Adult medaka (Oryzias latipes) were exposed to 17 beta-estradiol (E2) of the following concentrations 31.3 ng/L, 62.5ng/L, 123ng/L and 250ng/L for 21 days. The state of their testicular as well as the occurrence frequency of testis-ova were examined histologicaly in relation to their fertility. Results show that testis-ova appear in testes of males at all concentrations and their occurence frequency was highest at 62.5ng/L. Each developmental stage of the make germ cell was observed and the spermotazoa were produced actively at 31.3 ng/L, 62.5ng/L, 123ng/L and 250ng/L.. It was concluded that these concentration of E2 had no influence in the fertility of male medaka. At highest concentration (500 ng/L) it was found that the connective tissue was occupying most of the tetiscular tissue and decrease of the numver of the fertility. Although testis-ova were observed, concentrations below 250 ng/L may not give effect fertility of males.
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