Middle piece - Structure of the Sperm

Just behind the head, the small constriction (neck) carries a distal and a proximal centriole. The two lie at right angle to each other. The proximal centriole initiates the cleavage divisions of fertilized egg while the distal centriole provides attachment to axial filament of tail. The middle piece has mitochondria surrounding the base of the axial filament of the flagellum (tail). The mitochondria form compact and isolated clumps throughout the middle piece, or there may be a tightly coiled spiral of mitochondria around the proximal centriole of filament and distal centriole.

The mitochondria carry the oxidative enzymes responsible for oxidative phosphoryIation. Therefore, it is the 'power plant' supplying the energy to the flagellum to be used for propulsion of the sperm through the fluid medium. A dark ring is sometimes seen at the posterior end of the middle piece as a boundary between middle piece and the tail. It is known as 'ring centriole' but electron microscopic study reveals that the ring centriole does not resemble a centriole in its structure. Its origin and function are unknown.