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.