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Ovary |
Department of Biology, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China
Our recent experiments showed that gonadotropin(s) stimulated activin ßA and follistatin expression through the cAMP-PKA pathway but suppressed ßB 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 ßA and ßB 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 ßA, ßB, 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 nearly full-grown follicles. Activin ßA 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 ßB could be easily detected in the ovary of all stages; however, it did not exhibit an obvious trend of variation during the development. The second experiment examined the stage-dependent expression of activin ßA, ßB, and follistatin at the follicle level in the adult mature zebrafish. The expression of activin ßA was again low in the follicles during the primary growth stage, but exhibited a phenomenal increase after the follicles entered vitellogenesis with the peak level reached at midvitellogenic stage; in contrast, activin ßB 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 midvitellogenic stage. In the third experiment, we investigated the dynamic changes of the ovarian activin ßA, ßB, and follistatin expression during the daily ovulatory cycle. The expression of activin ßA and follistatin gradually increased from 1800 h onward and reached the peak level around 0400 h when the germinal vesicles had migrated to the periphery in the full-grown oocytes. In contrast, activin ßB expression steadily declined, although not statistically significant, during the same period, but increased sharply at 0700 h when mature oocytes started to appear in most of the ovaries collected. In conclusion, activin ßA and ßB exhibit distinct expression patterns during the development of the ovary and the daily ovarian cycle of the zebrafish. It seems that activin ßA is involved in promoting ovary and follicle growth, whereas activin ßB may have a tonic role throughout follicle development but becomes critical at the late stage of oocyte maturation and/or ovulation.
2 Correspondence: Wei Ge, Department of Biology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China. FAX: 852 2603 5646; weige{at}cuhk.edu.hk
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