The patterns of the tricholemmal follicular hamartomas (i.e., neoplasms in which the morphologic potentials of the tricholemma are variably expressed [at least as represented in this material]) are basically the same as those which are expressed, as morphologic variations, during the cycling of hair follicles. In this characterization, patterns of the inconstant portion of a hair follicle are represented; the expressed patterns include variable mixtures of patterns that are expressed in the cyclic anagen and catagen phenomena. Basic to a histologic definition of these lesions is a tendency, near the junction of the tumor with the epidermis, for tumor cells to form vertically oriented columns; the columns often are short; they seem to be mostly representative of patterns of the constant portion of a hair follicle (i.e., the infundibulum). In well developed examples, the epithelium in deeper portions of such a lesion tends to become confluent. Along the deep margin, the confluent epithelium forms an expansile interface with the dermis. Another basic quality is the representation of a dual population of cells in the columnar components near the dermal-epidermal interface; a germinative, basal unit-like component, and a terminally differentiating, superficial unit-like component are variably represented, but in varying proportions from lesion to lesion. Mostly in the superficial unit-like component (rarely in the basal unit-like component), some of the acidophilic cells form distinctive whorls (squamous eddies). The whorls, as they enlarge, form - by a process of central keratinization - keratin-filled lumens; these lumens generally are outlined by keratohyalin-rich, granular cells. On this basis, the patterns of keratinization have been characterized as being of epidermoid type; there are exceptions.
The morphologic and cytologic features of follicles correlate with the phase of the cycle. The phases, as expressed morphologically, basically have anagen, catagen, or telogen (resting) qualities; in normal events, the expression of one of these options basically excludes features of one of the alternates. In the follicular hamartomas of the butterfly of the face, such exclusivity is not a rule; anagen features, such as clear cell transformation of the type seen in an outer root sheath of an anagen follicle, may be combined with areas in which catagen-like features are expressed. The follicular neoplasms of the butterfly area of the face are not bound by the same genetic restrictions as are normal follicles.
If a follicular hamartoma of the butterfly area of the face finds almost exclusive expression in the patterns of a catagen-committed tricholemma, then the lesion, by current criteria, could be properly assigned to the category of “ inverted follicular keratosis.” On the other hand, an example, expressed in prominent clear cell patterns, could (and should) be characterized as tricholemmoma. Unfortunately, the decisions are not that simple; a good number of these lesions are expressed in patterns in which both basic categories are generously represented. Apparently, most observers, when confronted with these overlap patterns, simply assign the problem lesions to the tricholemmoma category. Many observers never have had an appreciation for the category of “inverted follicular keratosis.” This large group of biased observers accounts for the belief that inverted follicular keratosis is nothing more than an irritated seborrheic keratosis. It would appear that they may have been right for the wrong reasons. It seems reasonable to propose that the designation tricholemmoma can be divided into 3 subcategories: 1.) tricholemmoma, anagen predominance type, 2.) tricholemmoma, catagen predominance type, and 3.) tricholemmoma, mixed expressions.
There are more problems, other than those related to basic taxonomy. Some tricholemmomas show structural and cytologic atypia; they can be characterized as atypical; for some examples, the atypia is sufficient to characterize the lesion as carcinoma, or as showing intralesional carcinoma. Not all these tricholemmal neoplasms, showing evidence of progressive neoplasia, can be convincingly indentified as having had their origin in one of the common follicular hamartomas of the butterfly area of the face. Some of these lesions manifest basaloid (basosquamous) qualities.
Unusual features of some of these tricholemmomas include sclerosing entrapment, and focal lymphoepithelial transformation. Herein, the evolution of the patterns, leading to those of sclerosing entrapment have been detailed, pictorially and textually. The process of sclerosing entrapment is closely related to an initial expression of catagen-like phenomena; as a late manifestation, lytic defects are produced in the epithelial domain. These phenomena were described years ago, in an Letter to the Editor in the Am J Dermatopathol; the contribution has been ignored; currently, the pattern is characterized as desmoplastic change.
Clearly, the squamous cell patterns of “inverted follicular keratosis,” and the clear cell patterns of “tricholemmona” are commonly combined. The commonness of the patterns in individual lesions suggests that the lesions in which the patterns are purely those of one or the other extreme, are simply variations in a single neoplastic process; all the variations can be embraced as tricholemmomas. What then of the lesions in which patterns of hair matrix differentiation are also represented? They qualify as compound lesions, manifesting patterns of divergent differentiation.
Patterns, in keeping with tricholemmal differentiation, are encountered in uncommon carcinomas of the skin. Some atypical lesions, showing features of tricholemmal differentiation with the basic features of a tricholemmoma, qualify as “atypical tricholemmomas;” patterns of invasion are not a feature. It seems reasonable to anticipate the evolution of some of these atypical tricholemmomas into invasive carcinomas. It would, however, be most difficult to establish such a sequence. Some of the lesions with tricholemmal patterns appear to a variant of basosquamous carcinoma.
One lesion in this collection shows evidence of tricholemmal differentiation but is additionally distinguished by mixed or pleomophic patterns in which individual tumor cells are isolated in a prominent hyaline stroma. In addition, the isolated cells are associated with prominent acidophilic fibrils at the cell membrane level. While lesions might be examples of sweat gland tumors with focal hair sheath differentiation, there is also the option of classifying them as “mixed” or “pleomorphic” follicular tumors. Patterns of sweat duct differentiation are occasional a prominent feature of what are otherwise examples of classic tricholemmomas.