Development Of Gametes

In animals, the only cells that divide by meiosis are those that produce gametes within the reproductive organs. However, organisms vary in timing and structures associated with gamete formation. In humans, meiosis occurs in the testes (males) and the ovaries (females). Figure 8-13 shows a male human gamete joining with a female human gamete.

figure 8-13

When the female gamete (the egg) joins with a male gamete (sperm), the genetic instructions from the male and female are combined, and a new individiual is formed. (SEM 1,225x)

When the female gamete (the egg) joins with a male gamete (sperm), the genetic instructions from the male and female are combined, and a new individiual is formed. (SEM 1,225x)

In the testes, meiosis is involved in the production of male gametes known as sperm cells or spermatozoa. In the development of sperm cells, a diploid reproductive cell divides meiotically to form four haploid cells called spermatids. Each spermatid develops into a mature sperm cell. The production of sperm cells is called spermatogenesis, shown in Figure 8-14a.

Oogenesis (OH-oh-JEN-uh-sis) is the production of mature egg cells, or ova. During oogenesis, a diploid reproductive cell divides meiotically to produce one mature egg cell (ovum). During cytokinesis and cytokinesis II of oogenesis, the cytoplasm of the original cell is divided unequally between new cells. As Figure 8-14b shows, one cell, which develops into a mature egg cell, receives most of the cytoplasm of the original cell. As a result, one egg cell is produced by meiosis. The other three products of meiosis, called polar bodies, eventually will degenerate.

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