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.