Ch28P20-L3 Catagen-like Tricholemmoma
(sclerosing entrapment)

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F1Ch28 (R4074): To the right, the cytologic features include cytoplasmic vacuolization. This quality affects acidophilic, keratinizing cells; the affected cells are not the characteristic cells of a classic tricholemmoma; the area generally lacks the basophilia seen in a typical area of a classic tricholemmoma. The irregular patterns to the left are common in lesions of the type seen to the right. The pattern on the right apparently anticipates the changes on the left; in the transition from one to the other, cytolysis is the initial alteration; subsequently, the clefts are inlaid with fibrous tissue (sclerosing entrapment). To the right, there is some degree of papillomatosis with a prominent granular layer. In the same area, columnar patterns are represented but, focally, the epithelium of the vertically oriented columns is confluent. To the left, the epithelial component is interrupted by an island of pale fibrous tissue. Whorls and thin cords of plump, pale squamous cells are isolated within, or project into, the fibrous tissue from the more typical component to the right. There are mild infiltrates of lymphoid cells.


F2Ch28: Above, to the left, a “basaloid” component of small squamous cells abuts upon a population of larger squamous cells - the latter being of a type found in the superficial layer of the normal epidermis. There is an occasional dyskeratotic cell. To the left below the central plane of the field, keratinocytes are large, and have pale cytoplasm (they are somewhat reminiscent of the pale cells of an early keratoacanthoma); some of these plump, pale cells are clustered in whorls. Thin cords of squamous cells project into a fibrous matrix; many of the cells in the fibrous matrix are interpreted as histiocytes. The whorls and cords do not have a well-defined basal layer. The absence of a defined basal layer indicates that the component cells are at risk; they are likely to undergo lysis, or to keratinize and die.


F3Ch28: A portion of the pattern, that is more typical of tricholemmoma, is represented to the left of a centrally oriented vertical plane. The epithelial component at its extremity forms a uniform column that consists mostly of small (basaloid) cells. There is a thin glassy membrane. Cells of the column might be interpreted as representing germinative cells of the outer root sheath; some show perinuclear vacuoles. To the right, near the top of the field, keratinizing cells form small whorls; this population abuts upon stroma without a basal layer as an intermediary. Thin cords of these keratinocytes project into a cell-rich stroma.


F4Ch28: To the right of the irregular fissure, a pattern, commonly seen in a tricholemmoma, is represented; some of the cells are vacuolated in a characteristic fashion. There are scattered whorls of larger keratinocytes (squamous eddies). A lytic process, of a type that irregularly affects many tricholemmomas, has wiped out much of the epithelial component of this tricholemmoma. Whorls of plump, pale keratinocytes and thin, short cords of similar cells are all that remain of the epithelial component in this area. The lytic defects have been inlaid with a fibrous tissue, containing a loose infiltrate of chronic inflammatory cells. The patterns have a pseudo-invasive quality, but for the most part the process is better classified as sclerosing entrapment; lytic defects are the initial insult; the fibrous tissue is secondary. The character of the epithelium is reminiscent of the epithelium of an early keratoacanthoma. Like keratoacanthoma, the likely event is lysis and degeneration of the population of plump, pale keratinocytes. Perhaps, like KA, a rare squamous cell carcinoma may arise during the interplay between epithelium and stroma. 


F5Ch28: At the top of the field on the right, the fissure between isolated whorls and the vacuolated epithelium - epithelium that is typical of tricholemmoma - is represented. The plump cells of the whorls clearly show evidence of keratinization; as such, they appear destined to degenerate and die. Clefts are present among some layers of the whorls, and at the interface between neighboring whorls. The lytic process is seen in both inverted follicular keratosis and tricholemmoma. The resulting defects are not repaired by regenerating epithelial cells, but are invaded by fibrous tissue of adventitial type. The histiocytes are a reaction to degenerating cells.


F6Ch28: The epithelial patterns of a classic tricholemmoma are not represented in this field.  Epithelial nests in this field are isolated in the fibrous stroma; they stand as markers for a distinctive population of cells; the cells forming the isolated nests are relatively resistant to the lytic process, a process that has eliminated a large part of the pre-existing epithelial component. From this point of view, the cells of the whorls are least susceptible; it is the background component of keratinocytes that has borne the brunt of the initial lytic process; this is the same component that has the greatest proclivity to store glycogen; it is the population that undergoes lysis at the initiation of a catagen phase during the cyclic phenomona of hair growth. On this basis, the whorls may be characterized as “isthmic” in nature. Red arrows identify a nest of cells surrounded by a defect. This pattern is interpreted as a retraction artifact, rather than evidence of vascular invasion.


F7Ch28: The large nest at the top of the field shows scattered whorls, one of which centrally shows a collection of keratinized debris. The community, among the whorls, might be cited as representative of a population which, in the lower portion of the field, has undergone lysis; it has relinquished its domain to the invading fibrous tissue. An isolated nest with “retraction artefact” is represented at the bottom of the field.


F8Ch28: Whorls and small nests of plump, pale keratinocytes are entrapped in delicate fibrous tissue of adventitial type. A cluster of colloid bodies mark the site in which the cells of one whorl have degenerated and died. Two of the small nests of degenerating keratinocytes are outlined by retraction artefacts.

Additional features of sclerosing entrapment can be found in chapters 41a & 41b.



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