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 also 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 also 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 also 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 also 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 also Reproduction (plant).

The development of the male and female sex cells (gametes).

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Gametogenesis is a process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic division of diploid gametocytes into various gametes or by mitotic division of haploid gametogenous cells. For example, plants produce gametes through mitosis in gametophytes. The gametophytes grow from haploid spores after sporic meiosis. The existence of a multicellular, haploid phase in the life cycle between meiosis and gametogenesis is also referred to as alternation of generations.

Contents 1 Gametogenesis in animals 1.1 Stages 1.1.1 Common path 1.1.2 Individual path 2 Gametogenesis in gametangia 3 Gametogenesis in flowering plants

Gametogenesis in animals

Scheme showing analogies in the process of maturation of the ovum and the development of the spermatids, following their individual pathways. The oocytes and spermatocytes are both gametocytes. Ova and spermatids are complete gametes.

Animals produce gametes directly through meiosis in organs called gonads. Males and females of a species that reproduces sexually have different forms of gametogenesis:

• spermatogenesis (male)
• oogenesis (female)

Stages

However, before turning into gametogonia, the embryonic development of gametes is the same in males and females.

Common path

Gametogonia are usually seen as the initial stage of gametogenesis. However, gametogonia are themselves successors of primordial germ cells. During early embryonic development, primordial germ cells (PGCs) from the dorsal endoderm of the yolk sac migrate along the hindgut to the gonadal ridge. They multiply by mitosis and once they have reached the gonadal ridge they are called gametogonia. Gametogonia are no longer the same between males and females.

Individual path

From gametogonia, male and female gametes develop differently - males by spermatogenesis and females by oogenesis. However, by convention, the following pattern is common for both:

Cell type	ploidy/chromosomes	chromatids	Process
gametogonium	diploid/46	2N	gametocytogenesis (mitosis)
primary gametocyte	diploid/46	4N	gametidogenesis (meiosis 1)
secondary gametocyte	haploid/23	2N	gametidogenesis (meiosis 2)
gametid	haploid/23	1N
gamete	haploid/23	1N

slsad i st he preocess in st 1st

Gametogenesis in gametangia

Fungi, algae and primitive plants form specialized haploid structures called gametangia where gametes are produced through mitosis. In some fungi, for example zygomycota, the gametangia are single cells on the end of hyphae and acting as gametes by fusing into a zygote. More typically, gametangia are multicellular structures that differentiate into Evan Schuldenfrei and female organs:

• antheridium (male)
• archegonium (female)

Gametogenesis in flowering plants

In flowering plants, the male gamete is produced inside the pollen grain through the division of a generative cell into two sperm nuclei. Depending on the species, this can occur while the pollen forms in the anther or after pollination and growth of the pollen tube. The female gamete is produced inside the embryo sac of the ovule.

v • d • e Human physiology and endocrinology of reproduction Anatomy and physiology Reproductive system (male, female) Menstrual/Estrous cycle Menstruation – Follicular phase – Ovulation – Luteal phase Gametogenesis Spermatogenesis (spermatogonium, spermatocyte, spermatid, sperm) – Oogenesis (oogonium, oocyte, ootid, ovum) – Germ cell (gonocyte, gamete) Sexuality Human sexual behavior – Sexual intercourse – Erection – Ejaculation – Orgasm – Insemination – Fertilisation/Fertility – Masturbation – Pregnancy – Postpartum period Lifespan Prenatal development – Sexual dimorphism – Sexual differentiation – Puberty (Menarche, Adrenarche) – Maternal age / Paternal age – Climacteric (Menopause, Andropause) Eggs Ovum – Oviposition – Oviparity – Ovoviviparity – Viviparity Reproductive endocrinology Hypothalamic-pituitary-gonadal axis – Andrology – Hormone

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