Neuromesenchyme. The concept of a neurocristic effector cell for dermal mesenchyme.

The elusive dermal neurocristic effector cell is identified by its expression of fibrogenic functions in a variety of cutaneous neurocristic dysplasias and neoplasms. It is a normal resident of the dermis that disguises its embryonic heritage by the acquisition of fibrocytic (or fibrohistiocytic) functions. It shares with other cutaneous neurocristic derivatives the capacity to express variably three basic functions: 1) fibrogenesis, 2) melanogenesis, or 3) neurosustentation in the manner of the supportive cells of peripheral nerves. Its role in developing skin is prominently displayed in congenital nevi. The fibrogenic potentials of its embryonic relatives, the neurosustentacular cells and melanocytes, are expressed in perineurial fibromas and desmoplastic malignant melanomas. In the latter neoplasms, neurosustentacular functions are also often displayed. In the normal skin, the potentials of the cutaneous neurocristic migrants are usually restricted to one of three options. In dysplasias, the controls are derepressed and migrations may offer new, environmental influences that favor expression of latent properties. A dysplastic melanocyte loses the primary epigenic influence of epithelium by migration into the dermis. In the dermis, it encounters peripheral nerves and mesenchyme. In response, it may express its latent fibrogenic or neurosustentacular possibilities. The dermis is neuromesenchyme. The adventitial dermis is a special adaptation of mesenchyme to the metabolic needs of epithelium. Melanocytes and Merkel cells are situated ideally to function as mediators between epithelium and dermis. Pigmented melanocytes that concentrate in the bulbs of growing hairs probably are more important as mediators of epithelial-mesenchymal reactions than as sources of pigment. The reticular dermis and retinacula represent transformation from fetal type III collagen to adult type I collagen. In Mongolian spots, melanocytes are confined to the reticular dermis. They identify neurocristic effector cells that have been diverted from fibrogenic to melanogenic functions. In all likelihood, the transformation in the dermis from type III to type I collagen is induced by neurocristic migrants that have lost their identity in the population of fibrocytic cells.