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1、Developmental Biology,Chapter 3:The germ cell development and fertilization,The germ cell development and fertilization,How are the germ cells specified and determined?How are the primordial germ cells differentiated into eggs and sperm?(including meiosis)Fertilization and activation of the egg by t
2、he sperm that initiates the embryonic development.,The germ cell development and fertilization,1 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 Spe
3、rmatogenesis2.2 Oogenesis3 Fertilization3.1 The recognition of egg and sperm3.2 The prevention of polyspermy3.3 The activation of egg metabolism,The germ cell development and fertilization,1 The germ cell fate determination and development of gonads1.1 Specification and determination of the primordi
4、al 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 metabolism,Specification and determination of the PGCs,Gametes in all se
5、xually reproducing organisms arise from the primordial germ cells(PGCs)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 determinants
6、 are collectively referred to as the germ plasm（生殖质）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 granules in X
7、enopus,The germ cell fate of C.elegans is determined at the 16-cell embryo,The adult gonads and early development of the fertilized egg in C elegans,The cell lineage chart in the development of C elegans,558 cells present in the newly hatched larva,959 somatic cells in the adult,Origin of the PGCs i
8、n Drosophila,In Drosophila,the PGCs form as a group of pole cells at the posterior pole of the cellularizing blastoderm.The pole plasm（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.Tho
9、se are mRNA of gcl(germ cell less)gene,Oskar,Nanos,Vasa,mitochondrial ribosomal RNA(mtr RNA)etc.,Origin of the PGCs in Drosophila,The 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 plas
10、m and is distinguished by large organelles,the polar granules,which contains both proteins and mRNAs,The molecular mechanisms underlying the specification and determination of the PGCs in Drosophila(I),In Drosophila,the PGCs form as a group of pole cells at the posterior pole of the cellularizing bl
11、astoderm.The pole plasm（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.,In Drosophila,
12、several maternal gene(at least 8 genes)are functionally necessary and sufficient for the pole plasm formation,and the germ cell specification/determination Mutations in any of those genes,e.g oskar,result in the affected homozygous individual being“grandchildless”.Its offsprings(F1)lack a proper pol
13、e plasm,and although they may 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
14、gene oskar is sufficient to specify the germ cells in Drosophila,Functionally conserved genes in Germ-Cell Development,Origin of the PGCs in mammals,There is no obvious germ plasm in mammals,and mammalian germ cells are not morphologically distinct during early development.Rather,germ cells are indu
15、ced in gastrulating embryos 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 embryos,The molecular mechanisms underlying the sp
16、ecification and determination of the PGCs,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.In mice,At day 6.5 of embryonic development,BMP4 and BMP8b from the extraembr
17、yonic ectoderm give certain cells in this area the ability to produce germ cells.,Germ cell specification in the mouse involves cell-cell interactions,Fig 11.3 Germ-cell formation in the mouse.The precursors(white)of primordial germ cells(PGCs)and extra-embryonic mesoderm are induced in the proximal
18、 epiblast by signals from the extra-embryonic 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 mesod
19、erm.After their formation,the PGCs migrate to the gonads.,BMP4 is required for the generation of primordial germ cells in the mouse embryos,A: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
20、 with the wild type;D:Homozygous embryo.PGCs are entirely absent in the hindgut,The germ cell development and fertilization,1 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
21、 gonads2 Gametogenesis2.1 Spermatogenesis2.2 Oogenesis3 Fertilization3.1 The recognition of egg and sperm3.2 The prevention of polyspermy3.3 The activation of egg metabolism,In many animals,germ cells develop at some distance from the gonads,and only later migrate to them,where they differentiate in
22、to eggs and sperm.In the mouse gastrula,germ cells first become detectable at the posterior region of the epiblast（上胚层）.They become incorporated into the hindgut and then move from gut tube into the genital ridge.The mechanisms by which the PGCs know the route of migrating journey is still unknown.F
23、ibronectin is likely to be an important substrate for PGC migration.In vitro evidence suggests that TGF-beta like protein is capable of attracting mouse PGCs.2 genes,White spotting(W)and Steel,are involved in controlling proliferation of migrating germ cells.,Germ cell migration and the underlying m
24、echanisms(I),In Drosophila,the PGCs move from the posterior pole to the gonads in a manner similar to that of mammalian germ cells.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 fo
25、r the attracting the Drosophila PGCs to the gonads.,Germ cell migration and the underlying mechanisms(II),Pathway for the migration of mammalian PGCs,Gonads 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 1
26、0-100 cells to 2500-5000 cells in 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 testes,columbus gene is necessary for PGCs to migrate to gonads,a-d:anti-Vasa labels the
27、 PGCs:e,f:anti-Vasa labels the PGCs(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 associ
28、ate with the gonadal mesoderm(e),and instead scatter widely in the embryo(c),Ectopic expression of columbus gene is sufficient to attract PGCs to new locations,a:the embryo was immunostained with anti-b-galactosidase.b-e:the embryos were immunostained with anti-Vasa,The 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),
