Toward the end of maturation phase of spermatogenesis, elongated spermatids are released from Sertoli cells into the lumen of seminiferous tubule. This complex process, called spermiation involves progressive removal of specialized Sertoli-to-spermatid junctional complexes and disengagement of spermatids from the Sertoli cell. The presence of the 1-integrins in the Sertoli-to-spermatid junctions, as well as increased activity of the integrin-linked kinase at the time of spermiation, suggests an enzymatic control of spermatid  release. The rate of spermiation in the testis determines the number of sperm cells in the ejaculate of semen. Various pharmacologic treatments, toxic agents, and gonadotropin suppression result in spermiation failure, in which spermatids are not released but instead are retained and phagocytosed by the Sertoli cell.
Structure of the Mature Sperm
The events of spermiogenesis result in a structurally unique cell.
The mature human sperm is about 60 m long. The sperm head is flattened and pointed and measures 4.5 m long by 3 m wide by 1 m thick (see Fig. 22.12). The acrosomal cap that covers the anterior two-thirds of the nucleus contains hyaluronidase, neuraminidase, acid phosphatase, and a trypsin-like protease called acrosin. These acrosomal enzymes are essential for penetration of the zona pellucida of the outer dense fibers 4, 5, 6 outer doublets of axonemal complex central pair of microtubules of axonemal complex
FIGURE 22.12 • Diagram of a human spermatozoon. Regions of the spermatozoon are indicated on the left. Key structural features of the head (viewed in frontal and sagittal planes), the middle piece, and the principal piece of the spermatozoon are illustrated on the right. (Modified from Pederson PL, Fawcett DW. In: Hafez ESE, ed. Human Semen and Fertility Regulation in the
Male. St. Louis: CV Mosby, 1976.) ovum. The release of acrosomal enzymes as the sperm touches the egg is the first step in the acrosome reaction. This complex process facilitates sperm penetration and subsequent fertilization and prevents the entry of additional sperm into the ovum.
The sperm tail is subdivided into the neck, the middle piece, the principal piece, and the end piece. The short neck contains the centrioles and the origin of the coarse fibers. The middle piece is approximately 7 m long and contains the mitochondria, helically wrapped around the coarse fibers and the axonemal complex. These mitochondria provide the energy for movement of the tail and thus are responsible for the motility of the sperm. The principal piece is approximately 40 m long and contains the fibrous sheath external to the coarse fibers and the axonemal complex. The end piece, approximately the last 5 m of the flagellum in the mature sperm, contains only the axonemal complex.
Newly released sperm cells are processed in the epididymidis where they acquire motility and undergo further maturation.