AN INVESTIGATION OF CYTOKERATIN EXPRESSION IN SKIN EPITHELIAL CYSTS AND SOME UNCOMMON TYPES OF CYSTIC TUMORS USING CHAIN-SPECIFIC ANTIBODIES

The differentiation state of skin epithelial cysts and some uncommon types of epithelial skin tumours was investigated by immunohistochemical staining, mainly using cytokeratin (CK) polypeptide-specific monoclonal antibodies. Samples of interfollicular epidermis, hair follicles and eccrine sweat glands were included as reference tissues. The CK reactivity in epidermoid cysts and milia is not restricted to CKs involved in epidermal-type differentiation, i.e. CK1, 5, 10 and 14, but in addition CK16, a hyperproliferative keratinocyte marker is suprabasally expressed. CK1 and 10 are other prominent suprabasal markers, while CK14 seems to be preferentially expressed in the basal cell layer. Of the non-epidermal CKs, only CK4 was focally or more extensively detected in about 50% of the cases. In terms of CK reactivity, keratinization of trichilemmal cysts corresponds to the keratinization of the anagen-phase hair follicle in the isthmus. The CK reactivity is again restricted to CK1, 5, 10, 14 and 16. However, the CK1 as well as CK10 reactivity is subject to serious limitations, since both CKs were only convincingly observed in foci of terminal differentiation. Eccrine hydrocystoma obligatorily expresses a complex CK set, including CK7, 8, 14, 18 and 19. This CK set perfectly corresponds to the CK composition observed in acini of eccrine sweat glands. In addition, a discontinuous CK4 and 16 reactivity was seen in about 50% of the sites, while CK1 and 10 displayed a strictly focal appearance. On the other hand, syringoma produces in its distinct structures, a CK pattern reminiscent of the one observed in eccrine sweat gland ducts and includes CK1, 5, 10, 14, 16 and 19. Finally, the CK expression pattern of pilomatricoma includes CK1, 8, 10, 14 and 19, and is reminiscent of the CK staining of hair bulb matrix cells differentiating in the keratogenous zone in the direction of hair cortex. The reactivity of CK1 and 10 was mainly restricted to foci of squamoid differentiation and also to transitional cells bordering on shadow cells, as far as it concerns CK10. Occasionally, CK7 and 16 were observed in individual cells or small cell groups. In our view these CK reactivity patterns are useful to judge the differentiation state reached in pathological conditions, but so far do not allow us unequivocally to determine the site of origin of these lesions. © 1990 Springer-Verlag.