Ovary

Developmental Profiles of Activin A, B and Follistatin Expression in the Zebrafish Ovary: Evidence for Their Differential Roles During Sexual Maturation and Ovulatory Cycle

Abstract
Our recent experiments showed that gonadotropin(s) stimulates activin A and follistatin expression through the cAMP-PKA pathway but suppresses B via a cAMP-dependent but PKA-independent pathway in cultured zebrafish follicle cells. Given that pituitary gonadotropins are the major hormones controlling the development and function of the ovary, the differential expression of activin A and B as well as follistatin in response to gonadotropin(s) raises an interesting question about the temporal expression patterns of these molecules in vivo during sexual maturation and ovulatory cycle. Three experiments were performed in the present study. In the first experiment using sexually immature zebrafish, we followed the expression of activin A, B and follistatin at the whole ovary level during a 10-day period in which the ovary developed from the primary growth stage to the one with full-grown follicles. Activin A expression was very low at the primary growth stage but significantly increased with the growth of the ovary, and its rise was accompanied by an increase in follistatin expression. In contrast, the expression of activin B could be easily detected in the ovary of all stages; however, it did not exhibit obvious trend of variation during the development. The second experiment examined the stage-dependent expression of activin A, B and follistatin at the follicle level in the adult mature zebrafish. The expression of activin A was again low in the follicles of primary growth stage, but exhibited a phenomenal increase after the follicles entered vitellogenesis with the peak level reached at mid-vitellogenic stage; in contrast, activin B mRNA could be easily detected at all stages with a slight increase during follicle growth. The expression of follistatin, on the other hand, also increased significantly during vitellogenesis; however, its level dropped sharply after reaching the peak at the mid-vitellogenic stage. In the third experiment, we investigated the dynamic changes of the ovarian activin A, B and follistatin expression during the daily ovulatory cycle. The expression of activin A and follistatin gradually increased from 18:00 onwards and reached the peak level around 4:00 when the germinal vesicles had migrated to the periphery in the full-grown oocytes. In contrast, activin B expression steadily declined, although not statistically significant, during the same period, but increased sharply at 7:00 when mature oocytes started to appear in most of the ovaries collected. In conclusion, activin A and B exhibit distinct expression patterns during the development of the ovary and the daily ovarian cycle of the zebrafish. It seems that activin A is involved in promoting ovary and follicle growth, whereas activin B may have a tonic role throughout follicle development but becomes critical at the late stage of oocyte maturation and/or ovulation.

Key words: Ovary • Activin • Follicle • Follistatin • Oocyte development

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