Ch6T5 Tricholemmoma
(blastematous component)

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Chapter 6

Neoplasm of the Inconstant Outer Root Sheath

(Tricholemmoma and its Blastomatous [Germinative] Precusor as manifested in a “Hamartoma”):

Tricholemmoma is a common benign neoplasm of the skin, relatively restricted in a peculiar manner to the butterfly area of the face. It is common as a sporadic lesion. In Cowden’s syndrome, the cutaneous manifestations include tricholemmomas. The histologic diagnosis of tricholemmoma is relatively easy; on encountering an example, excitement is not an expected response of a practicing pathologist. The lesion is verrucoid, papillomatous, and broadly and confluently columnar. Focally, the patterns of inverted follicular keratosis and tricholemmoma overlap; squamous eddies, papillomatosis, and the prominence of zones showing hyperplasia of granular keratinocytes (cells rich in keratohyaline granules) are common features. These features have been promoted as evidence of a viral infection. On the other hand, they may merely recapitulate certain phenomena that are manifested in certain stages of follicular development and growth, particularly at the proximal extremity of the outer root sheath.

Basal cell hyperplasias are focal features of tricholemmoma at their advancing deep margins. The hyperplasia may more or less conform to the overall configuration of the affected column or may be manifested in miniature columnar patterns at the advancing margin of a primary column of differentiated cells. These basal cell components might be characterized as blastomatous. This characterization carries with it the implication that the basal cell components anticipate the downward growth of a follicular precusor - a precursor that, in the normal development of a follicle, will contribute the inconstant portion of a developing follicle. In combination, the outer root sheath, the inner root sheath, and the hair bulb with its derivative, the hair shaft, comprise the inconstant portion of a mature hair follicle. In this approach, we might to expect to find in some tricholemmomas that the components with features of the constant and inconstant portions of a hair follicle are variably represented, but the chief expression of a cell type is that of the outer root sheath.

It is not uncommon to find an increased number of immature (miniaturized) follicles in the dermis beneath a tricholemmoma. Some of the blastomatous components of a tricholemmoma manifest a striking relationship with some of these miniaturized follicles. One might postulate that tricholemmomas are prone to develop in the sites of small hair follicle nevi. On the other hand, and perhaps more plausibly, some of the blastomatous components might develop into the miniaturized follicles and, in addition, the small follicles may cycle in much the same manner as normal follicles.

In so-called inverted follicular keratosis (and in “irritated seborrheic keratosis”) zones of cytolysis, dyskeratosis, and acantholysis are prominent in the background population of     non-glycogenated keratinocytes - among, rather than within, the squamous eddies. They occur in cells that do not have the clear cytoplasms of cells of the outer root sheath, but many of the cells show perinuclear vacuoles of the type characterized as a marker for intracellular edema. It is in this population of cells that the degenerative changes progress to form rounded and angulated clefts; the whorls of squamous cells, in varying degrees, survive in the vicinity of the lytic defects. Perhaps, the lytic changes represent cytolytic changes as seen in the formation of sebum, but the process has been perverted; the affected cells do not accumulate lipids prior to their dissolution.

Some tricholemmomas focally show an increased number of sweat ducts at the advancing margin. This phenomena is expressed within the epithelial confines of the tumor and not in the adjacent dermis (although some of the intralesional sweat duct components can be traced into the dermis and then can be identified as being in continuity with a sweat duct. The blastomatous portion of a tricholemmoma may include cells with the potential to form sweat ducts - ducts that may, or may not, have continuity with underlying, more normal sweat ducts. I believe that a documentation of such a pattern was included in a article in the Archives in the 1960’s.

A more significant solitary tumor showing patterns of outer root sheath differentiation is separate and distinct from the small lesions of the butterfly area of the face. These larger, solitary tumors qualify as giant tricholemmomas.

In chapters 23 & 24, a tumor, with the histologic features of a tricholemmoma, is additionally associated with buds of primitive epithelium of basal cell type; these buds resemble primitive hair germs. In addition, there is a localized representation of an increased number of miniaturized hair follicles in the dermis, beneath the tumor.



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