Evidence for the sexual bipotentiality of spermatogonia in the fish, Oryzias latipes.

The origin of the oocyte-like cells (testis-ova) induced by the administration of estradiol in the testis of the fish, Oryzias latipes, was examined by light and electron microscopy. A small number of testis-ova at the zygotene stage was seen on the sixth day after the beginning of treatment with estradiol, and on the 12th day testis-ova at the pachytene and diplotene stages were observed. Ultrastructural observations revealed that the testis-ova at the zygotene stage were organized as cysts and that all testis-ova within a given cyst developed synchronously. The number of these testis-ova in each cyst varied but cysts containing 16-20 or 26-30 testis-ova were most frequently observed. Spermatogonia of O. latipes are classified morphologically into two types: the type A and the type B spermatogonia (Grier, '76 Cell Tissue Res., 168:419-431). Type A spermatogonia, which were separated from each other by Sertoli cells, were of the stem type and type B spermatogonia appeared to divide clonally 9-10 times in each cyst before entering meiosis. The present observations on zygotene testis-ova indicate that the testis-ova originate from type B spermatogonia which have divided clonally four or five times, suggesting that male germ cells remain sexually bipotent through the early stages of development of type B spermatogonia.