Histological and immunohistochemical analysis of gonadal development in the corn snake ( <i>Pantherophis guttatus</i> )

Snakes are a group of vertebrates with many unique features, such as significant body elongation, asymmetric positioning of viscera, and the loss of limbs. They belong to the squamate reptiles, one of the vertebrate groups with the most poorly studied gonadogenesis. This study presents the results of histological analyses on gonadal development in embryos of the corn snake (Pantherophis guttatus), covering the period from the initial formation of gonadal primordia, through the phase of undifferentiated gonads, sexual differentiation, to the establishment of the structures of testes and ovaries. The findings indicate that gonadal ridges in this species emerged on the ventral surface of the mesonephros on the first day after oviposition. Undifferentiated gonads were typically composed of primitive cortex and medulla, both containing germ cells. By Day 9 of egg incubation, sexual differentiation of gonads commenced. Testes were recognizable by the presence of distinct testis cords. The developing testes were covered by a thin epithelium devoid of germ cells. In differentiating ovaries, germ cells persisted in the cortex that became thickened in the region opposite to the gonadal mesogonium. Sex cords in the ovarian medulla were not clearly differentiated, and a lumen began to form in them. In subsequent stages, germ cells in the ovarian cortex entered meiosis, and the lumen expanded in the medulla. Additionally, the gonadal primordia and sexually differentiating gonads are symmetrically positioned. However, in later development, the left ovary and testis are positioned posteriorly relative to the right counterparts. The gonadal development of the corn snake shares several features with other squamate species, particularly geckos.