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1、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 the sperm that initiates the embryonic development.,The germ cell developm
2、ent and fertilization,1 The germ cell fate determination and development of gonads 1.1 Specification and determination of the primordial germ cells (PGCs) 1.2 The germ cell migration and development of gonads 2 Gametogenesis 2.1 Spermatogenesis 2.2 Oogenesis 3 Fertilization 3.1 The recognition of eg
3、g and sperm 3.2 The prevention of polyspermy 3.3 The activation of egg metabolism,The germ cell development and fertilization,1 The germ cell fate determination and development of gonads 1.1 Specification and determination of the primordial germ cells (PGCs) 1.2 The germ cell migration and developme
4、nt of gonads 2 Gametogenesis 2.1 Spermatogenesis 2.2 Oogenesis 3 Fertilization 3.1 The recognition of egg and sperm 3.2 The prevention of polyspermy 3.3 The activation of egg metabolism,Specification and determination of the PGCs,Gametes in all sexually reproducing organisms arise from the primordia
5、l 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 are collectively referred to as the germ pla
6、sm（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 granules P granules in C. elegans Polar granules in Drosophila Germinal granules in Xenopus,The germ cell fate of C. elegans
7、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 in Drosophila,In Drosophila, the PGCs f
8、orm 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. Those are mRNA of gcl (germ cell les
9、s) 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 plasm and is distinguished b
10、y 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 blastoderm. The pole p
11、lasm （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, several
12、 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 p
13、ole 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 (I
14、I),The 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 cell
15、s are induced 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 un
16、derlying the specification 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 BMP8
17、b from the extraembryonic 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 ar
18、e 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 ce
19、lls as extra-embryonic mesoderm. 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 m
20、utant. There are fewer PGCs compared 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 gonads 1.1 Specification and determination of the primordial germ cells (PGCs) 1.2
21、The germ cell migration and development of gonads 2 Gametogenesis 2.1 Spermatogenesis 2.2 Oogenesis 3 Fertilization 3.1 The recognition of egg and sperm 3.2 The prevention of polyspermy 3.3 The activation of egg metabolism,In many animals, germ cells develop at some distance from the gonads, and onl
22、y later migrate to them, where they differentiate into 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 PGC
23、s 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. 2 genes, White spotting (W) and Steel, are involved in controlling proliferation of
24、 migrating germ cells.,Germ cell migration and the underlying mechanisms (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 t
25、he 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),Pathway for the migration of mammalian PGCs,Gonads in mouse embryos at day 13.5 stained with antibodies to E-cadherin (red)
26、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 embryos Note: 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,columb
27、us gene is necessary for PGCs to migrate to gonads,a-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 associated with
28、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),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),