Cytokeratins are intermediate filaments present in all epithelial cells, and also in a number of non-epithelial cells. In fact, cytokeratin positively has been reported in almost every tumour type, including uterine smooth muscle tumours, most soft tissue sarcomas, melanomas, gliomas, plasmacytomas and occasionally lymphomas.
They form two group:
I: acidic cytokeratins, assigned Moll numbers 9 to 20
II: basic cytokeratins, assigned Moll numbers 1 to 8
Each type I cytokeratin forms a pair with a type II cytokeratin and all epithelial cells contain at least two cytokeratins. The exception is cytokeratin 19, which exists in an unpaired state. The type I cytokeratin usually pairs with a type II cytokeratin which is 7 to 9 kD larger.2
Cytokeratins may also be classified as low molecular weight (commonly 8, 18 and 19) and high molecular weight (commonly 1, 5, 10 and 14).
Diagnostic utility
Identification of carcinomas
Identification of sarcomas showing true epithelial differentiation:
synovial sarcoma
epithelioid sarcoma
Anomalous staining in a sarcoma may be a clue to a diagnosis of mesothelioma, melanoma, leiomyosarcoma or endothelial cell tumour. Positivity is usually only for cytokeratins 8 and 18 (as demonstrated by Cam5.2 and 35BH11). This ectopic expression of CK8 and CK18 is probably explained by the embryological expression of these two primordial cytokeratins by all tissues1.
Perinuclear "dot-like" staining may be seen in small cell carcinoma, Merkel cell tumour, and in sarcomas and myeloid neoplasms (Positivity with KL-1, Cam5.2 and AE1 in a case of refractory anaemia with excess of blasts in transformation [RAEB-t] and in two cases of AML FAB M4)3 showing anomalous immunoreactivity.
Many "small blue round cell tumours of childhood" express cytokeratin, including Ewing's sarcoma/PNET, rhabdomyosarcoma, Wilm's tumour and desmoplastic small round-cell tumour of childhood.
References
2Diagnostic Immunohistochemistry edited by Professor D. J. Dabbs, page 60.
©SMUHT/PW Bishop