Germ Plasm and Determination of Primordial Germ Cells:
All sexually reproducing organisms occur from fusion of gametes sperm and eggs. All gametes occur from the primordial germ cells. In most animal species, determination of primordial germ cells is produce by cytoplasmic localization of specific proteins and mRNAs in certain cells of early embryo (mammals being the major exception to this general rule). These cytoplasmic components are referred to as germ plasm.
Germ cell determination in nematodes:
This nematode contains just two chromosomes per haploid cell, permitting for thorough observations of individual chromosomes. Cleavage plane of first embryonic division is unusual in that it is equatorial, separating animal half from vegetal half of the zygote. Though, is the behavior of chromosomes in the subsequent division of these first two blastomeres.
The ends of chromosomes in animal-derived blastomere fragment in dozens of pieces just before this cell divides. This event is known as chromosome diminution, as only portion of the original chromosome survives. Many genes are lost in the cells when chromosomes fragment, and these genes are not included in newly formed nucle. In the meantime, in vegetal blastomere, chromosomes remain normal. During second cleavage, the animal cell splits meridionally whereas the vegetal cell again separates equatorially. Both vegetally derived cells have normal chromosomes. Though, chromosomes of more animally located of these two vegetal blastomeres fragment before third cleavage. Therefore, at the 4-cell stage, only one cell the most vegetal has a complete set of genes. Chromosomes are kept together only in those cells destined to form germ line. If this were not case, genetic information would degenerate from one generation to next. Cells which have suffered chromosome diminution generate somatic cells. The P-granules enter cell, and they appear vital for instructing it to become germ line precursor.
Germ cell determination in insects:
In Drosophila, PGCs form as the group of cells (pole cells) at posterior pole of cellularizing blastoderm. These nuclei migrate in the posterior region at ninth nuclear division, and they become enclosed by pole plasm, complex collection of fibrils, mitochondria, and polar granules. If pole cell nuclei are prevented from reaching pole plasm, no germ cells will be produced. Nature has given confirmation of significance of both pole plasm and polar granules. One of the components of pole plasm is mRNA of germ cell-less (gcl) gene. Wild-type gcl gene is transcribed in nurse cells of the fly's ovary, and its mRNA is transported into egg. Once in the egg, it is transported to posterior most portions and resides in what will become pole plasm. This message gets translated in protein during early phases of cleavage gcl-encoded protein appears to enter nucleus, and it is necessary for pole cell production. Oskar seems to be critical protein of this group, as injection of oskar mRNA in ectopic sites of embryo will cause nuclei in those areas to form germ cells. Genes which limit Oskar to the posterior pole are also essential for germ cell development. One of these RNAs is nanos message, whose product is necessary for posterior segment formation. Nanos is also necessary for germ cell formation. Another one of these RNAs encodes Vasa, an RNA-binding protein. mRNAs for this protein are observed in germ plasm of several species. A third germ plasm component was a big surprise: mitochondrial ribosomal RNA (mtrRNA). Furthermore, in normal fly eggs, small and large mitochondrial rRNAs are situated outside mitochondria only in pole plasm of cleavage stage embryos. Here, they appear as components of the polar granules. Fourth component of Drosophila pole plasm is a non-translatable RNA called polar granule component (Pgc).
Germ cell Determination in amphibian:
Cytoplasmic localization of germ cell determinants has also been seen in vertebrate embryos. Scientists illustrated that vegetal region of fertilized frog eggs has material with staining properties like those of Drosophila pole plasm. He was able to trace cortical cytoplasm in few cells in presumptive endoderm which would usually migrate in genital ridge. By transplanting genetically marked cells from one embryo in another of a differently marked strain,
They illustrated that these cells are primordial germ cell precursors. They discovered that germ plasm of unfertilized eggs comprises of tiny islands which appear to be tethered to yolk mass near vegetal cortex. These germ plasm islands move with vegetal yolk mass during cortical rotation of fertilization. After it, islands are released from yolk mass and start fusing together and migrating to vegetal pole. Later, periodic contractions of vegetal cell surface also come out to push germ plasm along cleavage furrows of the newly formed blastomeres, enabling it to enter the embryo. Very few primordial germ cells reach gonads; those few which do have approx one- tenth the volume of normal PGCs and have unusually shaped nuclei. Xenopus homologues of nanos and vasa are particularly localized to region. Components of germ plasm haven't all been catalogued. Certainly, in birds and mammals, such a list has barely even been started.
Germ Cell Migration:
Germ cell migration in amphibians:
Germ plasm of anuran amphibians (frogs and toads) gathers around vegetal pole in zygote. During cleavage, this material is brought upward by yolky cytoplasm, and finally becomes related with endodermal cells lining floor of blastocoels.
