At a late stage of alopecia in lupus erythematosus, the infiltrate of lymphocytes may be sparse (Figs. 1A–C) or absent (Figs. 2A–C). The interfollicular reticular dermis is largely normal in regard to size and arrangement of collagen bundles and number of fibroblasts, but some mucin may still be present in the reticular dermis (Fig. 1D). The fibrous tracts are devoid of follicles, but show slightly thickened eosinophilic collagen bundles (Figs. 1B and 2B). The papillary dermis between follicles is markedly thinned due to flattening of the rete ridges (Figs. 1C and 2C). Subtle vacuolar changes may still be encountered in the basal layer of the interfollicular epidermis (Fig. 1C), or the basement membrane may be thickened (Fig. 2C).

Just as in late lesions of alopecia in lupus erythematosus, the infiltrate of lymphocytes in late lesions of lichen planopilaris is sparse (Figs. 3A–D) or absent (Figs. 4A–C). Another feature in common between late lesions of alopecia in lupus erythematosus and late lesions of lichen planopilaris, is the normal thickness and arrangement of collagen bundles in the reticular dermis; but in contrast to late lesions of alopecia in lupus erythematosus, no deposits of mucin are usually encountered in the dermis of lesions of lichen planopilaris (Figs. 3D and 4D). The collagen in fibrous tracts of late lichen planopilaris may be fine (Fig. 3B) or slightly thickened (Fig. 4B), similar to that seen in late lesions of alopecia in lupus erythematosus. Contrary to the situation in lupus erythematosus, the interfollicular papillary dermis is preserved in lichen planus and the rete ridges, as well as the basement membrane, are normal (Figs. 3C and 4C).

In late lesions of folliculitis decalvans (Figs. 5A–C), the collagen in the entire reticular dermis is altered markedly in the manner of a scar. Thickened collagen bundles are oriented horizontally and fibroblasts are present between collagen bundles (Fig. 5C). A sparse infiltrate in the dermis mostly consists of plasma cells. A papillary dermis is not discernible, and the rete ridges are flattened, but vacuolar alteration is absent and the basement membrane is normal in thickness. Below the zone of fibrosis in the reticular dermis, the subcutis is largely normal. Fibrous tracts are similar to those seen in late lesions of alopecia in lupus erythematosus or lichen planopilaris.

In longstanding lesions of alopecia areata (Figs. 6A–B), epidermis, dermis, and subcutis are largely normal. Virtually no infiltrate of inflammatory cells is present. The papillary dermis is preserved and collagen bundles in the reticular dermis are normal in thickness and arrangement. Fibrous tracts are typified by thickened blue-gray bundles of collagen which represent the glassy membrane residual of late catagen of involuting follicles in alopecia areata (Fig. 6B).