Description: Image of a three-dimensional graphic illustration of the distal middle piece and the proximal principal piece of a bull sperm based upon electron microscopy with added labeling. The plasma membrane was removed to reveal the underlying helical mitochondrial sheath. The cross section through the middle piece shows the arrangement of 9 large dense outer fibers, 9 smaller doublets and the central pair within the tail. The 9 + 9 + 2 bundle of fibers is surrounded by a fibrous sheath with longitudinal columns. The 9 large outer fibers gradually disappear as they proceed down the principal piece of the tail until none is found in the end piece. Collectively, the 9 doublets and the 2 central fibers constitute the axoneme within the sperm tail, the contractile mechanism that powers sperm motility. This 9 + 2 fiber arrangement also is found in flagella of most other organisms. NAL #5087 shows the relationship of this image to the proximal parts of a sperm, and also reveals that each of the 9 doublets consists of a small A tubule and a larger B tubule with attached dyneim arms that energize sperm motility.