The cells that give rise to the gametes are often set aside during cleavage. During development, these cells will differentiate into primordial germ cells, migrate to the location of the gonad, and form the germ line of the animal.

Creation of germ plasm & primordial germ cells

Cleavage in most animals segregates cells containing Germ plasm from other cells. The germ plasm effectively turns off gene expression to render the genome of the cell inert. Cells expressing Germ plasm

Germ plasm

Germ plasm or polar plasm is a zone found in the cytoplasm of the egg cells of some model organisms , which contains determinants that will give rise to the germ cell lineage. As the zygote undergoes mitotic divisions the germ plasm is ultimately restricted to a few cells of the embryo...

 become primordial germ cells (PGCs) which will then give rise to the gametes. The germ line development in mammals, on the other hand, occurs by induction and not by an endogenous germ plasm.

Germ plasm in fruit fly

Germ plasm has been studied in detail in Drosophila. The posterior pole of the embryo contains necessary materials for the fertility of the fly. This cytoplasm, pole plasm, contains specialized materials called polar granules and the pole cells are the precursors to primordial germ cells.

Pole plasm is organized by and contains the proteins and mRNA of the posterior group genes (such as oskar

Oskar

oskar is a gene required for the development of the Drosophila embryo. It defines the posterior pole during early embryogenesis....

, nanos gene, tudor, vasa, and valois). These genes play a role in germ line development to localize nanos mRNA to the posterior and localize germ cell determinants. Drosophila progeny with mutations in these genes fail to produce pole cells and are thus sterile, giving these mutations the name 'grandchildless'. The genes Oskar

Oskar

oskar is a gene required for the development of the Drosophila embryo. It defines the posterior pole during early embryogenesis....

, nanos and germ cell-less (gcl) have important roles. Oskar is sufficient to recruit the other genes to form functional germ plasm. Nanos is required to prevent mitosis and somatic differentiation and for the pole cells to migrate to function as PGCs (see next section). Gcl is necessary (but not sufficient) for pole cell formation. In addition to these genes, Pgc polar granule component blocks phosphorylation and consequently activation of RNA polymerase II and shuts down transcription.

Germ plasm in amphibians

Similar germ plasm has been identified in Amphibians in the polar cytoplasm at the vegetal pole. This cytoplasm moves to the bottom of the blastocoel and eventually ends up as its own subset of endodermal cells. These cells eventually become PGCs. The presence of homologs of nanos and vasa also implicate this germ plasm as germ-determining.

Fruit flies

The first phase of migration in Drosophila occurs when the pole cells move passively and infold into the midgut invagination. Active migration occurs through repellents and attractants. The expression of wunen in the endoderm repels the PGCs out. The expression of columbus and hedgehog attracts the PGCs to the mesodermal precursors of the gonad. Nanos is required during migration. Regardless of PGC injection site, PGCs are able to correctly migrate to their target sites.

Zebrafish

In zebrafish, the PGCs express two CXCR4 transmembrane receptor proteins. The signaling system involving this protein and its ligand, Sdf1, is necessary and sufficient to direct PGC migration in fish.

Frogs

In frogs, the PGCs migrate along the mesentry to the gonadal mesoderm facilitated by orientated extracellular matrix with fibronectin. There is also evidence for the CXCR4/Sdf1 system in frogs.

Birds

In birds, the PGCs arise from the epiblast and migrate to anteriorly of the primitive streak to the germinal ridge. From there, they use blood vessels to find their way to the gonad. It is possible that the CXCR4/Sdf1 system is used.

Germ line development in mammals

There is no evidence of a germ plasm in mammals. This is evidence for specification of germ cells by induction. BMP

Bone morphogenetic protein

Bone morphogenetic proteins are a group of growth factors also known as cytokines and as metabologens . Originally discovered by their ability to induce the formation of bone and cartilage, BMPs are now considered to constitute a group of pivotal morphogenetic signals, orchestrating tissue...

 (Bone Morphogenetic Protein) signals from the extraembryonic ectoderm activate expression of fragilis and bias the cells towards PGC. The cells expressing fragilis accumulate at the posterior of the primitive streak

Primitive streak

The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...

 and collectively express stella via additional signals or interactions amongst themselves. Blimp1, a general repressor of transcription is also expressed. These cells become the PGCs. The migration of these PGCs is similar to amphibians along the dorsal mesentery

Dorsal mesentery

The portion of mesentery attached to the greater curvature of the stomach is named the dorsal mesentery , and the part which suspends the colon is termed the mesocolon....

 to the genital ridges. The CXCR4

CXCR4

C-X-C chemokine receptor type 4 also known as fusin or CD184 is a protein that in humans is encoded by the CXCR4 gene.- Function :...

/Sdf1

SDF-1 (biology)

SDF-1 is small cytokine belonging to the chemokine family that is officially designated Chemokine ligand 12 ....

 system and orientation fibronectin fibers again play an important role as well as filopodia

Filopodia

Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the...

 extension. Analysis of PGC migration in different organisms concludes that the cells are guided to their target sites by cues from somatic cells along the migratory pathway.

Differentiation of primordial germ cells

In the gonads, the germ cells undergo either spermatogenesis or oogenesis depending on whether the sex is male or female respectively.

Spermatogenesis

Mitotic germ stem cells, spermatogonia, divide by mitosis to produce spermatocytes committed to meiosis. The spermatocytes divide by meiosis to form spermatids. The post-meiotic spermatids differientate through spermiogenesis

Spermiogenesis

Spermiogenesis is the final stage of spermatogenesis, which sees the maturation of spermatids into mature, motile spermatozoa. The spermatid is more or less circular cell containing a nucleus Golgi apparatus, centriole and mitochondria...

 to become mature and functional spermatozoa.

Oogenesis

Mitotic germ stem cells, oogonia, divide by mitosis to produce primary oocytes committed to meiosis. Unlike sperm production, oocyte production is not continuous. These primary oocytes begin meiosis but pause in diplotene of meiosis I while in the embryo. All of the oogonia and many primary oocytes die before birth. After puberty in primates, small groups of oocytes and follicles prepare for ovulation by advancing to metaphase II. Only after fertilization is meiosis completed. Meiosis is asymmetric producing polar bodies and oocytes with large amounts of material for embryonic development.