Figure 2112

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.)

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 (im 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 (Jim long and contains the fibrous sheath external to the coarse fibers and the axonemal complex. The end piece, approximately the last 5 |xm of the flagellum in the mature sperm, contains only the axonemal complex.

Newly released sperm are nonmotile

Newly released sperm are carried from the seminiferous tubules in a fluid secreted by the Sertoli cells. The fluid and sperm flow through the seminiferous tubules, facilitated by peristaltic contractions of the peritubular contractile cells of the lamina propria. They then enter the straight tubules,

tibrous sheath head-neckü

tail-

- end piece acrosomal cap postacrosomal region

- middle piece principal piece acrosome nuclear .vacuoles

Mil If Reproductive System 695

nucleus nucleus post-acrosomal sheath lliïïh connecting |||jjf piece r^-outer dense. \ fibers axonemal complex centriole mitochondria

- end piece acrosomal cap postacrosomal region

- middle piece tibrous sheath outer dense fibers 4, 5, 6 outer doublets of axonemal complex central pair of microtubules of axonemal complex mitochondrial sheath principal piece

______________of the axonemal complex mitochondria outer dense fibers a short segment of the seminiferous tubule where the epithelium consists only of Sertoli cells. At the mediastinum testis the fluid and sperm enter the rete testis, an anastomosing system of ducts lined by simple cuboidal epithelium. From the rete testis, they move into the extratesticu-lar portion of the efferent ductules (ductuli efferentes), the first part of the excurrent duct system, and then into the proximal portion of the duct of the epididymis (ductus epididymis). As the sperm move through the 4 to 5 m of the highly coiled duct of the epididymis, they acquire motility. Contractions of the smooth muscle that surrounds the progressively distal and larger ducts continue to move the sperm by peristaltic action until they reach the distal portion of the duct of the epididymis where they are stored before ejaculation.

Sperm can live for several weeks in the male excurrent duct system, but they will survive only 2 to 3 days in the female reproductive tract. They acquire the ability to fertilize the ovum only after some time in the female tract. This process, which involves removal and replacement of glycocalyx components (glycoconjugates) on the sperm membrane, is called capacitation.

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