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Germ cell
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===Vertebrates=== In the aquatic frog ''[[Xenopus]]'' egg, the germ cell determinants are found in the most [[vegetal pole|vegetal]] [[blastomere]]s. These presumptive PGCs are brought to the endoderm of the [[blastocoel]] by [[gastrulation]]. They are determined as germ cells when gastrulation is completed. Migration from the hindgut along the gut and across the dorsal [[mesentery]] then takes place. The germ cells split into two populations and move to the paired gonadal ridges. Migration starts with 3-4 cells that undergo three rounds of cell division so that about 30 PGCs arrive at the gonads. On the migratory path of the PGCs, the orientation of underlying cells and their secreted molecules such as [[fibronectin]] play an important role.{{citation needed|date=December 2011}} Mammals have a migratory path comparable to that in ''Xenopus''. Migration begins with 50 gonocytes and about 5,000 PGCs arrive at the gonads. Proliferation occurs also during migration and lasts for 3β4 weeks in humans.{{citation needed|date=December 2011}} PGCs come from the [[epiblast]] and migrate subsequently into the mesoderm, the endoderm and the posterior of the [[yolk sac]]. Migration then takes place from the [[hindgut]] along the gut and across the dorsal mesentery to reach the gonads (4.5 weeks in human beings). [[Fibronectin]] maps here also a polarized network together with other molecules. The somatic cells on the path of germ cells provide them attractive, repulsive, and survival signals. But germ cells also send signals to each other.{{citation needed|date=December 2011}} In [[reptile]]s and [[bird]]s, germ cells use another path. PGCs come from the epiblast and move to the [[hypoblast]] to form the germinal crescent ([[anatomical terms of location#Anterior and posterior|anterior]] extraembryonic structure). The [[gonocyte]]s then squeeze into [[blood vessel]]s and use the [[circulatory system]] for transport. They squeeze out of the vessels when they are at height of the [[gonadal ridge]]s. [[Cell adhesion]] on the [[endothelium]] of the blood vessels and molecules such as [[chemoattractant]]s are probably involved in helping PGCs migrate.{{citation needed|date=December 2011}} ====The ''Sry'' gene of the Y chromosome==== The Sex-determining Region of the Y [[chromosome]] (''[[SRY]]'') directs male development in mammals by inducing the somatic cells of the gonadal ridge to develop into a testis, rather than an ovary.<ref name="Alberts">{{cite book |chapter=Primordial Germ Cells and Sex Determination in Mammals | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26940/|title=Molecular Biology of the Cell. | edition = 4th . | vauthors = Alberts B, Johnson A, Lewis J, etal |publisher=Garland Science|year=2002}}</ref> ''Sry'' is expressed in a small group of [[somatic cell]]s of the gonads and influences these cells to become [[Sertoli cells]] (supporting cells in testis). Sertoli cells are responsible for sexual development along a male pathway in many ways. One of these ways involves stimulation of the arriving primordial cells to differentiate into [[sperm]]. In the absence of the ''Sry'' gene, primordial germ cells differentiate into [[egg (biology)|eggs]]. Removing genital ridges before they start to develop into [[testes]] or [[ovary|ovaries]] results in the development of a female, independent of the carried [[sex chromosome]].<ref name="Alberts"/> ====Retinoic Acid and Germ cell differentiation==== Retinoic acid (RA) is an important factor that causes differentiation of primordial germ cells. In males, the mesonephros releases retinoic acid. RA then goes to the gonad causing an enzyme called CYP26B1 to be released by sertoli cells. CYP26B1 metabolizes RA, and because sertoli cells surround primordial germ cells (PGCs), PGCs never come into contact with RA, which results in a lack of proliferation of PGCs and no meiotic entry. This keeps spermatogenesis from starting too soon. In females, the mesonephros releases RA, which enters the gonad. RA stimulates Stra8, a critical gatekeeper of meiosis (1), and Rec8, causing primordial germ cells to enter meiosis. This causes the development of oocytes that arrest in meiosis I.<ref name="pmid28853925">{{cite journal | vauthors = Spiller C, Koopman P, Bowles J | title = Sex Determination in the Mammalian Germline | journal = Annual Review of Genetics | volume = 51 | pages = 265β285 | date = November 2017 | pmid = 28853925 | doi = 10.1146/annurev-genet-120215-035449 }}</ref>
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