Common Final Pathway
In this field, the characteristic changes of the “common final pathway” are manifested in the epidermis (and in the sparse representation of nests of atypical cells). For the dermal component, the representation of loosely and widely spaced nests of atypical cells, as well as the confinement of those nests to an inflamed, widened papillary dermis are requisites in the definition of the common final pathway - a distinction seen in well advanced melanocytic premalignant dysplasias. The epidermal response is lichen planus-like. In addition, the epidermis shows lentiginous and junctional, marked melanocytic dysplasia. Individual neoplastic cells and small nests of neoplastic cells have migrated into the epidermis (i.e., pagetoid spread. The cells in the junctional components show loss of cohesion. The stroma of the papillary dermis is loosely fibrous and delicate. To the left of the center of the field, a small nest of neoplastic cells is represented in the dermis (level II invasion; level II pattern. The spaces among neoplastic cells in the epidermal as well as the dermal nests can be characterized as an expanded stroma (intercellular matrix). Certainly, the nests are “large.” There are lytic defects at the dermal-epidermal interface with loss of a defined basal layer; the superficial unit of the epidermis is hyperplastic (a defensive alteration associated with a lytic process affecting the basal unit - a lichen planus-like alteration). The fibrous stroma about the junctional and dermal nests is loose and delicate; it is an activated tumor stroma. To the left at the interface between the papillary dermis and the reticular dermis, there is a band of condensed firbous tissue in which the fibers and the fibroctytes are parallel to the epidermis; a band such as this is common at the deep margin of level III lesions in association with a vertical growth component; the patterns in this field do not qualify as vertical growth.