INTERMEDIATE FILAMENT PROTEINS AND EPITHELIAL DIFFERENTIATION IN THE EMBRYONIC OVARY OF THE RAT

The development and sexual differentiation of gonads in female rat emb ryos and fetuses between the ages of 11 and 17 days was studied by imm unocytochemical analysis of intermediate filament proteins and laminin by light and electron microscopy. In the ii-day-old pregonadal embryo , the surface epithelial cells in the ventral cortex of the mesonephro s contained desmin but not cytokeratin or vimentin. The development of the gonad began on the following day by proliferative growth of the m esonephric surface cells, which like the subepithelial cells soon expr essed vimentin in addition to desmin. The differentiation continued by formation of separate epithelial cell clusters, which joined into cor ds, irregular in shape and size. Desmin disappeared from the cord cell s and cytokeratins appeared while vimentin remained in all somatic cel l types. Desmin was especially abundant in some stromal cells adjacent to the epithelial tissues. After the segretion of the basic ovarian t issues, vimentin and desmin decreased and cytokeratins appeared in the surface epithelial cells. New changes in cytokeratin expression appea red with the differentiation of the embryonic cords in a sex-specific manner with gradual decrease of reactivity for cytokeratin 18. No immu noreaction to the neurofilament proteins was found at the present ages , and the germ cells were negative for intermediate filaments. The res ults show that desmin is expressed in several primitive ovarian and me sonephric cells even though they are not myogenic. The sexual differen ces emerge after the incipient formation of the genetically female gon ad, as different organization of the internal epithelial tissue with d ifferent timing of changes in intermediate filament proteins when comp ared with the male gonad.