S2C5VA2-1: A distinguishing feature of psoriasis, as opposed to the common, immune-mediated psoriasiform disorders, is the relative lack of spongiosis in psoriasis. It is not simply the relative absence of widened intercellular spaces which distinguish psoriasis: there is also a relative lack of infiltrating lymphoid cells in psoriatic epidermis. Basal unit hyperplasia is common to both psoriasis and the psoriasiform disorders. T-cells and migratory histiocytes apparently induce the changes which are referred to as spongiosis. In psoriasis, as in the psoriasiform disorders, dendritic histiocytes can be identified in the epidermis. In psoriasis, they somehow fail to attract lymphocytes and migratory histiocytes to their domain.

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S2C5VA2-2: Histologic definition of psoriasis.

S2C5VA2-3: Various patterns in which inflammatory cells intermingle among keratinocytes in the psoriasiform disorders are diagramatically represented. For all these peregrinations, an alteration in the nature of the inter-cellular matrix probably facilitates the migrations of inflammatory cells. Exocytosis generally refers to the migration of lymphocytes and histiocytes into the epidermis. The migration of neutrophils into the epidermis does not appear to be as clearly related to the nidal role of dendritic histiocytes. It is tempting to propose that a compromise in the impervious character of the keratin layer and the related superficial functioning unit is, in part, the defect that promotes the immigration of neutrophils.

Psoriasis

1. modifying features

a. effaced rete ridges and stratified, but columnated, zones of parakeratosis (guttate variant)

b. focal hyperkeratosis; irregular granular layer; conspicuous ectasia of vessels (resolving lesion)

c. spongiosis and angiodermatitis (anatomic site [lower extremity] influences histologic patterns)

S2C5VA2-4: Features which deviate from those classically associated with lesions of psoriasis, but are occasionally encountered in the setting of psoriasis.

S2C5VA2-5: In the epidermis, mucinous avenues (the inter-cellular spaces) are present among keratinocytes of the basal unit. In psoriasiform processes, the avenues may be open all the way to the keratin layer. Inflammatory cells in the epidermis, in psoriasiform processes, migrate along the open mucinous avenues to the keratin layer. In the process of keratinization, the inflammatory cells can be discharged into the keratin layer (exfoliated along with the keratinized debris) in areas of parakeratosis. In the keratin layer, as the interstitial fluid is lost, the inflammatory cells degenerate; the resulting pattern qualifies as an inspissated pustule (i.e., Munro microabscess).

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