Gametogenesis

Gametogenesis is the formation of the gametes (germ or sex cells, i.e., oocytes and spermatozoa). In theory, gametogenesis consists of proliferation of immature diploid germ cells, meiosis, and maturation of the haploid germ cells produced by meiosis, but there are marked sex differences in gametogenesis. Gametogenesis in females is called oogenesis, and in males, spermatogenesis.

Oversimplified schematic of gametogenesis in females and males. Human female gametes are secondary oocytes, not ova. The first polar body usually does not divide and the second polar body is not produced until fertilization. Gray's Anatomy (1918) figure 7. Source: Wikimedia Commons

Types

• Oogenesis
• Spermatogenesis

Description

Although few animals produce mature ova, gametogenesis is often described in textbooks as the production of mature gametes (ova and spermatozoa). Gametogenesis has three stages: gametocytogenesis, gametidogenesis. and maturation of the gametes. In humans, oogenesis concludes partway through gametidogenesis.

Gametocytogenesis

Gametocytogenesis is the mitotic proliferation of gametogonia (primordial germ cells) and the differentiation of some of these cells into primary gametocytes. Gametocytogenesis occurs during the fetal period in females, and after puberty in males.

Gametidogenesis

Gametidogenesis is the stage during which meiotic cell division produces haploid germ cells. In mature males, primary spermatocytes produce four spermatids through a series of two meiotic divisions (meiosis I and II). In females, the story is more complicated. Prior to birth, all primary oocytes begin meiosis I and become arrested in prophase. During the reproductive period (roughly thirty years from menarche to perimenopause), monthly ovarian cycles include resumption of meiosis by usually one primary oocyte. Meiosis II typically occurs before ovulation, but is not completed until fertilization.

Gamete maturation

Immature male gametes (spermatids) transform into spermatozoa during spermiogenesis, which includes compaction of chromatin, reduction of cytoplasm, development of an acrosome, and formation of a flagellum.

Role of Gametogenesis in the Body

Gametes are haploid; they have half the number of chromosomes in a somatic cell (i.e., n chromosomes). This is important because fertilization in sexual reproduction then results in a diploid zygote, containing the standard number of chromosomes (2n).

Sexual reproduction is said to provide evolutionary benefits - hybrid vigor and genetic recombination (independent assortment and chromosomal crossing-over). Another evolutionary perspective suggests that somatic cells exist to aid in the propagation of the germ line.

Life cycle of Humans Source: Wikimedia Commons