Histopathologic evaluation of alopecias

The alopecias can be broadly classified into nonscarring and scarring forms. The latter are divided into primary and secondary scarring types. In primary scarring alopecias, the hair follicle is the prime target of destruction as opposed to secondary scarring alopecias in which it is involved in a neighboring nonfollicular process that impinges upon the follicle and ultimately destroys it. After an initial overview and a critique on the concept of scarring versus nonscarring, we outline in detail the histopathologic presentation of the major forms of alopecias. Because the correct histopathologic diagnosis often requires close clinicopathologic correlation, a brief description of the clinical presentation is included. This review also discusses important pathogenetic concepts, such as stem cell failure.