BIOLOGY Volume 3 - A Textbook of General Biology - 2004

21. REPRODUCTION

21.7. Human Reproductive Systems

21.7.3. Gametogenesis

The reproductive process is divided into three main stages: gametogenesis, Fertilization, and embryonic development. Gametogenesis refers to The formation of Gametes: sperm via Spermatogenesis and egg Cells (ova) via oogenesis. Both processes take place in the Gonads—the Testes in males and the Ovaries in females. Both processes involve Meiosis, a type of nuclear division that halves the chromosome number from two sets (the diploid state) to a single set (the haploid state). The cells undergoing meiosis are known as parent cells. The parent cells of sperm are called spermatocytes, while those of egg cells are called oocytes. Fig. 21.35 provides a schematic Overview of spermatogenesis and oogenesis, clearly illustrating the key Similarities and differences between the two processes; these are examined in greater detail in Sections 21.7.4 and 21.7.5.

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Fig. 21.35. Diagram of human gametogenesis.

Note that both processes begin with Cells of the so-called germinal epithelium, which forms the outer layer of the gonads. In both males and females, gametogenesis is divided into three phases: the multiplication phase, the growth phase, and the maturation phase. During the multiplication phase, numerous spermatogonia and oogonia are produced through repeated mitotic divisions. Each of these then enters a growth period in preparation for the first meiotic nuclear and Cell Division. This marks the onset of the maturation phase, during which the First and Second meiotic divisions occur, followed by the final differentiation into mature haploid gametes.

The gametes produced by any given individual exhibit some degree of variation resulting from independent assortment and Crossing Over during meiosis (see Section 23.4).