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cat3Pa1: the basic pattern of a lentiginous process is represented along, and at the extremity of this rete ridge. At the extremity, at least 3 dendritic melanocytes are spaced in a pattern that qualifies as lentiginous hyperplasia. The cells and their dendrites are pigmented (Fontana-Masson stain). The middle cell has migrated upward a short distance. The pale cell embraced on the deep side by two dendrites of this middle cell is interpreted as a dendritic epidermal histiocyte (Langerhans cell?). On the right side of this rete ridge, near the base of the ridge, an “epidermal clear cell” is represented at the basement membrane level; this cell also may be a dendritic histiocyte. There are too many dendritic histiocytes per unit area; this close spacing qualifies the lesion as having lentiginous qualities.
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cat3Pa2: rete ridges are elongated. Along the sides of the ridges, atypical melanocytes are arranged in lentiginous patterns. The atypical cells are not confined to the basement membrane zone; they have migrated into the epidermis; each migrating cell appears to lie within a lacuna. There is some degree of nuclear atypism. Near the extremities of the ridges, there are rounded or irregular clusters of atypical cells in “junctional” patterns. The central nest is irregular in outline; the pattern might be better characterized as cavitation of the epidermis by the population of atypical cells. the stroma of the papillary dermis is activated. Infiltrates of lymphocytes are mild.
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cat3Pa3: the nests outlined by red arrows is somewhat rounded and might be easily accepted as a nest of tumor cells that established a domain at the interface between the epidermis and stroma. The nests outlined by blue arrows is elongated; its interface with epithelium is irregular. Again, the cluster appears to represent a displacement, or lysis, of keratinocytes, and the formation of a cavity in which neoplastic cells have aggregated.
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cat3Pa4: on the right side of this ridge, the migrating cells have irregularly cavitated the epidermal domain. I assume that keratinocytes that become detached from their neighbors will eventually undergo lysis.
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cat3Pa5: at the extremity of this ridge, the proliferating neoplastic melanocytes have produced a cluster that conforms to the outlines of the respective ridge. The neoplastic cells are loosely attached to their neighbors. They have dendritic processes. Focally, the basal unit of the epidermis is interrupted, and the cavity abuts upon an hyperplastic superficial unit of the epidermis.
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