The cutaneous lesions in pellagra consist of an early erythema, or, in occasional cases, of vesicles or bullous formations which are followed by hyperkeratosis and pigmentation, resulting in a dry, dark brown scaliness. These various lesions are similar to those normally produced by the action of sunlight, but are much more marked. The histological phenomena of the erythematous and bullous stage are those of a mild acute inflammatory reaction, together with a degeneration in the superficial layers of the corium.
Following this degeneration, which involves not only the general connective tissue but the connective tissue of the blood vessels, there is a reparative change evidenced histologically by an increased cellularity of the corium and the presence of fibroblasts. The capillaries also are increased in number and much dilated. Apparently as a result of this increased vascularity of the corium, there is an increased proliferation of the epithelium resulting in a thickening of the epidermis. This increase in thickness of the epithelial layer is especially marked in the prickle cells and the stratum granulosum. In the later stages, in an effort to secure a firm basement membrane, the epithelium is seen to dip down deeply into the rarefied connective tissue. About the blood vessels during the reactionary process are found collections of lymphoid cells, a few plasma cells, but no mast cells or eosinophiles.
That the irritant producing the degeneration in the corium is sunlight in the presence of some predisposing factor, is suggested by the enormous increase in pigment formation in the epithelial cells and by the large number of chromatophores in the superficial layers of the corium. This pigmentation is autochthonous in both types of cell. There is no reason for believing that the pigment is formed in the cells of the corium and thence discharged into the epithelium, or that the reverse process takes place.
The predisposing factor inducing the changes in the corium is, apparently, a lessened resistance of the epithelium to the violet and ultra-violet rays, due to some metabolic insufficiency on the part of the epithelial cells.
Further observation may justify the conclusion that throughout the body, pellagra is a disease essentially of the epithelium, including the nervous system, this pathological condition manifesting itself by an insufficient or altered function.