The fibrous papule is distinguished by a preference for the butterfly area of the face. It shares this preferential distribution and its histologic patterns with the angiofibroma (adenoma sebaceum) of the tuberous sclerosis complex. A fibrous papule is a superficial lesion of the dermis; generally, it extends to the dermal-epidermal interface (F2-6Ch2a, F7Ch3, F1-3Ch4, F1-3Ch5, F1,2Ch12). Not all the fibrous hamartomas of the skin, as expressed in the tuberous sclerosis complex, are as confined to a limited portion of the dermis (F3-5Ch16). The shagreen patch is not so limited in its dermal distribution; it tends to be pan-dermal.
The fibrous component of a fibrous papule often can be defined as a peculiar process primarily affecting the components of the adventitial dermis (the papillary dermis, the perifollicular fibrous sheaths, and perivascular adventitia) (F1,2Ch16). Involvement of the papillary dermis seems to be an extension from the primary site into the upper portion of the reticular dermis. Collagenolysis of collagen bundles of the reticular dermis usually is found at the interface between the expanding fibrous lesion and the reticular dermis (F2Ch5, F5Ch8, F4Ch12, F1-5Ch20, F1-4,8Ch21).
In attempting to define the histogenesis of a fibrous papule, it is tempting to place emphasis on the fibrosing process. In support of this position, some small examples are expresed in band-like patterns with a fibrous band that is only slightly wider than a normal papillary dermis. Over some small fibrous papules, the epidermis is shows an intact basal layer with mild hyperplasia of the superficial unit and hyperkeratosis.
Many examples show more extensive involvement of the basal layer and the basal unit of the epidermis. The patterns are such that they can be accommodated in the definition of a cell-poor lichenoid reaction (F5Ch2a, F3Ch4, F1,2Ch10, F4,6Ch13, F3,4Ch17, F2,3,5Ch18, F4-7Ch19, F3Ch20, F4-7Ch21, F1,2Ch23, F1-4Ch25). In addition, many lesions are associated with some degree of melanocytic hyperplasia in the basal layer of the epidermis. In the fibrous component near the D-E interface, hyperplasia of dendritic cells are a prominent feature; morphologically, these cells are interpreted as fibroblasts (with the implication that fibroblasts of the papillary dermis are dendritic cells). In some examples, the dendritic fibroblasts are activated; in some examples, some of these cells are cytologically bizarre (F6Ch23, F1-5Ch24). In some areas in some lesions, it is difficult to exclude the possibility that some of the dendritic cells near the D-E interface are melanocytes which have “dropped-off” into the fibrous component.
Alterations of the basement membrane are common. They are variable in distribution and in character. The changes include a pale zone rather than a defined membrane (F3-6Ch2a, F1Ch23, F2,3,&5Ch24); a coarsened and hyalinized membrane; duplication of a hyalinized membrane; and membranous loops (F1Ch10, F6Ch13, F2,3Ch23, F3,4&6Ch23). One of the rather unique alterations at the D-E interface is a reduplication of the basement membrane to form a short loop. Often, one or two cells reside within the confines of the loop. One of the cells is spindle shaped; it could be a dendritic histiocyte (reticulum cell), or even a melanocyte. The other cell, which, when represented, will be found in close apposition to the spindle cell, probably is a lymphoid cell. The patterns are such that it is tempting to propose that a migrating histiocyte, normally destined for residence in the epidermis, has been restained and confined in the domain of the loop.
In addition, it must be emphasized that a fibrous matrix that resembles a normal papillary dermis is not a significant feature. Coarse, wavy collagen bundles extend to the D-E interface (F4-7CH3, F3Ch5, F1-3Ch13, F4,5Ch19, F1-3Ch24, F1,3-4Ch25). Some of the vertically oriented bundles extend to the D-E interface. In some examples, vertically oriented bundles form small clusters at the D-E interface. The abnormal “papillary dermis” seemingly has a role in the expression of an abnormal basement membrane. As a consequence, the altered basement membrane may have a deleterious effect on immune responses of the reactive superficial unit.
The epidermis of some examples of fibrous papule shows an inconspicuous basal unit with focal lysis of basal keratinocytes. In such areas, the superficial unit is hyperplastic with compact hyperkeratosis along the surface. The epidermal response is of the type associated with a cell-poor lichenoid reaction (F2,5Ch18, F1-7Ch19, F3Ch20, F4-7Ch21).
The second component of the adventitial dermis, the perifollicular fibrous sheath, is also altered in fibrous papules. The degree of perifollicular fibrosis is variable, with the variability possibly a reflection of the age of the respective lesion (as measured by size of a lesion, and extent of dermal fibrosis. For well-developed examples, the sheath consists of concentrically arranged lamellae; the lamellae are uniform in thickness and brightly acidophilic (F1-3Ch3, F1,2Ch6, F1-3Ch8, F4-6Ch8, F4Ch16, F6Ch17, F2,4-6Ch20, F1-2Ch21, F1Ch22). The follicles mostly are immature without mature sebaceous components; there are exceptions. Often the deep extremity, terminating in the bulb, is not vertically oriented.
The third component, the vascular adventitia, also is characterized by concentrically arranged fibrous lamellae; the lamellae have a hyaline quality (F1Ch2a, F5,6Ch3, F1-6Ch7, F5Ch8, F3,4Ch10, F1Ch13, F1-3Ch20, F1,2&7,8Ch21, F3-7Ch22). For some examples, probably more mature lesions, the lamellae become confluent; they lose their identity as independent units. Vessels are increased in number in irregular patterns; often, endothelial cells are hyperplastic (F4,5,7Ch12). Vascular changes may also involve the altered reticular dermis.