What is called, conventionally, alopecia mucinosa is but one cutaneous expression of mycosis fungoides, that disease being a systemic lymphoma from the outset, presenting itself first in the skin in many different morphologic guises, from digitate dermatosis to Sézary syndrome, from parapsoriasis en plaque to granulomatous slack skin, from pagetoid reticulosis to poikiloderma vasculare atrophicans, and from xantherythroderma perstans to parakeratosis variegata. One of the most dramatic and characteristic manifestations of mycosis fungoides, clinically and histopathologically, is mycosis fungoides with epithelial mucinosis, a condition that for nearly 50 years has been called alopecia mucinosa. That designation is utterly misleading, and, that being the case, we advise jettisoning it, just as we urge be done for a host of other equally opaque terms, such as parapsoriasis (unmodified), eczema, dysplasia, atypical Spitz's nevus, and borderline melanoma, to mention but five of hundreds of them. Because it is inevitable that "new" faces of mycosis fungoides, clinically and histopathologically, will be encountered and reported on in years to come, it behoves those who give new names to them to do that with concern for lucidity borne firmly in mind.

Mycosis fungoides has for many years been considered an epidermotropic T-cell lymphoma because lymphocytes so commonly are found in surface epidermis, "-tropism" being a suffix used as a combining form with any kind of anatomical structure, e.g., all kinds of epithelium. The idea of "tropism" is not restricted to any type of cell; it applies equally to inflammatory cells and neoplastic cells that may be present within an epithelium. The term "folliculotropic" or "follicular" mycosis fungoides was introduced because some students of the subject noted lymphocytes in what they regarded to be follicular epithelium when, in actuality, the epithelial structures affected by the neoplastic process of so-called follicular mycosis fungoides are, in most instances, infundibula (which are not the uppermost portion of follicles, but are as much epidermis as is surface epidermis). Nevertheless, in those expressions of mycosis fungoides known colloquially, as alopecia mucinosa and follicular mycosis fungoides, neoplastic lymphocytes may be found within follicles, that is, in the outer sheath at the isthmus, the stem, and the bulb, in sebaceous lobules, and in eccrine ductal (and, to lesser extent, glandular) epithelium, as well as in infundibular and surface epidermis. In reality, mycosis fungoides with epithelial mucinosis and follicular mycosis fungoides are simply representatives of mycosis fungoides marked by extraordinary infundibulotropism by lymphocytes (and to a much lesser extent by folliculotropism). It is as illogical to make a distinction between mycosis fungoides with epithelial mucinosis and follicular mycosis fungoides as it is to attempt to distinguish between granulomatous mycosis fungoides as it occurs in patches and plaques of small and large plaque parapsoriasis and as it appears in pendulous folds of granulomatous slack skin; in both circumstances, clusters of epithelioid histiocytes are representatives of granulomatous inflammation of the foreign body type that occurs in response to elastic fibers having been altered by the effects on them of the products of neoplastic lymphocytes.

Although it long has been known that mycosis fungoides was typified by epidermotropism, in the popular mind that phenomenon pertained solely to surface epidermis. But the infundibulum is just as much epidermis as is surface epidermis and, that being the case, it should come as no surprise that that funnel-shaped constituent of epidermis may attract to it lymphocytes of mycosis fungoides as mightily as does the epithelium of follicles, sebaceous lobules, and eccrine units. For reasons not yet known, epitheliotropism in the particular expression of mycosis fungoides known in the past as alopecia mucinosa is joined by mucin manufactured by cells of the infundibulo-folliculo-sebaceous unit. The amount of mucin does not seem to be related either to the density of the infiltrates of cells or to the composition of those infiltrates.

The penetration of eccrine glands and ducts by lymphocytes of mycosis fungoides may induce hyperplasia of those stuctures, particularly of ducts. Sometimes in mycosis fungoides, the nail unit may be affected by lymphocytes that make their way into that epithelium. When the matrix of the nail is caught up in the process, repercussions may be manifested in the nail plate, anonychia being one of them.

In summary, mycosis fungoides is not just an epidermotropic lymphoma, but an epitheliotropic lymphoma. Neoplastic lymphocytes may affect not only surface epidermis, but any other kind of epithelium of the skin, chief among them being infundibular epidermis, but also the entire length of follicles, as well as sebaceous lobules, eccrine units, and nail units, in each of which changes are induced by the effects of products of those mononuclear cells. The tropism of lymphocytes may affect a single type of epithelial structure only, a combination of them, or all of them together. Involvement of the follicular bulb by lymphocytes and, more important, deposits of mucin in the bulb, account for loss of hair in patches and subtle plaques of mycosis fungoides characterized by epithelial mucinosis. Why eosinophils are so numerous in some patches and in most plaques of mycosis fungoides with epithelial mucinosis is not known. That phenomenon is particularly intriguing in the context of eosinophils usually being absent entirely from patches of small and large plaque parapsoriasis and usually, too, from other kinds of subtly elevated plaques of mycosis fungoides. But mycosis fungoides with epithelial mucinosis is singular in several respects, not the least of which is the presence of mucin in infundibular, sebaceous, and follicular epithelium, and the presence often of eosinophils, sometimes in large number. Eosinophils are found not only in the dermis, but often in epithelial structures that house mucin. The site of predilection for eosinophils in the reticular dermis is the same as that in the manifestation histopathologically of mycosis fungoides termed "interstitial."