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A Progestin and an Estrogen Regulate Early Stages of Oogenesis in Fish¹

Laboratory of Fish Reproductive Physiology, Faculty of Agriculture,³ Ehime University, Ehime 790-8566, Japan Aomori Prefecture Fisheries Research Center, Inland Water Fisheries Institute,⁴ Aomori 034-0041, Japan

ABSTRACT

Using two species of teleost fish, Japanese huchen (Hucho perryi) and common carp (Cyprinus carpio), we investigated whether sex steroids are involved in early oogenesis in vitro. Ovarian fragments were cultured to examine the effects of a progestin, 17alpha, 20beta-dihydroxy-4-pregnen-3-one (DHP), and an estrogen, estradiol-17 beta (E2). DHP and E2 significantly promoted DNA synthesis in ovarian germ cells, as judged by 5-bromo-2-deoxyuridine (BrdU) incorporation into these cells. Furthermore, to detect the initiation of the first meiotic division of early oogenesis, we assessed ultrastructurally the occurrence of synaptonemal complexes (SCs) and analyzed by immunohistochemistry the expression of a meiosis-specific marker, Spo11. In huchen, a higher percentage of oocytes with SC was seen in DHP-treated ovarian fragments than in control or E2-treated ovarian fragments. Spo11 was expressed in germ cells after DHP treatment of carp ovarian explants. These data suggest that the progression of germ cells through early oogenesis involves two sex steroids: E2, which acts directly on oogonial proliferation, and DHP, which acts directly on the initiation of the first meiotic division of oogenesis.

17, 20ß-dihydroxy-4-pregnen-3-one, estradiol, estradiol-17ß, first meiotic division, gametogenesis, in vitro culture, meiosis, ovary, progesterone, Spo11

¹Supported by grants-in-aid from the Ministry of Agriculture, Forestry and Fisheries of Japan, the Global COE program of Japan's Society for the Promotion of Science (JSPS), and the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government.

Correspondence: ²Takeshi Miura, Laboratory of Fish Reproductive Physiology, Faculty of Agriculture, Ehime University, 3-5-6 Tarumi, Matsuyama, Ehime 790-8566, Japan. FAX: 81 89 946 9818; e-mail: miutake{at}agr.ehime-u.ac.jp