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1、The 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 by the sperm that initiates the embryonic development.第1页/共28页The germ cell
2、 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 Spermatogenesis2.2 Oogenesis3 Fertilization3.1 The recognition of eg
3、g and sperm3.2 The prevention of polyspermy3.3 The activation of egg metabolism第2页/共28页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 developm
4、ent 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第3页/共28页Specification and determination of the PGCsn Gametes in all sexually reproducing organisms arise from the primord
5、ial 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 determinants are collectively referred to as the germ plasm（生
6、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 granules in Xenopus第4页/共28页The germ cell fate of C.elegans i
7、s determined at the 16-cell embryo第5页/共28页The adult gonads and early development of the fertilized egg in C elegans第6页/共28页The cell lineage chart in the development of C elegans558 cells present in the newly hatched larva,959 somatic cells in the adult第7页/共28页Origin of the PGCs in Drosophilan In Dro
8、sophila,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.n The pole plasm includes several components that are crucial for the PGC determination.Those are mRNA of gc
9、l(germ cell less)gene,Oskar,Nanos,Vasa,mitochondrial ribosomal RNA(mtr RNA)etc.第8页/共28页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 plasm and is di
10、stinguished by large organelles,the polar granules,which contains both proteins and mRNAs第9页/共28页第10页/共28页The 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 cellulariz
11、ing 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.Those are mRNA of gcl(germ cell less)gene,Oskar,Nanos,Vasa,mitochondrial ribosomal RNA(mtr RNA)etc.第11页/共28页
12、n In Drosophila,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
13、 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)第12页/共28页The gene oskar i
14、s sufficient to specify the germ cells in Drosophila第13页/共28页Functionally conserved genes in Germ-Cell Development 第14页/共28页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 ar
15、e induced in gastrulating embryos n 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.第15页/共28页The mammalian PGCs appear to be induced in the early gastrulating embryos第16页/共28页The molecular mech
16、anisms underlying the specification and determination 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 a
17、nd BMP8b from the extraembryonic ectoderm give certain cells in this area the ability to produce germ cells.第17页/共28页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 a
18、re induced in the proximal 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 cell
19、s as extra-embryonic mesoderm.After their formation,the PGCs migrate to the gonads.第18页/共28页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 mut
20、ant.There are fewer PGCs compared with the wild type;D:Homozygous embryo.PGCs are entirely absent in the hindgut第19页/共28页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 Th
21、e 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第20页/共28页n In many animals,germ cells develop at some distance from the gonads,and only
22、 later migrate to them,where they differentiate into eggs and sperm.n 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.n The mechanisms by which the PGCs
23、know the route of migrating journey is still unknown.Fibronectin 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.n 2 genes,White spotting(W)and Steel,are involved in controlling proliferation of migrat
24、ing germ cells.Germ cell migration and the underlying mechanisms(I)第21页/共28页n In Drosophila,the 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
25、the mesoderm;2 genes,columbus and hedgehog,appear to be critical for the attracting the Drosophila PGCs to the gonads.Germ cell migration and the underlying mechanisms(II)第22页/共28页Pathway for the migration of mammalian PGCs第23页/共28页Gonads in mouse embryos at day 13.5 stained with antibodies to E-cad
26、herin(red)and laminin(green)Proliferation of PGCs from an initial population of 10-100 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第
27、24页/共28页第25页/共28页columbus gene is necessary for PGCs to migrate to gonadsa-d:anti-Vasa labels the 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 asso
28、ciated with the endoderm(a).In addition,many PGCs that do migrate to the mesoderm fail to associate with the gonadal mesoderm(e),and instead scatter widely in the embryo(c)第26页/共28页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)第27页/共28页感谢您的观看！第28页/共28页