Phenotypes and interactions of human melanocytes and keratinocytes in an epidermal reconstruction model

The morphologic and antigenic phenotype of normal human melanocytes and keratinocytes was investigated in monolayer and 3-dimensional cultures in an effort to develop an epidermal model that resembles the normal human epidermis. When cultured for several passages in optimal growth medium, pure cultures of either cell type could be established as demonstrated by light and electron microscopy and with monoclonal antibodies defining melanocyte- and keratinocyte-associated antigens. Three-dimensional growth of keratinocytes on polycarbonate filters was induced by increasing calcium concentrations in the culture medium and exposing cultures to air. After 30 to 35 days incubation, the 3-dimensional keratinocyte cultures reached a total of 12 to 25 layers and keratinocytes of various stages of differentiation formed three morphologically and antigenically different strata. The basal layer of these constructs consisted of ovoid cells with desmosomes and hemidesmosome-like structures. These cells expressed low molecular weight cytokeratins similar to basal cells in situ. The intermediate layer, representing the stratum spinosum in situ, contained flat cells with keratohyaline granules and many desmosomes. These cells expressed gp 80 kilodaltons, gp 40 to 50 kilodaltons, involucrin, and filaggrin. The upper layer, the stratum corneum equivalent, contained large, flattened cells with keratohyaline granules. The majority of these cells were anucleate. When melanocytes were cocultured with keratinocytes in monolayer or in epidermal reconstructs, they assumed a multidendritic morphology and donated pigment to surrounding keratinocytes. The majority of pigmented cells localized singly within the basal layer of the reconstructs and their dendrites were intimately associated with keratinocyte plasma membranes. Pigment donation to keratinocytes appeared to occur through the uptake of melanosome-containing dendrite fragments and phagocytosis of individual melanosomes by keratinocytes. It is hypothesized that keratinocytes produce unique microenvironmental factors that regulate the melanocytic phenotype.