Evaluation of the seminiferous epithelial cycle, spermatogonial kinetics and niche in donkeys (Equus asinus)

Kinetics of spermatogonia as well as localization in niches have been described in rodents, but rarely in large animals or in species of economical interest. In this regard, and envisioning the possibility of spermatogonial transplantation from donkeys (Equus asinus) to mules (Equus mulus mulus), many variables that may contribute for an enhanced understanding of the spermatogonial biology in donkeys were investigated. Testes from five adult donkeys were routinely processed for high-resolution light microscopy. Donkey seminiferous epithelium can be divided in XII stages based on the development of the acrosomal system. In addition, spermatogonial morphology and morphometric analysis were performed allowing the characterization of two groups of spermatogonia: undifferentiated (A_und) and differentiating (A₁, A₂, A₃, B₁ and B₂). A_und spermatogonia were present along all XII stages of the seminiferous epithelium cycle of this species, whereas differentiating spermatogonia were only at specific stages. Number of differentiating spermatogonia gradually increased as the cycle progressed, despite the apparent rigid regulation of the balance between mitosis and apoptosis throughout the spermatogenic process. Understanding of spermatogonial biology and kinetics in donkeys, revealed that type A_und spermatogonia are located in specific microenvironments, the spermatogonial niches. The present results enhance understanding of spermatogonial biology in donkeys providing information about subtypes, morphology, number and mitosis/apoptosis along the seminiferous epithelium cycle.