Sexual reproduction is the formation of a new individual following the union of two gametes. In humans and the majority of other eukaryotes plants as well as animals the two gametes differ in structure ("anisogamy") and are contributed by different parents. Gametes need motility to be able to meet and unite food to nourish the developing embryo. In animals (and some plants), these two rather contrasting needs are met by anisogametes: sperm that are motile (and small) eggs that contain food. The reproductive system of the male has two major functions: production of sperm delivery of these to the reproductive tract of the female. Sperm production spermatogenesis takes place in the testes.

Each testis is packed with seminiferous tubules (laid end to end, they would extend more than 20 meters) where spermatogenesis occurs.

The walls of the seminiferous tubules consist of diploid spermatogonia, stem cells that are the precursors of sperm.

Meiosis of each spermatocyte produces 4 haploid spermatids. This process takes over three weeks to complete.

Then the spermatids differentiate into sperm, losing most of their cytoplasm in the process.

This electron micrograph (courtesy of Dr. Don W. Fawcett and Susumu Ito) shows the sperm cell of a bat. Note the orderly arrangement of the mitochondria. They supply the ATP to power the whiplike motion of the tail.

An adult male manufactures over 100 million sperm cells each day. These gradually move into the epididymis where they undergo further maturation. The acidic environment in the epididymis keeps the mature sperm inactive.

The responsibility of the female mammal for successful reproduction is considerably greater than that of the male.

Egg formation takes place in the ovaries.

In contrast to males, the initial steps in egg production occur prior to birth. Diploid stem cells called oogonia divide by mitosis to produce more oogonia and primary oocytes. By the time the fetus is 20 weeks old, the process reaches its peak and all the oocytes that she will ever possess (~4 million of them) have been formed (*). By the time she is born, only about 1 million of these remain (the others eliminated by apoptosis). Each has begun the first steps of the first meiotic division stopping at the diplotene stage of meiosis I.

Read more from the original source:
Sexual Reproduction - RCN