In situ analysis of the changes in expression of ovarian inhibin subunit mRNAs during follicle recruitment after ovulation in pigs

In situ hybridization was used on frozen tissue sections with digoxigenin-l abelled antisense riboprobes to inhibin/activin alpha and beta(A) subunits to determine whether inhibin/activin subunit mRNA expression was associated with development of growing, steroidogenically active follicles during fol licle recruitment after ovulation. Cell proliferation-associated nuclear an tigen Ki-67 protein and cytochrome P450 aromatase expression in granulosa c ells were determined immunohistochemically and used as markers for granulos a cell proliferation and steroidogenesis, respectively, on days 3, 5 and 7 after the onset of oestrus. The amounts of inhibin/activin a and 13, subuni t mRNA and P450 aromatase protein were greater (102, 93, and 238%, respecti vely; P < 0.05) in medium than in small non-atretic follicles and were posi tively correlated with Ki-67 and with each other. Inhibin/activin alpha and beta(A) mRNA, P450 aromatase, and Ki-67 in granulosa cells were reduced by 66-83% (P < 0.001) in atretic follicles compared with non-atretic follicle s. In addition, inhibin/activin alpha and beta(A) mRNA and P450 aromatase i n small (1-2 mm) non-atretic follicles decreased (P < 0.05) between day 3 a nd day 7 independently of morphological or biochemical signs of atresia. Th e pattern of inhibin/activin subunit mRNA expression supports the notion th at activin and inhibin have roles in growth and steroidogenesis in follicle recruitment during the early luteal phase of the oestrous cycle.