Keratinization

KERATINIZATION, horny transformation, the process of formation of horny substance, i.e., keratin (see) in various tissues. In humans, keratinization normally occurs in the epidermis of the skin (see Skin, histology, Keratin, Keratohyalin). Keratin stains well with Gram's method, as well as with eosin in pink and with van Gieson's in yellow; in addition, the scales of the epidermis are sudanophilic, which indicates that they also contain a fatty substance, the so-called horny fat. The chemistry of individual intracellular transformations (from keratohyalin to eleidin and from eleidin to keratin), as well as the question of where the fat comes from, has been little studied. Keratinization of cells under normal conditions proceeds extremely rapidly and culminates in the shedding of the keratinized scales. It is most markedly expressed on the palms and soles, and least of all in flexural areas. Under the influence of various endogenous and exogenous causes, the course of normal keratinization can be disrupted. In this case, there may be observed either excessive keratinization, hyperkeratosis, which occurs, for example, in ichthyosis, on warts, corns, calluses, etc., or incomplete and irregularly proceeding keratinization, parakeratosis, the classic example of which is psoriasis. The pathology of keratinization, however, may also consist in the fact that it appears in places where keratinization is normally never encountered, for example, in ichthyosis on the tongue, in chronic irritations in the esophagus and vagina (pachydermia), in spontaneous and experimental vitamin A deficiency in humans and animals (chickens, white rats) on the corneal epithelium, in the esophagus, larynx, and other places. The mucous membranes of the middle ear, renal pelvis, and urinary bladder also sometimes produce a huge amount of horny substance, which accumulates in the cavities in the form of tumor-like masses (cholesteatoma). The same "tumors" are also encountered in the membranes of the brain and spinal cord, arising from rudiments of squamous epithelium that have entered there in the order of improper embryonic development. Accumulations of horny masses can also form in dermoids and teratomas. Squamous cell carcinomas are also capable of producing horny substance deep within their growths, which in such cases have the appearance of layered pearls. If the horny masses deep within the skin extend beyond the limits of the epithelial tissue and come into contact with connective tissue, they play the role of a foreign body, around which granulation tissue arises, sometimes with an admixture of numerous "foreign body giant cells" (epithelioma adenoides cysticum Brook and keratosis follicularis in cutem penetrans Kyrle).