ACANTHOSIS (G. akantha "thorn, prickle," G. -sis "condition")

Increased thickness of the spinous layer of the epidermis as a result of an increased number (hyperplasia) of spinous cells, an increase in size (hypertrophy) of those cells, or an increase in both number and size of them. It occurs in diseases such as psoriasis and in psoriasiform dermatitides such as pityriasis rubra pilaris and long-standing allergic contact dermatitis and nummular dermatitis. Mechanisms responsible for acanthosis vary and include such disparate phenomena as markedly increased proliferation of generative keratinocytes in psoriasis and the effects of persistent vigorous rubbing of the skin in lichen simplex chronicus.