Dermatitis (continued)

This silly drawing is a diagramatic representation of progressive hyperplasia of the basal unit of the epidermis in the evolution of an immunostimulatory immune reaction involving the reactive superficial unit of the epidermis. To the left at the far margin of the drawing, the epidermis is normal; the related dermal papilla contains a small capillary; fibrocytes are few in number; the plate of epidermis above the papilla (the supra-papillary plate) is comprised of 2-3 cell layers. The connective tissue of the papilla is delicate; it composed of unit fibrils of collagen. With stimulation, as a consequence of an inflammatory reaction, the rete ridges become elongated; this alteration is a consequence of hyperplasia of cells of the basal unit of the epidermis. In the basal unit, the cells tend to be polygonal and, often, their long axes appear to be perpendicular to the surface of the skin. With hyperplasia of this unit, there is a greater need for exposure to the weeping portion of the capillary loop. This is accomplished by elongation of the dermal papillae and, in turn, of the weeping capillary loops in the area of involvement. More cells in the epidermis require an increase in nutrients; the nutrients weep from the vessels in greater amounts. In addition to an effect on keratinocytes of the basal unit of the epidermis, the inflammatory process may stimulate fibroblasts of the papillary dermis. The fibroblasts are increased in number, per unit area, and the fibrous matrix is expanded. The fibrous matrix  may be altered qualitatively, as well as quantitatively, as illustrated by the appearance of fibrous lamellae along the contours of the elongated rete ridges. The extreme of this type of reaction is illustrated on the right and might be characterized as a hypertrophic phase. Missing from the diagram are the effects of basal unit hyperplasia on the character of the keratin layer; often basal unit hyperplasia is associated with zones of parakeratosis.

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Introduction

Spongiotic and psoriasiform inflammatory diseases have the qualities of an immunostimulatory process; as a consequence of the antigen-T cell interaction, the epidermis undergoes varying degrees of hyperplasia (acanthosis). In turn, the papillary dermis undergoes compensatory hyperplasia. The reactions in this category involve both the epidermis and the papillary dermis as a functioning unit ( the reactive superficial unit) (VA1-2). The relationships are such that one might anticipate a role for helper T cells in the pathogenesis of the related disorders. The reactions in these two categories are the most common manifestations of the inflammatory diseases of the skin - a wide spectrum of disorders are manifested in spongiotic, or spongiotic and psoriasiform patterns (VA1-3). The epidermal dendritic histiocyte - the Langerhans’ cell (a cell that is immunoreactive for S-100 protein and CD1a) -  has a pivotal role; it serves as the nidus for the presentation of antigens. Once an antigen is attached to a dendritic histiocyte, the ensuing phenomena attract lymphocytes and migratory histiocytes to the vicinity of the antigens  (VA1-1). One of the phenomena, that is important in the migration of inflammatory cells into the epidermis, is a modification of the fluidity of the inter-cellular matrix. The modication in fluidity is primarily the result of an alteration in the hydrophilic capacity of epidermal interstitial glycosaminoglycans. Glycosaminoglycans are hydrophilic. Apparently, the greater the degree of hydrophilia, the more readily inflammatory cells can migrate into epithelium among keratinocytes - perhaps the expanding space disrupts inter-cellular cohesion.

The functioning units of the epidermis are variably affected in this category, but hyperplasia of the basal unit of the epidermis and spongiosis (inter-cellular edema) are the most characteristic features (VA1-4 & 5).

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