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

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Abstract
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.