLUSIONS

Terminal follicles of the human scalp may also present this variant of proximal insertion of the AP muscle.