Remodelling of mitochondria during spermiogenesis of Chinese soft-shelled turtle (Pelodiscus sinensis)

Abstract

Mitochondria are vital cellular organelles that have the ability to change their shape under different conditions, such as in response to stress, disease, changes in metabolic rate, energy requirements and apoptosis. In the present study, we observed remodelling of mitochondria during spermiogenesis and its relationship with mitochondria-associated granules (MAG). At the beginning of spermiogenesis, mitochondria are characterised by their round shape. As spermiogenesis progresses, the round-shaped mitochondria change into elongated and then swollen mitochondria, subsequently forming a crescent-like shape and finally developing into onion-like shaped mitochondria. We also noted changes in mitochondrial size, location and patterns of cristae at different stages of spermiogenesis. Significant differences (P < 0.0001) were found in the size of the different-shaped mitochondria. In early spermatids transitioning to the granular nucleus stage, the size of the mitochondria decreased, but increased subsequently during spermiogenesis. Changes in size and morphological variations were achieved through marked mitochondrial fusion. We also observed a non-membranous structure (MAG) closely associated with mitochondria that may stimulate or control fusion during mitochondrial remodelling. The end product of this sophisticated remodelling process in turtle spermatozoa is an onion-like mitochondrion. The acquisition of this kind of mitochondrial configuration is one strategy for long-term sperm storage in turtles.