The PGCs become concentrated in posterior region of larval gut, and as abdominal cavity forms, they migrate along dorsal side of gut, first along dorsal mesentery (that connects gut to region where mesodermal organs are forming) and then along abdominal wall and in genital ridges. They migrate up this tissue until they reach developing gonads. Xenopus PGCs move by extruding single filopodium and then streaming their yolky cytoplasm in that filopodium while retracting their tail. Moreover, PGC adhesion and migration can be reserved if mesentery is treated with antibodies against Xenopus fibronectin. Therefore, pathway for germ cell migration in these frogs seems to be made up of the oriented fibronectin-containing extracellular matrix. Primordial germ cells of urodele amphibians (salamanders) have actually different origin that has been traced by reciprocal transplantation experiments to regions of mesoderm which involute through ventrolateral lips of blastopore. Furthermore, there doesn't appear to be any specific localized germ plasm in salamander eggs. Rather, interaction of dorsal endoderm cells and animal hemisphere cells creates conditions required to form germ cells in areas which involute through ventrolateral lips.
Germ cell migration in mammal:
There is no obvious germ plasm in mammals, and mammalian germ cells aren't morphologically distinct in early development. Though, by using monoclonal antibodies which identify cell surface differences between PGCs and their surrounding cells, localized the region to area which becomes extraembryonic mesoderm just posterior to primitive streak of 7-day mouse embryo. Here, approx eight large, alkaline phosphatase-staining cells are seen. If this region is removed, remaining embryo becomes devoid of germ cells, while isolated region develops the large number of PGCs. In normal mouse embryos, germ cell precursors migrate from extraembryonic mesoderm back in embryo, by way of allantois. Route of mammalian PGC migration from allantois resembles that of anuran PGC migration. PGCs then move caudally from yolk sac through newly formed hindgut and up the dorsal mesentery into genital ridge. Most of the PGCs have reached developing mouse gonad by eleventh day after fertilization. Like PGCs of Xenopus, mammalian PGCs appear to be strongly related with cells over which they migrate, and they move by extending filopodia over the underlying cell surfaces. These cells are also capable of penetrating cell monolayers and migrating through the cell sheets. Mechanism by which PGCs know route of this journey is still unknown.
Germ cell migration in birds and reptiles:
In birds and reptiles, primordial germ cells are derived from epiblast cells which migrate from central region of area pellucida to the crescent-shaped zone in hypoblast at anterior border of area pellucida. This extraembryonic region is known as germinal crescent, and PGCs multiply there. Unlike those of amphibians and mammals, PGCs of birds and reptiles migrate to gonads mainly through bloodstream. When blood vessels form in germinal crescent, PGCs enter those vessels and are carried by circulation to region where hindgut is forming. Here, they exit from circulation, become related with mesentery, and migrate in genital ridges. PGCs of germinal crescent appear to enter blood vessels by diapedesis, a kind of movement common to lymphocytes and macrophages which allows cells to squeeze between endothelial cells of small blood vessels.
Germ cell migration in Drosophila:
During Drosophila embryogenesis, primordial germ cells move from posterior pole to the gonads. First step in this migration is passive one, in which 30 40 pole cells are displaced in posterior midgut by the movements of gastrulation. In second step, gut endoderm triggers active amoeboid movement in PGCs that travel through blind end of posterior midgut, migrating in visceral mesoderm. In the third step, PGCs split in two groups, each of which will become related with developing gonad primordium.
In fourth step, PGCs migrate to gonads, that are derived from lateral mesoderm of parasegments 10-12. This step involves both attraction and repulsion. Product of wunen gene seems to be responsible for directing migration of PGCs from endoderm in mesoderm. This protein is expressed in endoderm immediately before PGC migration, and it repels PGCs. In loss- of-function mutants of this gene, PGCs wander randomly. Another gene required for proper migration of Drosophila PGCs is the product of Columbus gene.
Embryonic germ (EG) cells:
Stem cell factor increases proliferation of migrating mouse primordial germ cells in culture, and this proliferation can be more increased by adding another development factor, leukemia inhibition factor (LIF). Though, life span of these PGCs is short, and cells soon die. But if the extra mitotic regulator basic fibroblast growth factor (FGF2) is added, extraordinary change occurs. Cells continue to proliferate, producing pluripotent embryonic stem cells with features resembling cells of the inner cell mass. These PGC-derived cells are known as embryonic germ (EG) cells, and they have potential to distinguish in all the cell types of body. EG cells are frequently considered as embryonic stem (ES) cells and distinction of their origin is ignored.
Embryonic stem (ES) cells:
Embryonic stem (ES) cells are cells which are derived from inner cell mass. ES cells and EG cells can be transfected with recombinant genes and inserted in blastocyst to develop transgenic mice. Such a mammalian germ cell or stem cell has within it all information required for subsequent development. In one kind of tumor, germ cells become embryonic stem cells, like FGF2-treated PGCs in experiment above. This kind of tumor is known as a teratocarcinoma. Whether spontaneous or experimentally produced, teratocarcinoma has undifferentiated stem cell population which has biochemical and developmental properties extraordinarily like those of inner cell mass. Furthermore, these stem cells not only split, but can also distinguish in wide variety of tissues, comprising gut and respiratory epithelia, nerve, muscle, cartilage, and bone. Once distinguished, these cells no longer split, and are thus no longer malignant. Such tumors give rise to most of the tissue types in the body. Therefore, teratocarcinoma stem cells mimic early mammalian development, but tumor they form is illustrated by chance, haphazard development.
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