Spermatozoon

This picture shows the middle piece and the transition to the principal piece. The axoneme (the 9 + 2 microtubular structure) is responsible for movement of the tail, and runs troughout the three parts of the tail: the middle piece (with mitochondria), the principal piece (with fibrous rings outside the axoneme), and the end piece (only axoneme, not seen in this picture).