what is GERMLINE?

Question

Could someone explain what exactly germline is?

Thank you.

Answer 1

In sexually reproducing organisms, cells of the germ line form gametes and establish a physical link, an unbroken chain, from generation to generation. C. elegans germline development can be conceptually divided into three phases: specification, growth, and maintenance. Early in embryogenesis, germ cells are specified as distinct from somatic cells. During postembryonic larval stages, the germ line proliferates and undergoes meiotic development. During the remainder of reproductive life the mature germ line continues proliferation and meiotic development, and produces gametes. Chapters in the Germ line section focus on specific molecular/developmental aspects of these events. Here, we briefly introduce these chapters in the context of the temporal progression of germline development in hermaphrodites (Figure 1).

Answer 2

In addition to what dimpleso copied from somewhere, sperm germ cells can be transplanted from one animal to another (even different species) and continue to multiply with their original genetic make-up. They live and multiply where they are competely isolated from the host animal's genetic system.

Answer 3

In biology and genetics, the germline of a mature or developing individual is the line (sequence) of germ cells that have genetic material that may be passed to a child.

For example, sex cells, such as the sperm or the egg, are part of the germline. So are the cells that produce sex cells, called gametocytes, the cells that produce those, called gametogonia, and all the way back to the zygote, the cell from which the individual developed.

Cells that are not in the germline are called somatic cells. For example, all cells of the mammalian liver are somatic. If there is a mutation or other genetic change in the germline, it can be passed to offspring, but a change in a somatic cell will not be.

Germline cells are immortal, in the sense that they can reproduce indefinitely. This is enabled by a special enzyme called telomerase. This enzyme is dedicated to lengthening the DNA primer of the chromosome, allowing for unending duplication. Somatic cells, by comparison, can only divide around 30-50 times, as they do not contain telomerases.

"Germline" can refer to a lineage of cells spanning many generations of individuals; for example, the germline that links any living individual to the hypothetical first eukaryote of about one billion years ago, from which all plants and animals descend.

In biology and genetics, the germline of a mature or developing individual is the line (sequence) of germ cells that have genetic material that may be passed to a child.

For example, sex cells, such as the sperm or the egg, are part of the germline. So are the cells that produce sex cells, called gametocytes, the cells that produce those, called gametogonia, and all the way back to the zygote, the cell from which the individual developed.

Cells that are not in the germline are called somatic cells. For example, all cells of the mammalian liver are somatic. If there is a mutation or other genetic change in the germline, it can be passed to offspring, but a change in a somatic cell will not be.

Germline cells are immortal, in the sense that they can reproduce indefinitely. This is enabled by a special enzyme called telomerase. This enzyme is dedicated to lengthening the DNA primer of the chromosome, allowing for unending duplication. Somatic cells, by comparison, can only divide around 30-50 times, as they do not contain telomerases.

"Germline" can refer to a lineage of cells spanning many generations of individuals; for example, the germline that links any living individual to the hypothetical first eukaryote of about one billion years ago, from which all plants and animals descend.

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 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, 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, 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, a non-coding mRNA, polar granule component (Pgc) 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 cytopolasm 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.



Migration of primordial germ cells

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 PCG 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) 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 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 to the genital ridges. The CXCR4/Sdf1 system and orientation fibronectin fibers again play an important role as well as filopodia 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
Main article: 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 to become mature and functional spermatozoa.


Oogenesis
Main article: 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.