Differential expression of acidic cytokeratins 18 and 19 during sexual differentiation of the rat gonad

The expression of cytokeratins (CKs) 8, 18 and 19 was analyzed in male and female rat gonads from the undifferentiated stage (12.5 days of gestation) until two weeks after birth by indirect immunofluorescence, using specific monoclonal antibodies anti-CK 8 (LE41), anti-CK 19 (LP2K) and anti-CK 18 (LE65 and RGE53). In the undifferentiated blastema, the somatic cells were stained for CK 8 and CK 19, whereas no detectable immunoreactivity for CK 18 was obtained. The same staining CK pattern was observed in ovaries, in the somatic cells of ovigerous cords and in primary follicles. The staining was progressively decreasing in growing follicles after one week after birth. At the onset of testicular differentiation, when the first Sertoli cells differentiate in the gonad of 13.5-day old male fetuses, positive staining for CK 18 became evident, in addition to CK 8 and CK 19 expression. In the following days, CK 8, CK 18 and CK 19 were detected in Sertoli cells in the differentiating seminiferous cords, but progressively the reactivity for CK 19 decreased and was no longer observed after 18.5-19.5 days of gestation. In all cases, CKs were found to be coexpressed with vimentin, and germ cells were negative for both vimentin and CKs. The results reported here show first, that CKs are expressed before sexual differentiation in gonadal blastema in which no epithelial organization is observed, and second, that there is a CK 18/CK 19 shift in expression during morphogenesis of the testis which is not observed in the differentiating ovary. Future studies will have to determine whether these differences in CK expression are due to epitope-masking phenomena or to the regulation of CK synthesis.