The testis of the donkey was completely descended into the scrotum around birth. It was covered by a thick tunica albuginea consisting of outer and inner fibrous layers in addition to middle vascular one. Discrete bundles of smooth muscle cells were demonstrated in the outer fibrous layer of the tunica albuginea. These cells give the tunica albuginea a contractile function, which may aid in sperm transport. The present study emphasized an irregular arrangement of the testicular lobules of the donkey during various stages of the postnatal life. Such arrangement does not allow some lobules to have direct contact with the mediastinum testis. The latter was located paraxially, toward the epididymal border and extended about two-thirds of the testicular length; it is thick at the head extremity and gradually faded out caudal wards. The connection between the seminiferous cords and the initial part of the straight tubules exhibited a sharp cut contact in neonates, but it demonstrated exfoliated supporting and germ cells, as well as crowded population of peritubular cells in suckling animals. In premature ones, the connection between the seminiferous tubules and straight testicular tubules was characterized by a peculiar structure appearance that attained more modification and development. This connection included two regions; terminal segment of the seminiferous tubule and a dilated initial part of the straight tubule. In mature stage, the terminal segment was composed of three regions with different cellular composition. The duct system of the donkey testis included the intralobular, interlobular, mediastinal and tunical segments in addition to the efferent ductules. From the seminiferous tubules to the efferent ductules, the lining epithelium was essentially the same in cellular composition; however these passages differed in size and configuration from region to another.

In this investigation, testes of 20 donkeys ranging from birth to maturity were studied. The postnatal morphologic and morphometric characteristics of donkey's seminiferous epithelium during the postnatal period were studied. The volume percentage of the tubular compartment (Seminiferous tubules) was about 12.76% in neonates, progressively increased with postnatal age, reaching about 78.42% of the testicular parenchyma in mature donkeys. The seminiferous tubules measured about 205 μm in diameter, and their lining epithelium was about 73.7 μm in height in mature animals. The supporting (Sertoli) and germ cells (gonocytes) were the main components of the seminiferous cord or (tubules). The supporting type gradually decreased in number from neonates through suckling to the premature and mature stages, while they started to acquire the morphological characteristics of mature cells in late suckling period. The gonocytes maintained the same morphological characteristics during the neonatal and suckling periods; however, they showed a slight increase in number during the latter stage. In addition, dividing germ cells were frequently observed. The germ cells were mostly demonstrated in a central position within the testicular cords of neonatal donkeys. In suckling animals, some gonocytes started to contact the basement membrane, whereas in late suckling period, most of germ cells in contact with basement membrane. In premature donkeys, the gonocytes arranged in 2-3 layers between the supporting cells. In addition, the lumination of the seminiferous cords occurred in 1.5 year. The germ cells could be distinguished to spermatogonia, and primary, as well as secondary spermatocytes. Spermatids, at different stages of transformation, were also detected within some seminiferous tubules. In adults, the semniferous cords became more coiled and were totally luminated. All kinds of germ cells could be observed within the seminiferous epithelium, representing the complete seminiferous cycle and spermatogenesis is completed by 2 years of age. The present study provides baseline information for further experiment or quantitative studies exploring normal development of the testis and hormonal regulation of Sertoli cells, spermatogonial stem cells and spermatogenesis in donkey and other related species.