L-11   ANDROGENETIC ALOPECIA IN BIOPSIES

DA Whiting Baylor Hair Research and Treatment Center, Dallas, Texas, USA.

The histology of common baldness is the same in both sexes. It results from miniaturization of genetically predisposed hairs within the pattern of male or female baldness. Successive, abrupt miniaturization of different groups of hairs scattered symmetrically over the crown can account for the stepwise progression of the balding process. Follicular counts in horizontal sections of scalp biopsies from these patients confirm that increasing proportions of vellus hairs in the papillary dermis correlate with increasing baldness. Since hair size is determined by the relative distribution of a fixed number of mesenchymal cells in the dermal papilla and dermal sheath, the total follicular count in a representative biopsy should reflect the potential for possible future regrowth. All hair shafts thinner than their investing inner root sheath, with a diameter of 3-4 microns or less, are classified as vellus, whether primary or secondary to miniaturization from any cause. Secondary vellus hairs show streamers (stelae), which trail down to their site of origin in reticular dermis or subcutaneous tissue. Usually, their outer root sheath is thicker than that of a primary vellus hair, indicating their origin from a terminal hair. In diagnosing the cause of secondary vellus hairs, predominant upper follicle inflammation favors androgenetic alopecia and predominant lower follicle inflammation favors alopecia areata; without inflammation, higher telogen and vellus counts favor alopecia areata, depending on the stage of the process. A mild lymphohistiocytic infiltrate can be found around upper follicles in about 1/3 of normal controls and in androgenetic alopecia, chronic telogen effluvium, and alopecia areata. A moderate infiltrate around upper follicle affects at least another 1/3 of cases of androgenetic alopecia but only 10% of the other main types of noncicatricial alopecia. The notion that perifollicular inflammation increases hair loss in androgenetic alopecia needs further investigation. Scalp biopsies are usually not needed to diagnose common baldness in males, unless they have hair loss in a female pattern. Scalp biopsies may be needed in females to distinguish pattern alopecia from diffuse shedding such as chronic telogen effluvium or diffuse alopecia areata. Scalp biopsies can have prognostic significance in patients with severe alopecia and permanent follicular dropout to indicate the need for surgical hair replacement or a hairpiece. Follicular counts in horizontal sections of serial scalp biopsies can measure hair growth or loss in investigational studies.