Curry and Osteen (2001) reported that MMP-2 and MMP-9 were expressed in the developing follicles, where major diffusion of granulosa cells occurs. Chaffin and Stouffer (1999) also reported that the gelatinase activity of MMP-2 and MMP-9 was required for the modification of granulosa cells during follicle development. In this regard, the reduction in the ovulation rate in miniature pigs may be due to the low expression of MMP-9 in granulosa cells. Higher MMP-2 expression in the Graafian follicles of miniature pigs could not compensate for the lack of MMP-9, probably because MMPs are required before the LH level surge to ensure successful ovulation (Espey and Lipner 1994).
Based on the results observed in our study, MMP-9 appears to be more important than MMP-2 for the structural changes in granulosa cells. Decreased expression of TIMP-2 (MMP-2 inhibitor) and increased expression of TIMP-3 (MMP-9 inhibitor) observed in miniature pigs lead to the inhibition of MMP-9 and enhancement of MMP-2 activity and have been shown to correlate with induction of ovulation and transformation of follicular cells into luteal cells (Ray and Stetler-Stevenson 1994), thus supporting our hypothesis. Other reports have also suggested that MMP-9 is necessary for the reorganization of follicles (Curry and Osteen 2001). On the other hand, several studies demonstrated the effect of MMP-2 on follicular development. It can be speculated that the reorganization of ovarian cells in normal and miniature pigs could be modulated by different MMPs. Taken together, our results suggest that the differential expression of the gelatinases MMP-2 and MMP-9, which degrade the basement membrane and promote reorganisation of ovarian cells, may contribute to the differences in the rate of ovulation, and follicle development between normal and miniature pigs.