In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads.
After transport, involving passive movements and active migration, germ cells arrive at the developing gonads.
The second way is found in mammals, where germ cells are not specified by such determinants but by signals controlled by zygotic genes.
Specification of primordial germ cells in the laboratory mouse is initiated by high levels of bone morphogenetic protein (BMP) signaling, which activates expression of the transcription factors Blimp-1/Prdm1 and Prdm14.
After splitting into two populations, the germ cells continue migrating laterally and in parallel until they reach the gonads.
[citation needed] In the aquatic frog Xenopus egg, the germ cell determinants are found in the most vegetal blastomeres.
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] 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).
Cell adhesion on the endothelium of the blood vessels and molecules such as chemoattractants are probably involved in helping PGCs migrate.
[11] Gametogenesis, the development of diploid germ cells into either haploid eggs or sperm (respectively oogenesis and spermatogenesis) is different for each species but the general stages are similar.
Oogenesis and spermatogenesis have many features in common, they both involve: Despite their homologies they also have major differences:[citation needed] After migration primordial germ cells will become oogonia in the forming gonad (ovary).
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.
Meiosis stands by because of the follicular granulosa cells that send inhibitory signals through gap junctions and the zona pellucida.
The pituitary gland secrete follicle-stimulating hormones (FSHs) that stimulate follicular growth and oocyte maturation.
Meiotic division I goes on in the ovulated oocyte stimulated by luteinizing hormones (LHs) produced by the pituitary gland.
Large non-mammalian oocytes accumulate egg yolk, glycogen, lipids, ribosomes, and the mRNA needed for protein synthesis during early embryonic growth.
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.
During ovulation, the arrested secondary oocyte leaves the ovary and matures rapidly into an egg ready for fertilization.
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.
One of these mechanisms is to have extra copies of genes: meiotic division I is paused so that the oocyte grows while it contains two diploid chromosome sets.
[14] The long period of meiotic arrest at the four chromatid dictyate stage of meiosis may facilitate recombinational repair of DNA damages.
These spermatids differentiate morphologically into sperm by nuclear condensation, ejection of the cytoplasm and formation of the acrosome and flagellum.
Receptors belonging to the taste 2 family are specialized to detect bitter compounds including extremely toxic alkaloids.
Homologous recombinational repair of double-strand breaks occurs in mouse during sequential stages of spermatogenesis, but is most prominent in spermatocytes.
[22] Germ cell tumors are generally located in the gonads but can also appear in the abdomen, pelvis, mediastinum, or brain.
Germ cells migrating to the gonads may not reach that intended destination and a tumor can grow wherever they end up, but the exact cause is still unknown.
Efforts to create sperm and eggs from skin and embryonic stem cells were pioneered by Hayashi and Saitou's research group at Kyoto University.
Hayashi and Saitou's group was able to promote the differentiation of embryonic stem cells into PGCs with the use of precise timing and bone morphogenetic protein 4 (Bmp4).