Editing Germ cell (section)

==Oogenesis==
After migration primordial germ cells will become oogonia in the forming gonad (ovary). The oogonia proliferate extensively by mitotic divisions, up to 5-7 million cells in humans. But then many of these oogonia die and about 50,000 remain. These cells differentiate into primary oocytes. In week 11-12 ''post coitus'' the first meiotic division begins (before birth for most mammals) and remains arrested in prophase I from a few days to many years depending on the species. It is in this period or in some cases at the beginning of sexual maturity that the primary oocytes secrete proteins to form a coat called [[zona pellucida]] and they also produce [[cortical granules]] containing enzymes and proteins needed for fertilization. Meiosis stands by because of the [[follicular granulosa cell]]s that send inhibitory signals through [[gap junction]]s and the zona pellucida. Sexual maturation is the beginning of periodic ovulation. [[Ovulation]] is the regular release of one oocyte from the ovary into the reproductive tract and is preceded by follicular growth. A few follicle cells are stimulated to grow but only one oocyte is ovulated. A primordial follicle consists of an epithelial layer of follicular granulosa cells enclosing an oocyte. The [[pituitary gland]] secrete [[follicle-stimulating hormone]]s (FSHs) that stimulate follicular growth and oocyte maturation. The [[thecal cell]]s around each follicle secrete [[estrogen]]. This hormone stimulates the production of FSH receptors on the follicular granulosa cells and has at the same time a negative feedback on FSH secretion. This results in a competition between the follicles and only the follicle with the most FSH receptors survives and is ovulated. Meiotic division I goes on in the ovulated oocyte stimulated by [[luteinizing hormone]]s (LHs) produced by the [[pituitary gland]]. FSH and LH block the gap junctions between follicle cells and the oocyte therefore inhibiting communication between them. Most follicular granulosa cells stay around the oocyte and so form the cumulus layer. Large non-mammalian oocytes accumulate [[egg yolk]], [[glycogen]], [[lipid]]s, [[ribosome]]s, and the [[mRNA]] needed for protein synthesis during early embryonic growth. These intensive RNA biosynthese are mirrored in the structure of the [[chromosome]]s, which decondense and form lateral loops giving them a lampbrush appearance (see [[Lampbrush chromosome]]). Oocyte maturation is the following phase of oocyte development. It occurs at sexual maturity when hormones stimulate the oocyte to complete meiotic division I. The meiotic division I produces 2 cells differing in size: a small polar body and a large secondary oocyte. The secondary oocyte undergoes meiotic division II and that results in the formation of a second small polar body and a large mature egg, both being [[haploid]] cells. The polar bodies degenerate.<ref>{{cite journal | vauthors = De Felici M, Scaldaferri ML, Lobascio M, Iona S, Nazzicone V, Klinger FG, Farini D | title = Experimental approaches to the study of primordial germ cell lineage and proliferation | journal = Human Reproduction Update | volume = 10 | issue = 3 | pages = 197–206 | year = 2004 | pmid = 15140867 | doi = 10.1093/humupd/dmh020 | doi-access = free }}</ref> Oocyte maturation stands by at metaphase II in most vertebrates. During ovulation, the arrested secondary oocyte leaves the ovary and matures rapidly into an egg ready for fertilization. Fertilization will cause the egg to complete meiosis II. In human females there is proliferation of the oogonia in the fetus, meiosis starts then before birth and stands by at meiotic division I up to 50 years, ovulation begins at [[puberty]].{{citation needed|date=December 2011}}

===Egg growth===

A 10 - 20 μm large somatic cell generally needs 24 hours to double its [[mass]] for mitosis. By this way it would take a very long time for that cell to reach the size of a mammalian egg with a diameter of 100 μm (some insects have eggs of about 1,000 μm or greater). Eggs have therefore special mechanisms to grow to their large size. One of these mechanisms is to have extra copies of [[gene]]s: meiotic division I is paused so that the oocyte grows while it contains two diploid chromosome sets. Some species produce many extra copies of genes, such as amphibians, which may have up to 1 or 2 million copies. A complementary mechanism is partly dependent on syntheses of other cells. In amphibians, birds, and insects, yolk is made by the liver (or its equivalent) and secreted into the [[blood]]. Neighboring [[accessory cell]]s in the ovary can also provide nutritive help of two types. In some invertebrates some oogonia become [[nurse cell]]s. These cells are connected by cytoplasmic bridges with oocytes. The nurse cells of insects provide oocytes macromolecules such as proteins and mRNA. Follicular granulosa cells are the second type of accessory cells in the ovary in both invertebrates and vertebrates. They form a layer around the oocyte and nourish them with small molecules, no macromolecules, but eventually their smaller precursor molecules, by [[gap junction]]s.{{citation needed|date=December 2011}}

===Mutation and DNA repair===

The [[mutation]] frequency of female [[germline]] cells in mice is about 5-fold lower than that of [[somatic cell]]s, according to one study.<ref name="pmid23153565">{{cite journal | vauthors = Murphey P, McLean DJ, McMahan CA, Walter CA, McCarrey JR | title = Enhanced genetic integrity in mouse germ cells | journal = Biology of Reproduction | volume = 88 | issue = 1 | pages = 6 | date = January 2013 | pmid = 23153565 | pmc = 4434944 | doi = 10.1095/biolreprod.112.103481 }}</ref>

The mouse [[oocyte]] in the [[dictyate]] (prolonged diplotene) stage of [[meiosis]] actively repairs [[DNA damage (naturally occurring)|DNA damage]], whereas [[DNA repair]] was not detected in the pre-dictyate ([[leptotene]], [[zygotene]] and [[pachytene]]) stages of meiosis.<ref name="pmid3380109">{{cite journal | vauthors = Guli CL, Smyth DR | title = UV-induced DNA repair is not detectable in pre-dictyate oocytes of the mouse | journal = Mutation Research | volume = 208 | issue = 2 | pages = 115–119 | date = June 1988 | pmid = 3380109 | doi = 10.1016/s0165-7992(98)90010-0 }}</ref>  The long period of meiotic arrest at the four [[chromatid]] dictyate stage of meiosis may facilitate [[homologous recombination|recombination]]al repair of DNA damages.<ref name="pmid9778439">{{cite journal | vauthors = Mira A | title = Why is meiosis arrested? | journal = Journal of Theoretical Biology | volume = 194 | issue = 2 | pages = 275–287 | date = September 1998 | pmid = 9778439 | doi = 10.1006/jtbi.1998.0761 | bibcode = 1998JThBi.194..275M }}</ref>