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1、Developmental BiologyChapter 3:The germ cell development and fertilizationThe germ cell development and fertilizationn How are the germ cells specified and determined?n How are the primordial germ cells differentiated into eggs and sperm?(including meiosis)n Fertilization and activation of the egg b
2、y the sperm that initiates the embryonic development.The germ cell development and fertilization1 The germ cell fate determination and development of gonads1.1 Specification and determination of the primordial germ cells(PGCs)1.2 The germ cell migration and development of gonads2 Gametogenesis2.1 Sp
3、ermatogenesis2.2 Oogenesis3 Fertilization3.1 The recognition of egg and sperm3.2 The prevention of polyspermy3.3 The activation of egg metabolismThe germ cell development and fertilization1 The germ cell fate determination and development of gonads1.1 Specification and determination of the primordia
4、l germ cells(PGCs)1.2 The germ cell migration and development of gonads2 Gametogenesis2.1 Spermatogenesis2.2 Oogenesis3 Fertilization3.1 The recognition of egg and sperm3.2 The prevention of polyspermy3.3 The activation of egg metabolismSpecification and determination of the PGCsn Gametes in all sex
5、ually reproducing organisms arise from the primordial germ cells(PGCs)n In many instances(including nematodes,flies,and frogs),the PGCs are specified and determined autonomously by the cytoplasmic determinants in the egg that are comprised of specific proteins and mRNAs.These cytoplasmic determinant
6、s are collectively referred to as the germ plasm（生殖质生殖质）n Germ plasm can be identified morphologically by the presence of conspicuous membrane-unbound organelles with an electron-dense granulofibrillar appearance called germ granulesP granules in C.elegansPolar granules in DrosophilaGerminal granule
7、s in XenopusThe germ cell fate of C.elegans is determined at the 16-cell embryoThe adult gonads and early development of the fertilized egg in C elegansThe cell lineage chart in the development of C elegans558 cells present in the newly hatched larva,959 somatic cells in the adultOrigin of the PGCs
8、in Drosophilan In Drosophila,the PGCs form as a group of pole cells at the posterior pole of the cellularizing blastoderm.The pole plasm（极质极质）at the posterior pole is critical for determination of the PGCs in flies.n The pole plasm includes several components that are crucial for the PGC determinati
9、on.Those are mRNA of gcl(germ cell less)gene,Oskar,Nanos,Vasa,mitochondrial ribosomal RNA(mtr RNA)etc.Origin of the PGCs in DrosophilaThe PGCs known as pole cells become distinct at the posterior pole of the egg about 90 minutes after fertilization.The cytoplasm at the posterior pole is called pole
10、plasm and is distinguished by large organelles,the polar granules,which contains both proteins and mRNAsThe molecular mechanisms underlying the specification and determination of the PGCs in Drosophila(I)n In Drosophila,the PGCs form as a group of pole cells at the posterior pole of the cellularizin
11、g blastoderm.The pole plasm（极质极质）at the posterior pole is critical for determination of the PGCs in flies.The pole plasm includes several components that are crucial for the PGC determination.Those are mRNA of gcl(germ cell less)gene,Oskar,Nanos,Vasa,mitochondrial ribosomal RNA(mtr RNA)etc.n In Dros
12、ophila,several maternal gene(at least 8 genes)are functionally necessary and sufficient for the pole plasm formation,and the germ cell specification/determination oskar,result in the affected homozygous individual being“grandchildless”.Its offsprings(F1)lack a proper pole plasm,and although they may
13、 develop normally in other ways,they lack germ cells and therefore are sterile(F2).Expression of oskar alone is sufficient for ectopically specifying the germ cells The molecular mechanisms underlying the specification and determination of the PGCs in Drosophila(II)The gene oskar is sufficient to sp
14、ecify the germ cells in DrosophilaFunctionally conserved genes in Germ-Cell Development Origin of the PGCs in mammalsn There is no obvious germ plasm in mammals,and mammalian germ cells are not morphologically distinct during early development.Rather,germ cells are induced in gastrulating embryos n
