Morphometric analysis of the sperm midpiece during capacitation

Background In mammalian sperm, mitochondria are very densely packed and form a helical sheath located in the midpiece of the flagellum. Mitochondria play multiple roles in the cell and can rapidly change shape to adapt to environmental conditions. During capacitation, mammalian spermatozoa undergo morphological and physiological changes to acquire fertilization ability. This is evidenced by changes in sperm motility patterns (hyperactivation) and the ability to perform the acrosome reaction. Whether there are changes in sperm mitochondrial shape or dimensions during capacitation is unknown. This work aimed to quantify morphometric changes in the sperm midpiece during capacitation based on computational analysis and image processing. Results Using mitochondrial fluorescent probes and a combination of freely available software, we quantified the dimensions and fluorescence intensity of the midpiece of the sperm flagellum. After capacitation, the area occupied by the mitochondria decreased. This decrease was due to a reduction in the width but not the length of the midpiece. A reduction in the area and width of the midpiece occurred in spermatozoa that underwent the acrosome reaction, suggesting a shrinkage of the mitochondria during the process of capacitation. Conclusion These results suggest that the flagellar structure is remodeled during sperm capacitation and the acrosome reaction, which is consistent with the observed changes in mitochondrial organization. The application of image processing to fluorescence microscopy images may help to identify morphological changes during capacitation.