Ch21 Basement Membrane
& Membranous Loop

Buster (the plight of feral cats)


F1Ch2a (116089): Features of a fibrous papule include epidermal changes, fibrosis of the adventitial domain of the dermis (including the domain normally characterized as papillary dermis), the peri-follicular adventitial domain, and the adventitial domain of blood vessels (blue arrow). The fibrosing process tends to extend from the altered adventitial domains into the reticular dermis, with changes progressing from superficial locations to deeper locations. The extent of involvement of the reticular dermis is variable from case to case. The changes at the dermal-epidermal interface are variable.


F2Ch2a (116089): For some examples, the epidermal changes are subtle and commonly have been ignored. In this field, the epidermis shows hypergranulosis, hyperplasia of the superficial unit, and variable representation of a basal unit (representation of basal unit most evident in the rete ridges. There is hyperplasia of basal melanocytes in the epidermis; most of the basal “clear’ cells are interpreted as melanocytes. Some of the small “clear’ cells above the basal layer are interpreted as dendritic histiocytes. Many of the keratinocytes of the basal unit show peri-nuclear vacuoles. The basement membrane is irregularly coarsened and brightly acidophilic. Basement membrane-like material is stacked in lamellar patterns in the adjacent fibrous adventitial domain (yellow arrows). The blue arrow on the left that appears to be dropping into the dermis; this might be a melanocyte caught in the act of “dropping off.” The vacuoles at the bottom of the field on the right, are an artefact.


F3Ch2a (116089): To the left, the basement membrane is coarsened and hyalinized; it also is duplicated. In this same area, some of the mononuclear cells adjacent to the basement membrane (but on the epidermal side) are interpreted as histiocytes. To the right of the center of the field, a short dermal papilla is represented. Along the surface of this papilla, the most superficial, hyalinized lamella of the reduplicated basement membrane is separated from the basal layer of the epidermis by a relatively clear space; delicate, wavy fibers and tapered cell processes extend into this space. Basal keratinocytes above the space show a zone of basal cytoplasmic acidophilia. This acidophilia might be taken as evidence that these basal keratinocytes are in the act of producing new basement membrane material. Two dendritic cells are present among the most superficial lamina of the reduplicated basement membrane. These relationships suggest that the separation between the epidermis and the basement membrane has been mediated at a cellular level. There is an alternate choice; the two cells may, in concert with the basal keratinocytes, be engaged in the formation of a new lamina. Could the two cells be melanocytes rather than histiocytes; the answer awaits an investigator with access to a proper laboratory. Note that there is evidence of dyskeratosis (increased cytoplasmic acidophilia) in many of the cells of the basal unit of the epidermis (Note: the interface between the basal unit and the superficial unit is that level where the orientation of cells, with reference to the long axis of the cells, changes from vertical to horizontal; cells of the basal unit have their long axis perpendicular to the surface of the sking, and those of the superficial unit are oriented horizontally.


F4Ch2a (116089): There is no evidence of a normal papillary dermis. Coarse collagen bundles, resembling those of the reticular dermis extend close to the dermal- epidermal interface. The coarse bundles show hyper-eosinophilia. The pattern, in this field, is not that of the normal reticular dermis; there is a background meshwork of smaller fibers. There is an increased number of cells per unit area, and the spindle cells among the collagen bundles are individually hypertrophied. A basement membrane is not well-defined; there is a finely vacuolated zone of pallor in the basement membrane domain. To the left, an ill-defined basement membrane is represented. Many of the basal keratinocytes show a basal zone of cytoplasmic acidophilia; this may represent basal keratinocytes that are in the process of producing basement membrane material. To the left, the dermis bulges upward in the pattern of a poorly developed dermal papilla. To the far left, in this same area, there is an ill-defined duplication of basement membrane material. A small histiocyte extends into this area to the dermal-epidermal interface. Scattered keratinocytes of the basal unit of the epidermis show zones of cytoplasmic acidophilia compatible with dyskeratosis.


F5Ch2a (116089): Dyskeratosis of scattered keratinocyte of the basal unit is a feature. Some of the keratinocytes show peri-nuclear vacuoles. There is hypergranulosis. The patterns in the epidermis are those associated with damage to the basal unit and the basal layer of the epidermis. In this approach, the changes take the qualities of a cell-poor lichenoid reaction (see Section on lichen planus-like reactions (see navigation aid in Masterborder to the left: Inflammation). Some of the small cells in the epidermis above the basal layer are interpreted as dendritic histiocytes. In the area between the two blue arrows to the left, the basement membrane is irregularly coarsened; there are liquefactive changes. Small clefts just beneath the partially reduplicated basement membrane contain dendritic cells. Between the two blue arrows to the right, some of the basal keratinocytes show basal cytoplasmic acidophilia


F6Ch2a (116089): In the area that is outlined by red arrows, two cells are present in the space that is formed by a “loop” of basement membrane material; each cell is distinctive. The cells are in close apposition; the deeper one with the elongated nucleus has a dendrite that extends through the basement membrane into a widened intercellular space between two basal cells. The other cell within the loop could represent a histocyte. This uncommon pattern and association is not uncommon in the setting of fibrous papule (melanocytic angiofibroma). The basement membrane is not well-defined over the arms of the loop; the possibility that the dendritic is a melanocyte that is in the act of “dropping” into the dermis should be considered. The intimacy between the two cells suggests that the two are interacting. In this general area, basement membrane material is reduplicated. It is tempting to propose that the two cells, in their embrace, effect changes in the basement area and influence the fibrous character of the adventitial dermal domain. The designation, papillary dermis, has been avoided. It appears that a basic defect of fibrous papule is an inability to induce a proper stroma (i.e., adventitial dermis, including papillary dermis, peri- follicular sheath, and vascular adventitia). The spindle cell in the membranous loop extends a dendrite into the epidermis to the level of the junction between the basal and the superficial units.


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