The ultrastructure of spermatozoa in cauda epididymidis of soft-shelled turtle, P. sinensis during breeding season was investigated by light microscopy (LM) and electron microscopy (TEM and SEM). The mature spermatozoa appeared elongated and filiform. In general, the turtle spermatozoon contains a characteristic head, midpiece and tail, similar in morphology to that of birds, amphibians and other reptiles. However, several features are unique. These include (1) three intranuclear tubules containing dense core extend from the subacrosomal cone through the rostral nucleus and deep into the nuclear body; (2) the midpiece is composed of 40 mitochondria which present a staggered rings-and-columns arrangement (8 parallel rings and 5 columns); (3) unusual spherical mitochondria with a dense core are surrounded by 8-10 concentric layers of cristae. Surprisingly, about 21.4±3.6 percent immature spermatozoa with normal morphology are also observed in this season. Different from the mature spermatozoa, a variable amount of cytoplasm droplets are attached to the immature spermatozoa under SEM. Some spermatozoa still show the tail coiled tightly around the cytoplasm. These spermatozoa in transverse sections under TEM, showed a large amount of cytoplasm wrapped by plasma membrane; even some free mitochondria and higher electron density material still seen in the cytoplasm. Among the immature spermatozoa, most of them possess a cytoplasmic droplet which is located eccentrically on the midpiece, and contains a lot of lipid droplets in addition to hollow vesicles. Lipid droplets are closely associated with mitochondrial membranes and may function in the formation or degradation of mitochondria. These immature spermatozoa may be the dormant cells, but whether or not they can fertilize the ovum or not is unknown. Thus, in the present study we hypothesized that the cauda epididymidis might be involved in the sperm maturation in this species.
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