CHAPTER 19, THE SAGA OF THE GERM LINE ZOO3603 Development of germ cells  The germ cells do not arise within the gonad itself.  Their precursors, the primordial germ cells (PGCs), arise elsewhere and migrate into developing gonads. Fig. 17.2 The points of gametogenesis 1. The formation of germ plasm & the determination of the PGCs. 2. The migration of the PGCs into the developing gonads. 3. The process of meiosis & the modifications of meiosis for forming sperm & eggs. 4. The differentiation of the sperm & eggs. 5. The hormonal control of gamete maturation & ovulation.Formation of germ plasm and determination of PGCs  In many instances, the PGCs are specified autonomously by cytoplasmic determinants in the egg.  In other instances, the PGCs are specified by interactions among neighboring cells.  The determination caused by the autonomous localization of specific proteins and mRNAs, and these cytoplasmic components are the germ plasm. Germ cell determination in frogs and fishes  The germ plasm of unfertilized eggs consists of tiny “islands” tethered to the yolk mass near the vegetal cortex.  After the cortical rotation, the islands are released and fuse together.  Migrate to the vegetal pole.  The germ plasm of amphibians contains many of the RNA and proteins, which repress transcription and translation. Germ plasm at the vegetal pole of frog embryos In situ hybridization to Nanos, marker of PGC 1st cleavage 4th cleavage Localized to the vegetal region Germ plasm movement during zebrafish cleavage The germ plasm forms a nuage-like structure characterized by polar granules, mitochondria and concentrates mRNA At late cleavage, only four cells have the germ plasm Germ cell determination in mammals In mammals, there is no obvious germ plasm.  BMP4 & 8b from extra-embryonic ectoderm induce the precursor cells of PGCs express fragilis, trans membrane protein.Specification & migration of mammalian PGCs  During gastrulation, PGC precursor cells come to reside in the posterior epiblast.  The cells emerge from the posterior or primitive streak. Specification & migration of mammalian PGCs  The cluster of precursor PGCs express fragilis, trans membrane protein.  The small group of cells in the cluster expresses blimp1 & stella. Specification & migration of mammalian PGCs  Stella protein is involved in early cleavage event.  Bimp1 is a general transcriptional inhibitor; might be critical for specifying the PGCs.  Cells the express blimp1 are restricted the germ cell fate. The inert genome hypothesis  The components of the germ plasm might inhibit both transcription and translation.  This could prevent the cell from differentiation into anything else.  Many of the components in the germ plasm (i.e. Nanos & Vasa) are found in all groups of animals.  PGCs are specified outside of the embryo; are protected from paracrine factors in the embryo. The points of gametogenesis 1. The formation of germ plasm & the determination of the PGCs. 2. The migration of the PGCs into the developing gonads. 3. The process of meiosis & the modifications of meiosis for forming sperm & eggs. 4. The differentiation of the sperm & eggs. 5. The hormonal control of gamete maturation & ovulation.Specification and migration of germ cells in zebrafish In situ hybridization of Vasa mRNA Germ plasm along the cleavage planes during initial cleavages Specification and migration of germ cells in zebrafish The germ plasm remains in 4 clusters. Movement of the clusters is caused by attractive and repulsive, but the migration to the gonad is directed by the Sdf1 chemo-attractant. Migration of Xenopus germ plasm Germ plasm associates with the endoderm. Migration of Xenopus germ plasm A germ plasm-containing cell in endodermal region of blastula Migration of PGCs along the dorsal mesentery PGCs are accumulated in the posterior region of the larval gut; and migrate to dorsal side of the gut, and to the dorsal mesentery; and migrate along the abdominal wall and into the genital ridges. Mechanism directing to gonads is unknown. May be fibronectin & CXCR4? PGC migration in the mouse PGCs migrate directly into the endoderm from the posterior primitive streak, and are in the hingut Primoridal germ cell migration in the mouse From hindgut, PGCs migrate through the gut and into the genital ridges. Mechanism of migration into gonad is still unknown. Fibronectin? Sdf1? Fgf7 & Scf for PGC survival. The germinal crescent of the chick embryo PGCs are in the extra-embryonic region, germinal crescent PGCs Migration of primordial germ cells in the chick embryo A chic PGC in a capillary of a gastrulating embryo PGCs enter the vessels in the genital crescent and are carried by the hindgut with unknown mechanism. Migration of primordial germ cells in the chick embryo PGCs are instructed to exit the blood vessels by unknown mechanism, and enter the genital ridges by Sdf1/CXCR4 chemotactic system. The points of gametogenesis 1. The formation of germ plasm & the determination of the PGCs. 2. The migration of the PGCs into the developing gonads. 3. The process of meiosis & the modifications of meiosis for forming sperm & eggs. 4. The differentiation of the sperm & eggs. 5. The hormonal control of gamete maturation & ovulation.Characteristics of meiosis  The germ cells reduce their chromosomes from the diploid to the haploid condition <Meiosis>.  Two different points of meiosis from mitosis. 1. Meiotic cells undergo two sell divisions without an intervening period of DNA replication. 2. Homologous chromosomes pair together and recombine genetic material.  Meiosis is at the center of sexual reproduction.2.5 Gametogenesis --- Meiosis-I 1. Chromosome replication 2. Holding together by kinetochores (centromeres) The nuclear envelope breaks down and homologous chromosomes align in pairs. Chromatin: complex of DNA and protein Chromosomal rearrangements Five stages of prophase 1. Leptotene  Allthechromosomescondense.  Pairing.Homologousdyads(pairsofsisterchromatids) 2. Zygotene  Synapsis.Thesynaptonemalcomplexbeginstoform.  DNAstrandsofnon-sisterchromatidsbegintheprocessofrecombination.