Basaloid Matrical (inconstant epithelial component) and Catagen-like Tumor of Hair Follicle Type:
This lesion shows patterns of follicular differentiation; a portion of the lesion resembles an “irritated” seborrheic keratosis with scattered patterns of “ infundibular” differentiation, and with a component of smaller cells of germinative type. The patterns could also be compared to those of “inverted follicular keratosis.” A lytic process focally affects a component of basal unit-like squamous cells; lytic defects are the result. The cytolytic process, being of a type that commonly is encountered in “irritated” seborrheic keratosis,” affects cells that basically have no other distinguishable characteristics other than squamous cells qualities.
With epidermis as a model, squamous epithelium consists of two functioning units; a superficial unit and a basal unit; the basal layer is a component of the basal unit, and the keratin layer is a product of the superficial unit. Although structurally a follicle is a unit, an examination of the cyclically expressed phenomena provides insight into a duality: A portion of each follicle can be characterized as constant throughout a cycle; the other portion is inconstant - the inconstant portion is considerably altered by the changes that, in a sequence, are associated with anagen phase, catagen phase, and resting phase. A plethora of phenotypes are expressed during the anagen phase. During the catagen phase, cells of the outer root sheath are progressively depleted. The other distinctive anagen-related cells - those clearly derived from the several cell lines that are derived from the bulb, revert to the nondescript pattern of stratified squamous epithelium. They form a bulge that moves superiorly to eventually reach the level of the constant portion of the follicle. In this bulge, many of the cells have vacuolated cytoplasm; cytolysis and dyskeratosis are prominent features. The categen phase is distinguished by the emergence of the regressive phenomena, as manifested by intracellular edema, dyskeratosis, and cytolysis (physiologic apoptosis).
The squamous epithelium of the constant portion of a follicle (i.e., that portion above the isthmus) can be conceptualized as consisting of two functioning units in close apposition; each unit is mutually dependent on the other. In certain stages of the cycling of hair follicles, the epithelium of the both the constant, and inconstant components of a follicle is of a nondescript type. For the inconstant component, the epithelium of a catagen follicle progressively acquires a nondescript quality, particularly in the tail that is distal to the catagen bulge. If, in the bulge of a regressing, inconstant portion, the phenomena of cytolysis and dyskeratosis are represented, then the epithelium can be characterized as being of catagen-committed type.
In a variety of follicular hamartomas and neoplasms, areas of squamous epithelium display catagen-like phenomena. These properties are recognized by the identification of dyskeratotic cells and cytolytic phenomena; they are common in the tricholemmal hamartomas of the butterfly area of the face. The classification of these tricholemmal hamartomas should reflect the structural duality of each follicle, and the manner in which structure is influenced by cyclic phenomena - phenomena that are an intrinsic property of follicular epithelium, but are expressed in disorganized patterns in these follicular hamartomas.
Chapters 16, 17, 18, & 19 along tier 3, are children of this parent chapter (Ch4). A portion of the tumor that is illustrated in chapters 16-19 (F1Ch16), is composed of what basically is nondescript squamous epithelium. As manifested in these 4 chapters, there is no continuity between the columns of basaloid germinative cells, and the separate, but surrounding, component that is composed of squamous epithelium (F1-7Ch17). The basaloid component appears as isolated columns in a loosely cellular, delicate fibrous matrix. Catagen phenomena - lysis of cells in the squamous cell component near the interface between the squamous and basaloid components - might be offered as an explanation for the lack of continuity. The columnar patterns might be characterized as evidence of follicular differentiation along the lines of the inconstant portion of the follicle (that portion that cycles, and in stages of progressive growth tends to be manifested initially in solid, columnar patterns). In addition, the cytologic features of the basaloid cells of the columns can be compared to the features of cells of the germinative portion of a hair bulb (F1Ch9). The lesion is not trichogenic (there is no evidence of the formation of a hair shaft).
This lesion certainly exceeds the limits that have been defined for the diagnosis of either tricholemmoma, or inverted follicular keratosis. If attention is given solely to the character of the squamous cell component, the patterns more closely resemble those of an “irritated” seborrheic keratosis.
Dr. Pam Martin, Dr. Donald Pultized, and I studied a group of basaloid tumors that shared features with the basaloid portion of this tumor. We interpreted the lesions as hair matrical tumors (non-trichogenic). The work was submitted to several journals but not accepted for publication. Reviewers interpreted the lesions as either common basal cell carcinomas or as trichoblastomas. The opinion that the lesions were common basal cell carcinomas is too immature to deserve additional comment. The diagnosis of trichoblastoma is one of personal preference - whatever is currently being promoted as such is acceptable as an example. There is no discrimination in the diagnosis. Originally, lesions, reported as such, could have equally qualified, or did qualify, as giant trichoepithelioma; immature hair bulbs with distorted papillae were a feature. Our collected material, including the photomicrographs, were a victim of Katrina. A copy of one revision of the manuscript is available (HMA).