S1C18cP8c-3: The cells of the follicular epithelium on the left show the features related to terminal differentiation; there is a flattened basal layer, but no basal unit. Blue arrows point to bright red elastic fibers in the exudate in the lumen of the follicle (a marker for a “perforating” folliculitis. Perforation is a common feature of acneiform folliculitis. The exudate consists of neutrophils and cellular debris. For the elastica to have found its way into the lumen of the follicle, the epithelium must have extended beyond the follicular sheath into the reticular dermis (“transepidermal elimination” is the result of an encounter between passive connective tissue fibers and invasive squamous epithelium). Perforating elastic fibers are more brightly acidophilic (fibrin-like) than are the undisturbed elastic fibers of the adjacent dermis (dark blue arrows).

S1C18cP8c-4: A protruding nest of squamous cells is irregular in outline. The nest, at the advancing margin, is composed of hyperplastic cells of the basal unit of squamous epithelium. In the upper right hand corner, the squamous cells are individually enlarged and pale, but more acidophilic. These enlarged, acidophilic cells show features related to the process of terminal differentiation (cells committed to keratinization of terminal differentiation type). The red membrane at the interface between the epithelium and the stroma is fibrin. The stellate cells of the granulation tissue are fibroblasts. In this manner, epithelium intrudes into the dermis to entrap connective tissue fibers. In a sequence, there is, first, epithelial hyperplasia; entrappment of connective tissue fibers follows; and, finally, entrapped fibers move upward to be extruded along the epithelial surface.