S7C13P10-1: The interstitial infiltrates of the reticular dermis in this lesion of “hydroa aestivale” are composed of neutrophils and histiocytes. There are fragments of nuclear debris (leukocytoclasia) (same lesion as illustrated in S7C12P9-4 & 5). Perivascular infiltrates are mixed. There is an occasional plasma cell.

S7C13P10-3:  The moth-eaten appearance of the epidermis is evidence of lysis of individual keratinocytes. The vesicle represents a site in which the lytic changes have become confluent. Degenerating keratinocytes form epithelial bridges which irregularly partition the vesicle (a reticulated vesicle). The basal unit of the epidermis is hyperplastic.

S7C13P10-4: The vesicle is reticulated; the pattern in the neighboring epidermis is spongioform. Neutrophils are present in the vesicle, but occasional eosinophils also are represented at other levels.

S7C13P10-5: Blue arrows identify epithelial bridges forming the walls of the epithelial defects. The cells forming the bridges show degenerative changes with increased cytoplasmic acidophilia ( vesiculation in reticulated patterns).

S7C13P10-6: The keratinocytes in the spongioform regions show vacuolization of cytoplasm (cytopathic changes). To the right, epithelial bridges, which are remnants of the keratinized walls of degenerating keratinocytes, irregularly partition the vesicle. Some of the keratinocytes adjacent to the vesicle contain melanin granules in their cytoplasm. In this case, immunohistochemical reactions for the demonstration of herpes simplex virus (type I & II) were positive in some nuclei of keratinocytes in the affected areas. The clinical course and the clinical features were interpreted as consistent with hydroa aestivale.The basic character of the reaction pattern, with the exception of the neutrophic component, is erythema multiforme-like.