Seasonal changes of sperm storage and correlative structures in male and female soft-shelled turtles, Trionyx sinensis

Reproductive ducts of male and female soft-shelled turtles, Trionyx sinensis were examined throughout the year (March, May, September, December) using brightfield and electron microscopes (TEM and SEM), to determine the location and histomorphological characteristics of sperm storage structures as well as their changes at different phases of the seasonal reproductive cycle. Sperm stored in the epididymis were also examined. In the male, spermatogenesis is initiated in spring (May), and then the mature sperm are released in autumn as an episodic event. Spermatogenesis is inactive in winter. However, in this species, the epididymis contains sperm throughout the entire year. Sperm observed in the epididymis are intact and some structures are uniquely different from other reptiles, and is characterized by 35–40 concentric mitochondria with a dense core in the centre. Many glycogen granules are observed in the cytoplasm of the midpiece. However, the epithelial cell type of epididymal duct change in different seasons. The cells are fully developed with a highly secretory activity in September. The materials secreted from the epithelium might have the function as nourishment for the stored sperm. Sperm storage structures in the form of tubules are observed in the wall of the isthmus of the oviduct in hibernating females but are absent in the groups of May and September. These tubules develop either by folding or fusion of the oviductal mucosal folds and are lined by both ciliated and secretory cells. These tubules might provide a microenvironment for the sperm to enable its long-term storage. After being separated 4 months (December–March) from the male, sperm are observed in the tubules of the isthmus of the oviduct. The unique character of the sperm combined with the special sperm storage structures enable the sperm to maintain fertility and activity during their storage.