Ch40dP32d Tricholemmoma
(mixed patterns)

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F1Ch40d (Q5036): The lesion is polypoid and mostly sits above the surface of the adjacent skin. There is papillomatosis. The section is somewhat off the vertical axis of the tumor. In the upper 1/3 of the field, acidophilic squamous cells form columns. There are numerous squamous eddies (whorls). Along the columns in the upper 1/3 of the field the basal layer is accentuated. This finding might be taken as evidence that this portion of the lesion is representative of a population that shares attributes with the inconstant portion of a hair follicle; in addition, the rather pure squamous cell character of this portion might be cited as evidence that the cells are neoplastically committed to the phenomena of the catagen phase.


F2Ch40d: Squamous eddies are numerous; the squamous cells among the squamous eddies show some degree of cytoplasmic pallor. In the deeper portion (i.e., the middle level of the  lesion) the columns are composed of cells with basaloid qualities, and with pale or clear cytoplasm. If emphasis is simply placed on common criteria, the lesion might be dismissed a compound hamartoma with a component of inverted follicular keratosis, and a second component of tricholemmoma. Blue arrows identify an area of sclerosing entrapment; the entrapped cords and nests of this area are composed of cells with brightly acidophilic cytoplasm, a feature of advanced keratinization.


F3Ch40d: An interface between the squamous cell component and the clear cell component is represented in the middle of the field. Much of the epithelial component among the whorls are cells with the characteristics of cells of the basal unit of the epidermis. It is tempting to also characterize the clear cell component as being representative of a population with germinative qualities and being more akin to cells of a peculiar basal unit - they are not cells committed to terminal differentiation. As such, they might be ideally suited to serve as a the source of a special mucinous interstitium. What would be the advantage of such a matrix? It might serve a lubricated interface between the inner and outer root sheaths; as such, the upward advance of the inner root sheath would be independent of the more stationary outer root sheath.


F4Ch40d: Components, each consisting of distinctive cells (one component having qualities of the basal unit and the other having qualities of the superficial unit of the epidermis), outline a fibrous stroma. In this field, the cells of the basal unit-like population are basaloid but do not have the fully developed qualities of clear cells. On the other hand, to the left, some of the cells with superficial unit-like qualities have vacuolated cytoplasm. As discussed for other examples, this alteration is common; it seems to anticipate catagen-like phenomena with cytolysis and dyskeratosis. In the process, lytic defects will develop. If, in the process of cytolysis, the basal unit-like component is involved, interruptions in the basal layer may be produced. In this event, the cleft come to abut upon mesenchyme. The response is an ingrowth of connective tissue as an inlay for the empty defect. The nipple-like protrusion, from the population of cells on the left, seems to be an extension from a basal unit-like component but the cells show dyskeratosis and loss of polarity; there is no basal layer. We should accept the islands and cords of acidophilic cells in the fibrotic stroma as marker for an epithelial domain, that, in the process of cytolysis, has lost part of its domain to stroma. The surviving cells are also degenerating, hence the intense cytoplasmic acidophilia. The lesion could be characterize as being, in part, in the grip of catagen phenomena - some of these follicular hamartomas of the butterfly area of the face may in foci, and in distorted patterns, recapitulate the phenomena of a catagen phase.


F5Ch40d: In this field, clear cell properties are well developed. The cells have thin, but distinct, cell walls. Nuclei tend to displaced eccentrically. Nuclei are enlarged, and nucleoli are variable. There is one questionable mitotic figure. Although the pattern is rather bland, there is cytologic atypia.


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