CORNEOCYTE or CORNIFIED CELL (L. col~~eum "horn-like," G. kutos "envelope")

A cellular end product of the process of cornification. A normal cornified cell is anuclear, thin, flat, and eosinophilic when stained by hematoxylin and eosin. By electron microscopy, the cytoplasm of a normal corneocyte is characterized by densely packed tonofilaments and is devoid of cellular organelles. Cornified cells of the normal stratum corneum protect an organism by acting as a barrier against noxious agents of various kinds. Cornified cells in hair follicles are present in the inner sheath, shaft, and infundibulum; in the nail unit they constitute the plate. The uppermost parts of normal eccrine and apocrine ducts (acrosyringia) also cornify. Squames are corneocytes that have becorne detached from one another, as happens to contents of an infundibular type of follicular cyst after it has ruptured.
Keratin, a fibrous protein, is present in variable amounts in all cells of cornifying epithelium, but cornified cells are made up almost entirely of keratin. It is inaccurate, however, to employ "keratinized cell" as a synonym for "cornified cell," not only because corneocyies are not the only cells to possess keratin, but also because keratinocytes need not be corneocytes. It follows that the cornified layer of the epidermis, i.e., the stratum corneum, should not be called the keratin layer. "Horn cells" and "horny cells" are imprecise designations for cornified cells.