Three dimensional rendering of the mitochondrial sheath morphogenesis during mouse spermiogenesis

In the middle piece of mouse sperm tail, the idea of the mitochondria wrapping in a sinistral (left-handed) double helical structure was generally accepted. In the existing model, mitochondria aligned in four longitudinal rows (stage 1) and twisted dextrally (right-handed) (stage 2) and began to stagger, where opposing rows of mitochondria contacted each other to form a sinistral double helix (stage 3), finally, the end-on touching mitochondria further elongated to their mature length (stage 4). However, in this model, mitochondria need to shift a long distance and reposition themselves. Since no gaps have been found in mitochondrial sheath, repositioning of most mitochondria along the middle piece is unlikely to happen. Therefore, we reapproached the questions through three-dimensional rendering to provide a new model for mitochondrial sheath formation. In our proposed model, four dextrally arranged spherical mitochondrial arrays were considered stage I (resembles stage 2 of the old model). In stage 2 (resembles stage 3 of the old model), a critical difference was found that pairs of mitochondria from the opposing arrays formed ring-like structures instead of a sinistral double helix. In stage 3, which was not observed in the old model, mitochondria staggered in a specific pattern to form the sinistral double helix. In stage 4, mitochondria elongated from crescent-shaped to rod-shaped structures. The new model proposed here would allow each mitochondrion to stay at where they attached first and elongate laterally from two directions to reach their final double helical structure without unreasonable great distance shift along the outer dense fibers.