The alterations in a fibrous papule are multifaceted. Emphasis has been placed primarily on an alteration in the fibrous component of the dermis. In this approach, fibrosis begins superficially in the dermis; with progression, a fibrous papule becomes polypoid. The pattern of collagen bundles in the lesion is abnormal but the collagen bundles are coarse and wavy. In addition, the bundles tend to be more brightly acidophilic than those of the normal reticular dermis. The bundles have a hyaline quality. The coarse bundles tend to be loosely spaced; more delicate bundles are interwoven among the coarse bundles. For some examples, the fibrosing process extends into the reticular dermis; for examples showing a prominent component in the reticular dermis, the process takes on some of the features of a fibrous hamartoma of the reticular dermis as seen in shagreen patch. For many examples, the involvement of the reticular dermis remains limited to the superficial portion of the dermis; for many of the these lesions, the collagen bundles of the underlying dermis are loosely spaced and multinucleated histiocytic giant cells can be found among these collagen bundles.
A feature, that is seldom, if ever, emphasized, is the extension of the fibrosing process to the basement membrane zone of the epidermis. In this violation of anatomic boundaries, the abnormal fibrous tissue comes to replace the stroma of the epidermis. In addition, in well-developed examples, there are areas in which the coarse collagen bundles of the lesion insert into a thickened, hyalinized basement membrane. The stroma of the epidermis almost certainly has a role in allowing the epidermis to adapt to minor shearing forces. In addition, it probably has a role in the transfer of nutrients from blood vessels across the basement membrane to the epidermis. It certainly has a role in immunoreactions involving the reactive superficial unit of the epidermis.
The cell-poor lichenoid reactions encountered in some but not all examples of fibrous papule probably provide evidence that an altered or absent papillary dermis (stroma) adversely affects immune responses. Alterations of the stroma almost certainly are associated with alterations in the immunologic functions of the basement membrane.
The membranous loops of basement membrane material are patterns of duplication of basement membrane. Commonly, in these loops, two cells, each of different cytologic character, are entrapped; in addition, these cells are found in a close embrace. The antigenic nature of the cellular components has not been defined but, in view of normal relationships, one cell might be a dendritic histiocyte (i.e., a Langerhans cell) and the other a lymphoid cell. In contrast to normal relationships, the cellular embrace is confined to a domain that is defined by basement membrane material.
It is of interest that the collagen bundles of a fibrous papule are more acidophilic than those of the normal reticular dermis; in addition, the bundles tend to have a hyaline quality. Perhaps, basement membrane materials have been deposited in these bundles.
As an alternative explanation, the dilated vessels of a fibrous papule may leak abnormal proteinaceous materials into the adjacent fibrous tissue. A feature of some of the dilated vessels is duplication of basement membrane material in concentric patterns about the vessels. Might it be possible that the leakage includes an immunostimulant affecting mesenchymal cells?