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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)
the process of development and formation of the sex cells, or gametes. The gametogenesis of the male gametes (spermatozoids, sperms) is called spermatogenesis; and that of the female gametes (the egg cells), oogenesis. Gametogenesis proceeds differently in animals and plants, depending on the place of meiosis in the life cycle of these organisms.
In multicellular animals gametogenesis takes place in special organs—the sex glands, or gonads (ovaries, testes, hermaphroditic sex glands), and takes place in three basic stages. The first stage is reproduction of the primordial sex cells—the gametogonia (spermatogonia and oogonia) by means of a series of consecutive mitoses. The second stage is the growth and maturation of these cells, now called gametocytes (spermatocytes and oocytes), which, like the gametogonia, possess a full (usually diploid) set of chromosomes. At this point the crucial event of gametogenesis in animals occurs: the division of the gametocytes by means of meiosis, which brings about a reduction (halving) of the number of chromosomes in these cells and their conversion into haploid cells—spermatids and ootids. The third stage is the formation of spermatozoids (or sperms) and egg cells; in this stage the egg cells take on a series of embryonic membranes while the spermatozoids acquire flagella which enable them to move. In many animal species meiosis and the formation of the egg are completed in the female after the penetration of the spermatozoid into the cytoplasm of the oocyte but before the fusion of the nuclei of the spermatozoid and the egg cell.
In plants, gametogenesis is separate from meiosis and begins in the haploid cells, in the spores (in higher plants, microspores and megaspores). The sexual generation of the plant, the haploid gametophyte, develops from the spores, and it is in the sex organs of the gametophyte, the gametangia (male, anther; female, archegonium), that gametogenesis takes place by means of mitoses. Gymnospermous and angiospermous plants, in which gametogenesis takes place directly in the germinating microspore, in the pollen cell, are an exception. In all lower and higher spore-bearing plants, gametogenesis in the anthers is the repeated division of cells which produces a large number of small, motile spermatozoids. Gametogenesis in the archegonia is the formation of one, two, or several egg cells. In gymnospermous and angiospermous plants male gametogenesis consists of the division (by means of mitosis) of the nucleus of the pollen cell into a generative nucleus and a vegetative nucleus and the further division of the generative nucleus (also by means of mitosis) into two sperms. This division takes place in the germinating pollen tube. Female gametogenesis in angiospermous plants is the isolation by mitosis of one egg cell inside the eight-nucleate embryonic sac. The major difference between gametogenesis in animals and plants is that in animals it includes the transformation of the cells from diploid cells to haploid cells and the formation of haploid gametes; and in plants gametogenesis is the formation of the gametes from haploid cells.
IU. F. BOGDANOV