Discussion (part 2)
chapter 4, tier 2, textual level
The phenotypic potentials of embryonic mesenchyme seem to be retained in the adventitial dermis; it finds expression in its neoplasms. This conceptualization provides insight into the relationships between some myxomas of the skin, and related epithelial dysplasias (ddd). In some myxoid neoplasms of the adventitial dermis, the primitive myxoid matrix, along with its adventitial fibroblasts, retains the capacity to induce, if not to orderly control, the proliferation of epithelium. The resulting patterns grotesquely recapitulate the formation of the skin appendages during embryogenesis. In this manner, the follicular distortions, embodied in designations such as “trichogenic adnexal myxomas,” find an accommodation.
In this conceptualization of fibroblasts and fibroblastomas, the fibroblast, not the character of the stroma, is afforded primacy. Fibroblasts are functionally diverse with the capacity to prescribe the nature of its extracellular product, which may be either mucinous or fibrous. For some examples, the extracellular product is scanty, and the cells are closely clustered; such a lesion is a cellular variant. The designation, myxoma, is appropriate for those lesions in which the mucoid matrix is abundant, and the cells are widely, but fairly regularly, spaced. On the other hand, this designation improperly emphasizes the character of the cellular product, rather than identifying the source of the product. The designation, fibroblastoma, if not modified to identify other qualities, is appropriate for lesions in which cells are widely spaced in a matrix which is uniformly fibrous; such lesions are fibrous variants. For lesions that are myxoid, or intermediate in character, the designation, myxofibroblastoma, gives recognition to diverse functions of fibroblasts.
Conceptually, fibroblasts of the adventitial dermis may be functionally independent of the population of fibroblasts of the reticular dermis. Utilizing these conceptualizations, one example of a fibrous lesion of the dermis, the so-called fibrous papule or angiofibroma, is an adventitial variant of fibroblastoma. Giant cell fibroblastoma is a polymorphic fibroblastoma, but is a lesion of the reticular dermis. Fascia and retinacula form the boundaries of anatomic compartments; they are of the same order as the reticular dermis. If its taxonomy were not so deeply ingrained, so-called malignant fibrous histiocytoma might be more appropriately characterized as “polymorphic malignant fibroblastoma.” Whether the apparent duality of fibroblastic types in the dermis is genetically determined, or merely a phenotypic expression by dermal firbroblasts in response to the influence of neighboring populations of cells, such as melanocytes and keratinocytes, is a mystery.
Ultrastructurally, the cells of adventitial cellular myxofibroblastomas have fibroblastic qualities. The ultrastructural identification of the fibroblastic nature of the tumor cells does not, in turn, determine the nature of the extracellular product. It may be either densely fibrous (type I collagen), or loose and myxoid (type III collagen or reticulum). In adventitial cellular myxofibroblastomas, the fibrogenic nature of the tumor cells is manifested in complex patterns of reticular fibers (type III collagen?). The matrical expressions of adventitial cellular myxofibroblastoma include mixtures of connective tissue mucins, reticulum, and collagen; the resulting patterns are cellular (scanty matrix), myxomatous, or fibrous.
Conceptually, the stroma of adnexal basal cell neoplasms, including trichoepitheliomas, and well differentiated basal cell carcinomas, is myxofibroblastic. The stroma cells are adventitia- like fibroblasts or myxofibroblasts. In these basal cell neoplasms, the epithelial components seemingly have the capacity to activate stromal fibroblasts and, in turn, induce a primitive stroma. The relationships between epithelium and stroma in these examples are interdependent. Either the adventitial fibroblast, or the epithelial cell may activate its cellular counterpart.
The inter-relationships of histiocytes and fibroblasts find expression in a variety of soft tissue and cutaneous lesions and tumors. It finds expression in the designation, fibrous histiocytoma. More recently, in a plastic manner, which lends itself to defensive taxonomic ploys, the interrelationships have found expression in the concept of “dermal dendrocyte,” and dermal dendrocytoma. They are expressed in lesions such as nodular fasciitis, but the histiocytic contribution is not given recognition in this designation. They have been responsible for conceptualizations of the histogenesis of tumors, and of the fibroblastic potentials of histiocytes.