III生殖细胞发生与受精I.ppt

Specification and determination of the PGCs
Gametes in all sexually 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 are collectively referred to as the germ plasm（生殖质）
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 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 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
Mutations in any of those genes, e.g oskar, result in the affected homozygous individual being “grandchildless”. Its offsprings (F1) lack a proper pole plasm, and although they may develop normally in other ways, they lack germ cells and therefore are sterile (F2).
The molecular mechanisms underlying the specification and determination of the PGCs in Drosophila (II)
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
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 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
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 by large organelles, the polar granules, which contains both proteins and mRNAs
Expression of oskar alone is sufficient for ectopically spBaidu Nhomakorabeacifying the germ cells
The gene oskar is sufficient to specify the germ cells in Drosophila
Germ cell specification in the mouse involves cell-cell interactions
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 the sperm that initiates the embryonic development.
P granules in C. elegans Polar granules in Drosophila Germinal granules in Xenopus
The germ cell fate of C. elegans is determined at the 16-cell embryo
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 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
Germ plasm can be identified morphologically by the presence of conspicuous membrane-unbound organelles with an electrondense granulofibrillar appearance called germ granules
Origin of the PGCs in 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. Those are mRNA of gcl (germ cell less) gene, Oskar, Nanos, Vasa, mitochondrial ribosomal RNA (mtr RNA) etc.
The mammalian PGCs appear to be induced in the early gastrulating embryos
The molecular mechanisms underlying 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 BMP8b from the extraembryonic ectoderm give certain cells in this area the ability to produce germ cells.
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 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.