The nucleus becomes smaller, denser and takes on a characteristic, flattened form. Seen from above, the nucleus is oval and, from the narrow side, is pear-shaped. The acrosome lies over the tip. Nucleus and acrosome form the sperm cell's head that is bound to the mid-piece by a short neck.
The Golgi complex engender the vesicles, which then merge into a larger formation that settles close to the cell nucleus and finally inverts itself like a cap over the largest part of the nucleus. The acrosome corresponds functionally to a lysosome and thus contains lysosomal enzymes (hyaluronidase among others).
Development of the flagellum
The future axonemal structure grows out of one centriole (distal). This consists of a bundle of nine peripheral double microtubules and two single ones in the center. During its development, through the rotation of the nucleus and acrosomal vesicle, the flagellum primordium comes to lie on the opposite side of the acrosome.
Four parts of the finished flagellum can be distinguished:
- The neck contains the two centrioles (proximal and distal) among other things.
- The mid piece consists of a sheath of ring-shaped mitochondria grouped around the axoneme to provide the energy for the flagellar movement.
- The principle piece has a sheath of ring fibers around the axoneme.
- The tail consists of only the 9+2 structure of the axoneme
The mature sperm cell is approximately 60 mm long and completely enveloped by the plasma membrane.