HYPERPLASIA (G. huper "over, above," G. plasso "to mold," G. -ia "forming")

An increase in the absolute number of nonneoplastic cells in a tissue (Fig. 3.10). Inherent in this term is the concept, derived from classic pathology, that hyperplasia ceases when the stimulus responsible for it ceases, even though the tissue may not revert completely to normal. Epithelial hyperplasia in skin can be divided roughly into epidermal and adnexal types (Fig. 3.11). Epidermal hyperplasia may be psorias~form, i.e., with elongated rete ridges of about equal length that alternate with derrnal papillae of about equal length, as in psoriasis, waeven psorias~forl7b, i.e., with elongated rete ridges that are not of uniform length but that alternate with dermal papillae to maintain an undulate pattern, as in nummular dermatitis, jagged, i.e., with a serrated base of a thickened epidermis, as in lichen planus, and mammillated, papillated, or digitated, i.e., with the surface of the epidermis resembling breasts, nipples, or fmgers, respectively. "Verrucous" is a synonym for ('digitate." Hyperplasia of adnexal epithelial structures, i.e., infundibula and the upper part of eccrine ducts, may be slight or marked, with the latter sometimes assuming the form of pseudocarciwomatous hypenplasia, as seen in association with halogenodermas, infections by deep fungi and atypical mycobacteria, and the verrucous stage of incontinentia pigmenti.

Melanocylic hyperplasia refers to an increase in the absolute number of normal-appearing melanocytes per unit area of basal layer of epidermis. An example is the temporary increase in number of melanocytes in this layer after sunburn. An acquired junctional melanocytic nevus and a melanoma in situ, however, are not hyperplasias, but neoplasms; the processes do not ordinarily cease and reverse themselves but, in the case of a nevus, persist and cease growing and, in the case of melanoma, progress relentlessly.

In the realm of morphologic observations, it is preferable to speak and write of "proliferation" of cells rather than of "hyperplasia" or "neoplasia" because correct application of the terms "hyperplasia" and "neoplasia" turns on first having made an accurate diagnosis. For example, if the diagnosis is verruca vulgaris, then the process is a hyperplasia, but if the diagnosis is verrucous carcinoma, then the process is a neoplasm. For that reason, the terIIL "proliferation," devoid as it is of implications of specific diagnosis or of cause, is more accurate.