Coronal section of a juvenile male zebrafish, total body length 17 mm; age 8w; H&E staining testis-ova

Early oocytes are observed in the developing testis at varying prevalences (1:350 in the RIVM zebrafish stock, a higher prevalence was reported in the stock of the Veterinary Faculty, Utrecht University; Raoul Kuiper, personal communication). This image shows the phenomenon at a relatively more advanced developmental stages, which includes mature sperm. There is an obvious clustered organisation (spermatocysts), combined with clusters of spermatogonia. Incidently, Sertoli cells can be discerned.

The presence of primitive oocytes in the developing testis has been interpreted as a feature of undifferentiated gonochorism or protogyny: the undifferentiated gonad develops into an ovary-like gonad in all individuals and then in about one-half of the population transforms into a testis¹. Takahashi described "intersexuality" similar to the images presented here with a maximal incidence of 1:4. These varying incidences indicate that the balance of sex-determining factors is delicate and environmental or genetical variations between stocks may produce a slightly different differentiation process. Our observations do not support the undifferentiated gonochoristic or protogyny paradigm in zebrafish.

Note: testis-ova is distinct from ovo-testis, where ovarian tissue is observed adjacent to testicular tissue.

Reference
¹ Takahashi-H. Juvenile hermaphroditism in the zebrafish, Brachydanio rerio. Bull.Fac.Fish.Hokkaido Univ.28:57-65;1977