Development and gonadal sex differentiation in the neotenic urodele: Ambystoma mexicanum

Abstract

Ambystoma mexicanum is an endemic neotenic urodele amphibian from Mexico; although it belongs to the Salamandridae family, it is characterized by retaining larval structures and hence has been widely used as an experimental model. In the present study, we describe the main events of gonadal morphogenesis in A. mexicanum and correlate these with stages in embryonic and larval development. In this way, it was established that during stage 41 (St41), the gonadal primordium is formed, consisting of primordial germ cells (PGC) and somatic cells. During St45, the undifferentiated gonad is formed from a larger number of PGC interacting with somatic cells. During St53, the germ and somatic cells arrange into the cortical and medullary region. As development proceeds between St55 and 57, morphological differentiation of the gonadal sex takes place, primarily manifested in ovarian differentiation. Our observations and the way these correlate with other urodeles suggest that gonadal morphogenesis in A. mexicanum does not depend on larval age. Besides, onset of gonadal sexual differentiation takes place from St53 onward, evidenced by ovarian structural changes, thus neotenic condition does not influence gonadal differentiation events. Finally, it has been established that gonadal development is controlled by chronological regulation that differs from that of somatic development which in the case of A. mexicanum suggests that gonadal development is completely independent of metamorphosis, thus implying a process of heterochrony.