An ultrastructural study of sex differentiation in the teleost Oryzias latipes

Electron-microscopic observations on sex differentiation during normal development in the medaka, Oryzias latipes, are presented in this report. Primordial germ cells in embryos 6–10 days after fertilization are clearly distinguishable from somatic cells by the presence in the cytoplasm of the former of germinal dense bodies, closely associated with large aggregations of mitochondria. The gonadal primordium is composed of the primordial germ cells and the enveloping somatic cells; no special ultrastructural relationships have been detected between these two cell types. The sex of an embryo cannot be decided by means of the electron microscope. The newly formed ovary is distinguishable in newly hatched fry of about 5 mm total body length. In the ovary, a layer of cells forming the ovarian wall encloses the ovarian matrix, consisting of oogonia and their surrounding follicle cells. There are often observable desmosomes between two neighbouring follicle cells. Although marked rearrangement of somatic cells takes place during sex differentiation of the gonad, few visible changes are observed in the ultrastructure of the primordial germ cells during their transition to oogonia. The transition from the oogonium to the oocyte is, however, characterized by a distinctive change in the nucleus, associated with the onset of meiosis. In the young ovary of 5-day-old fry, ooctye chromatin is visibly organized into electron-dense axial elements at the leptotene stage of meiotic prophase. But in the ovary of 10-day-old fry (about 6 mm body length), many oocytes at zygotene or pachytene stages are found, with synaptonemal complex configurations essentially the same as those described in numerous other meiosing plant and animal cells. In contrast, the testis of newly hatched male fry remains in an undifferentiated state, with the somatic cells simply enveloping the spermatogonia. No cells with the ultrastructural characteristics of Leydig cells (as observed in adult testis) can be distinguished in the young ovary or in the undifferentiated testis 10 days after hatching. Cells with the ultrastructural characteristics of adult Leydig cells are detectable for the first time in the matrix of the testis of 25-day-old young, about 8 mm in total length. When proliferation of spermatogonia has occurred, to form the typical testis of 45-day-old (about 15 mm), young, these cells appear in the interstitial region of the testis. Observations from the present study indicate that sex differentiation of germ cells and somatic cells in the gonad precedes the differentiation of steroid-secreting cells. Therefore, the hypothesis that sex hormones are natural sex-inducers is not supported by the present results. The possibility is emphasized that some intracellular mechanisms may be involved in the natural course of sex differentiation of the germ cells and the gonad in this fish.