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Germ Cells

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  1. Germ Cells • Specification • Migration • Meiosis • Gamete Formation

  2. Germ Cell Specification: Germ Plasm • Theodor Boveri (1862-1915) • Centrifugation and displacement of first cleavage.

  3. Germ cell specification: Drosophila • Pole cells • germ cell-less (gcl) • polar granule component (pgc) • Oskar, Vasa, Aubergine

  4. Germ cell specification: Drosophila Fig 16.4

  5. Germ cell specification: Frogs/Fish • Germ Plasm located in vegetal pole. • Nanos • Vasa

  6. Germ cell specification: mammals • Wnts make cells competent for BMPs • BMPs Induce blimp1 and prdm14 • Regulate Sox2, Nanog, and Nanos3.

  7. Cell Migration: Drosophila • Movement into posterior midgut • Gut endoderm triggers migration into visceral mesoderm • PGCs split, each will associate with developing gonad. • Germ cells migrate to gonads. • Attraction/repulsion • HMG-CoAreductase

  8. Cell Migration: Drosophila • Dpp- BMP2/4 homolog • Bag of Marbles (Bam)

  9. Cell migration: Zebrafish • Sdf1 and CXCR4 Figure 16.12

  10. Cell migration: Frogs Fig. 16.13

  11. Cell migration: Mammals Fig. 16.14

  12. Cell migration: Bird and Reptiles • Germinal crescent • Sdf1/CXCR4 Fig. 16.15 and 8.9

  13. Cell migration: Bird and Reptiles • Germinal crescent • Sdf1/CXCR4 Fig. 16.15 and 8.9

  14. Meiosis

  15. Prophase I • Chromatin condenses • Spindle forms • Nuclear membrane breaks down • Homologous chromosomes pair • Chromosomes attach to spindle as tetrads

  16. Homologous pairs form tetrads • While chromosomes are paired, crossing-over can occur Tetrad

  17. Metaphase I • Tetrads line up at equator • Homologous chromosomes face opposite poles

  18. Anaphase I • Homologous chromosomes separate • Pulled apart by spindle shortening • Sister chromatids stay together

  19. Haploid cell Telophase I • Chromosomes reach poles • Nucleus may re-form • Spindle breaks down • Cell divides (cytokinesis) • Two haploid daughter cells (N chromosomes) Haploid cell

  20. Meiosis I • Daughters have one chromosome from each pair • Haploid: no homologous pairs • Still two copies of each chromosome (sister chromatids) diploid cell: N = 2 2N = 4 haploid cell: 2 chromosomes

  21. Meiosis II • Very similar to mitosis • Sister chromatids separate Cytokinesis Telophase I Interphase II Prophase II Metaphase II Anaphase II Telophase II

  22. Results of meiosis • Four haploid gametes • One chromosome set each • Genetically different A b b A A A b a A b a b B B a B B a a B

  23. Gamete Maturation: Frogs • Vitellogenin • Progesterone Fig. 16.22 and 16.25

  24. Gametogenesis in Mammals Female oogenesis Male spermatogenesis Meiosis initiated continuously in mitotic stem cell Four gametes produced Meiosis completed in days or weeks Meiosis and differentiation proceed continuously w/o cell cycle arrest Differentiation of gamete occurs while haploid Sex chromosomes excluded from recombination and transcription • Meiosis initiated once in finite population of cells • One gamete produced • Completion of meiosis delayed months/years • Meiosis arrested in first prophase. • Differentiation of gamete occurs while diploid • All chromosomes exhibit equivalent transcription

  25. Gametogenesis in Mammals • Fig. 16.26

  26. Gametogenesis in Mammals • Fig 16.27

  27. Spermiogenesis

  28. Mammalian Oogenesis • Gonadotropin-releasing hormone (GRH) • Follicle –stimulating and lutenizing hormones (FSH and LH)