Submicroscopical observations on the differentiation of chick gonads.

The role of hormones in gonadal differentiation has not been fully elucidated. One of the main problems consists in determining the exact moment in which steroid synthesis begins. If, as has been claimed, sex hormones act as organizers and are responsible for the morphological changes which characterize gonadal differentiation, then they should appear before these changes take place. Although the morphological differentiation of chick gonads is evident only after the eighth day of incubation small differences in epithelium height permit sex identification on the seventh day. Biological (Wolff, 1946), biochemical (Gallien & Le Foulgoc, 1961) and histochemical (Scheib, 1959; Narbaitz & Sabatini, 1963; Narbaitz & Kolodny, 1964; Chieffi, Manelli, Botte & Mastrolia, 1964) evidence suggests that estrogen synthesis takes place in embryonic ovaries after the eighth day. On the other hand, steroid production by embryonic testes has not been proven. Wolff (1950) showed that a testicular secretion is responsible for the atrophy of Mullerian ducts on the ninth day; however histochemical techniques have given contradictory results. Thus, lipids are present after the eighth day but cholesterol does not appear until the tenth day (Narbaitz & Sabatini, 1963); Chieffi et al. (1964) claim that 3/?-hydroxysteroid dehydrogenase activity is present in 8-day testes, but Narbaitz & Kolodny (1964) obtained negative results with the same material. Regarding the undifferentiated gonads, the evidence for steroid synthesis is still scarce. Weniger (1961) claims that the feminizing action of embryonic ovaries described by Wolff (1946) is already present in 5-day gonads belonging to genetically female embryos, but no biochemical nor histochemical facts have yet supported this claim. Since cells producing steroid hormones have special submicroscopic characteristics an electron microscopic study of differentiating chick gonads was undertaken in the hope of finding additional evidence that may throw light on these problems.