Gametogenesis

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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)

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Gametogenesis

 

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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

gametogenesis

[gə‚mēd·ə′jen·ə·səs]
(biology)
The formation of gametes, or reproductive cells such as ova or sperm.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The spawning period in both years was followed by months of gametogenic inactivity during which GI values were minimum and gonads were mostly in the recovery stage, as described in several other tropical and temperate sea cucumbers (Mercier & Hamel, 2009).
In contrast to other regions in the Pacific (Palau, Penland et al., 2004; Great Barrier Reef, Stobart et al., 1992), no evidence of biannual spawning (two gametogenic cycles per year) was detected.
littoralis with no gametogenic activity predominated in November and December.
* The testis and ovary are similar in that they both consist of gametogenic and endocrine tissues.
There were also significant differences in the percentage of gametogenic tissue between resident offshore and resident nearshore females during the spring, summer, and fall (Table 4).
Because of its close proximity to Africa, Spain is one of the European countries most susceptible to the traffic of sub-Saharan migrant workers and the risk for transmission by mosquitoes migrating from other countries or indigenous anophelines infected by gametogenic hosts has increased substantially.
Although most unionids have been classified as either tachytictic or bradytictic, other aspects of their reproductive biology (e.g., gametogenic cycle and period of fertilization) are not well known (Zale and Neves, 1982a, b; Gordon and Layzer, 1989).
Gametogenic cycle of Rangia cuneata (Mactridae, Mollusca) in Mobile Bay, Alabama, with comments on geographic variation.
King first provides a review of a number of well-known systems in which meiotic behavior and fitness effects of specific rearrangements are well documented, including several in which nondisjunction and gametogenic impairment is high in hybrid crosses.
Three metrics were noted on gametogenic development: (1) sex, (2) percent coverage of gonadal area, and (3) stage of gametogenesis.
Likewise in Ecuador, different conditioning treatments have been attempted in which temperature has been deemed as the primary factor for starting the gametogenic cycle, (Arguello-Guevara et al., 2013).
The gametogenic cycle was asynchronous with continuous spawning except for December.