The interpretation of scalp biopsies from patients with hair loss is challenging. The pathologist depends on an adequate biopsy with sufficient hair follicles for quantification as well as on accurate clinical information. The site of the biopsy, pattern and duration of the hair loss, and location of the specimen relative to the hair loss are all important, and clinicopathologic correlation is essential in arriving at the correct diagnosis. The most common causes of acquired alopecia are listed in Table 10-1.
TABLE 10-1Common Causes of Acquired Alopecia |Favorite Table|Download (.pdf) TABLE 10-1 Common Causes of Acquired Alopecia
|Androgenetic alopecia (male and female pattern hair loss) |
|Alopecia areata |
|Telogen effluvium |
|Traction alopecia |
| Discoid lupus erythematosus |
| Lichen planopilaris/frontal fibrosing alopecia |
| Central centrifugal cicatricial alopecia |
| Folliculitis decalvans |
| Dissecting cellulitis of the scalp |
Longitudinal follicular anatomy
A basic understanding of the hair follicle and the hair cycle is needed to interpret scalp biopsies. In longitudinal anatomy, the human hair follicle consists of a permanent upper segment, comprised of the infundibulum and isthmus, and an impermanent lower segment, comprised of the lower follicle and hair bulb. The infundibulum is lined with keratinized epithelium that extends from the skin surface to the upper hair follicle, ending where the sebaceous duct enters the follicle (Fig. 10-1). The isthmus is a short segment extending from the insertion of the sebaceous duct to the insertion of the arrector pili muscle into the follicle. It is lined by the external root sheath (trichilemma, Fig. 10-2). The lower segment depends on which part of the cycle the hair follicle is in. In anagen, it surrounds a growing hair shaft with internal and external root sheaths, hyaline membrane, and fibrous sheath (Fig. 10-3). The bulb consists of the dermal papilla surrounded by hair matrix cells (Fig. 10-4). It generates the growing hair shaft and the surrounding inner and outer root sheaths. The dermal papilla is continuous with a fibrous sheath that surrounds the entire hair follicle.
Infundibulum of an anagen hair follicle in vertical section. Note the entry of the sebaceous duct. The follicular epithelium merges with the epidermis. (H&E, 10x)
Isthmus of an anagen hair follicle in vertical section. The segment of the hair follicle from the insertion of the sebaceous duct (above left) to the insertion of the erector pili muscle (lower right). (H&E, 4x)
The lower anagen hair follicle in vertical section just above the bulb. The blue hair matrix below becomes a nucleated hair shaft and the ...