spermatogenesis

Definition: The spermatogenesis is the developmental process which leads to the production of male gametes, termed spermatozoa or sperm.

Mammalian spermatogenesis is representative for most animals. In human males, spermatogenesis begins at puberty in seminiferous tubules in the testes and go on continuously.

The spermatogonia (spermatogonium) are immature germ cells. They proliferate continuously by mitotic divisions around the outer edge of the seminiferous tubules, next to the basal lamina. Some of these cells stop proliferation and differentiate into primary spermatocytes. After they proceed through the first meiotic division, two secondary spermatocytes are produced. The two secondary spermatocytes undergo the second meiotic division to form four haploid spermatids. These spermatids differentiate morphologically into sperm by nuclear condensation, ejection of the cytoplasm and formation of the acrosome and flagellum.

The developing male germ cells do not complete cytokinesis during spermatogenesis. Consequently cytoplasmic bridges assure connection between the clones of differentiating daughter cells to form a syncytium. In this way the haploid cells are supplied with all the products of a complete diploid genome.

Sperm that carry a Y chromosome, for example, is supplied with essential molecules that are encoded by genes on the X chromosome.

Steps

Spermatogonial stem cells divide by mitosis to form primary spermatocytes.

The primary spermatocyte is initially diploid and undergoes meiosis.

After the first meiotic division, the resulting daughter cells are termed secondary spermatocytes. These undergo the second meiotic division giving rise to haploid spermatids.

Spermatids mature further in a developmental process called spermiogenesis, becoming spermatozoa.

Spermatozoa are characterized by an elongated shape with a compact nucleus and a flagellum.

Additional maturation takes place in the epididymis, where the sperm become motile.

After capacitation in the female reproductive tract, sperm are capable of fertilization.

Pathology

 At least three nonoverlapping regions of the human Y chromosome-AZFa, AZFb, and AZFc ("azoospermia factors" a, b, and c)-are essential for normal spermatogenesis. These intervals are defined by interstitial Y-chromosome deletions that impair or extinguish spermatogenesis.