Gonadal morphogenesis and sexual differentiation in rhea (Rhea americana)

The rhea (Rhea americana) is a docile and highly adaptable ratite with notable zootechnical potential for the production of meat, eggs, feathers, and leather. Beyond its economic relevance, captive breeding plays an important role in conservation. However, information on its embryonic and gonadal development remains limited—knowledge that is essential for improving reproductive management. This study aimed to characterize gonadal differentiation during artificial incubation, focusing on developmental stages at days 9, 12, 15, 18, 21, and 24. We provide a detailed morphological and cytological description of germ and somatic cells, documenting the appearance of the gonadal crest at day 9, the formation of undifferentiated gonads by day 12, and the onset of sexual differentiation at day 15. Morphometric analyses of cortex thickness and gonad–mesonephros area relationships enabled comparisons between sexes. The gonads were positioned cranioventrally on the mesoneph, and testicular maturation occurred earlier than ovarian differentiation. Transmission electron microscopy revealed greater mitochondrial density in male germ cells than in female germ cells, indicating early functional divergence. These results contribute to the understanding of reproductive development in R. americana and support the development of conservation and breeding strategies for this and related species.