Gonadogenesis in the Bearded Dragon (Pogona vitticeps, Agamidae): A Comprehensive Histological Analysis from Gonadal Ridge Formation to Testicular and Ovarian Development

The bearded dragon (Pogona vitticeps) is the most commonly kept pet lizard and a promising model organism for studies of sex determination and gonadal development. Despite its potential, the morphological basis of gonadogenesis in this species remains poorly characterized. Here, we provide a comprehensive histological characterization of gonad development in P. vitticeps using serial paraffin and semi-thin sections, supplemented by morphometric analyses. Gonadal ridges first appeared at stage S28 as bilateral thickenings of the coelomic epithelium, coinciding with primordial germ cell colonization; by S28/29, a recognizable cortex and medulla were already present. The first morphological differences between male and female gonads appeared at S29/30. In differentiating testes, well-defined testis cords with a central lumen formed rapidly, while the cortex became thin and retained only scattered germ cells. Testicular development was characterized by rapid lumen formation within the testis cords, resulting in their early transformation into seminiferous tubules, followed by elongation and coiling of the tubules, maintenance of a stable tubule diameter, and a transient mitotic arrest of germ cells, with proliferation resuming at stage S36. In differentiating ovaries, the cortex remained thick and multilayered, with a progressive increase in germ cell number reflecting active oogonial proliferation. The ovarian medulla expanded substantially, and from S36 onward, lacunae developed within the medullary cords. No meiotic cells were observed at any examined stage. These results provide an essential morphological framework for future molecular and experimental studies of sex determination and gonadal differentiation in this species and in squamates more broadly.