gametogenesis

gametogenesis [gə‚mēd·ə′jen·ə·səs]

(biology)

The formation of gametes, or reproductive cells such as ova or sperm.

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Gametogenesis

The production of gametes, either eggs by the female or sperm by the male, through a process involving meiosis. In animals, the cells which will ultimately differentiate into eggs and sperm arise from primordial germ cells set aside from the potential somatic cells very early in the formation of the embryo.

The final products of gametogenesis are the large, sedentary egg cells, and the smaller, motile sperm cells. Each type of gamete is haploid; that is, it contains half the chromosomal complement and thus half as much deoxyribonucleic acid (DNA) as the somatic cells, which are diploid. Reduction of the DNA content is accomplished by meiosis, which is characterized by one cycle of DNA replication followed by two cycles of cell division. See Chromosome, Meiosis

The production of sperm differs from that of oocytes in that each primary spermatocyte divides twice to produce four equivalent spermatozoa which differ only in the content of sex chromosomes (in XY sex determination, characteristic of mammals, two of the sperm contain an X chromosome and two contain a Y). The morphology of sperm is highly specialized, with distinctive organelles forming both the posterior motile apparatus and the anterior acrosome, which assists in penetration of the oocyte at fertilization. See Spermatogenesis

The cytoplasm of the primary oocyte increases greatly during the meiotic prophase and often contains large quantities of yolk accumulated from the blood. Meiotic divisions in the oocyte are often set in motion by sperm entry, and result in the production of one large egg and three polar bodies. The polar bodies play no role, or a very subordinate one, in the formation of the embryo. See Oogenesis

After fertilization and the formation of the polar bodies, the haploid sperm and egg nuclei (pronuclei) fuse, thus restoring the normal diploid complement of chromosomes. See Reproduction (animal)

Meiosis in flowering plants, or angiosperms, is essentially similar to that in animals. However, the cells produced after meiosis are spores, and these do not develop directly into gametes. Female spores (megaspores) and male spores (microspores) develop into gametophytes, that is, female and male haploid plants that bear within them the egg and sperm, respectively. There is a wide range in the details of development and structure of gametes among the different groups of plants other than angiosperms. See Reproduction (plant)