15、In mice,the germ cells form at the posterior region of the epiblast,at the junction of the extraembryonic ectoderm,epiblast,primitive streak,and allantois.The mammalian PGCs appear to be induced in the early gastrulating embryosThe molecular mechanisms underlying the specification and determination
16、of the PGCsn There is no evidence for germ plasm being involved in germ cell formation in the mouse or other mammals.Instead,germ cell specification in the mouse involves cell-cell interactions.n In mice,At day 6.5 of embryonic development,BMP4 and BMP8b from the extraembryonic ectoderm give certain
17、 cells in this area the ability to produce germ cells.Germ cell specification in the mouse involves cell-cell interactions Germ-cell formation in the mouse.The precursors(white)of primordial germ cells(PGCs)and extra-embryonic mesoderm are induced in the proximal epiblast by signals from the extra-e
18、mbryonic ectoderm that include BMP-4.During gastrulation,these cells move to the posterior end of the embryo above the primitive streak.Here they form a cluster in which the central cells becomes specified as PGCs and the peripheral cells as extra-embryonic mesoderm.After their formation,the PGCs mi
19、grate to the gonads.BMP4 is required for the generation of primordial germ cells in the mouse embryosA:Wild type embryo;B:High magnification of part of A showing individual PGCs in the hindgut;C:Embryos heterozygous for BMP4 mutant.There are fewer PGCs compared with the wild type;D:Homozygous embryo
20、.PGCs are entirely absent in the hindgutThe germ cell development and fertilization1 The germ cell fate determination and development of gonads1.1 Specification and determination of the primordial germ cells(PGCs)1.2 The germ cell migration and development of gonads2 Gametogenesis2.1 Spermatogenesis
21、2.2 Oogenesis3 Fertilization3.1 The recognition of egg and sperm3.2 The prevention of polyspermy3.3 The activation of egg metabolismn In many animals,germ cells develop at some distance from the gonads,and only later migrate to them,where they differentiate into eggs and sperm.n In the mouse gastrul
22、a,germ cells first become detectable at the posterior region of the epiblast（上胚层上胚层）.They become incorporated into the hindgut and then move from gut tube into the genital ridge.n The mechanisms by which the PGCs know the route of migrating journey is still unknown.Fibronectin is likely to be an imp
23、ortant substrate for PGC migration.In vitro evidence suggests that TGF-beta like protein is capable of attracting mouse PGCs.n 2 genes,White spotting(W)and Steel,are involved in controlling proliferation of migrating germ cells.Germ cell migration and the underlying mechanisms(I)n In Drosophila,the
24、PGCs move from the posterior pole to the gonads in a manner similar to that of mammalian germ cells.n The wunen gene appears to be responsible for directing the migration of the PGCs from the endoderm to the mesoderm;2 genes,columbus and hedgehog,appear to be critical for the attracting the Drosophi
25、la PGCs to the gonads.Germ cell migration and the underlying mechanisms(II)Pathway for the migration of mammalian PGCsGonads in mouse embryos at day 13.5 stained with antibodies to E-cadherin(red)and laminin(green)Proliferation of PGCs from an initial population of 10-100 cells to 2500-5000 cells in
26、 the gonads by day 12 of mouse embryosNote:The PGCs of both sexes express high levels of E-cadherin and are arranged in cortical clusters in the ovary and in internal cord in the testescolumbus gene is necessary for PGCs to migrate to gonadsa-d:anti-Vasa labels the PGCs:e,f:anti-Vasa labels the PGCs
27、(purple);anti-Clift labels gonadal mesodermal cells(brown)In columbus mutant embryos,some PGCs fail to migrate from the endoderm to the mesoderm,and instead remain associated with the endoderm(a).In addition,many PGCs that do migrate to the mesoderm fail to associate with the gonadal mesoderm(e),and
28、 instead scatter widely in the embryo(c)Ectopic expression of columbus gene is sufficient to attract PGCs to new locationsa:the embryo was immunostained with anti-b-galactosidase.b-e:the embryos were immunostained with anti-VasaThe Gal4/Uas binary system was employed to ectopically express columbus gene in specific tissues including abdominal segments(a,b),epidermis(c),the nervous system(d)and the PGCs(e)