Gonadal Development in Larvae of the Paedomorphic Ambystoma mexicanum (Amphibia: Urodela)

The axolotl (Ambystoma mexicanum) is an endemic amphibian of the order Urodela, which has the ability to reproduce while retaining its larval characteristics. The primary objective of this study was to outline the stages of gonadal development leading to sexual maturity. Gonads from both male and female larvae, aged 4–12 months, were histologically processed and characterized according to their developmental stage. Histological analysis of the ovaries revealed that larvae aged 4–8 months were immature. At 9 and 10 months, the oocytes showed early signs of maturity, characterized by an increase in volume, yolk accumulation, and the presence of cortical pigment at one cellular pole. At 12 months, mature oocytes are predominant, as indicated by a significant increase in size and the migration of the germinal vesicle toward the pigmented animal pole. Male larvae have immature gonads at 4–5 months of age. By 7 to 8 months, the beginning of the spermatogenic process is evident, regionalizing the testis in a cephalocaudal and lateromedial direction. In 9-month-old males, the spermiation process begins, becoming dominant by 10 months. In 12-month-old males, a clear regionalization of the testis is evident; while spermiation continues in the caudal and medial regions, spermatogenesis restarts in the more cephalic and dorsal areas. This early and accelerated gametogenesis in males signifies a gonadal maturity that occurs sooner than in females, which reflects a process of heterochrony. The relatively early attainment of sexual maturity for both sexes, a direct result of their paedomorphic condition, is an advantageous adaptive strategy.