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Gametogenesis

Gametogenesis. Once upon a time…. The Germ Plasm. Embryos must establish the germ cells as cells which never differentiate until gametogenesis occurs Germ plasm 1 st proposed and studied by Theodore Boveri . Germ Plasm Prevents Chromosome Diminution.

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Gametogenesis

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  1. Gametogenesis Once upon a time….

  2. The Germ Plasm • Embryos must establish the germ cells as cells which never differentiate until gametogenesis occurs • Germ plasm 1st proposed and studied by Theodore Boveri

  3. Germ Plasm Prevents Chromosome Diminution • Boveri observed chromosome diminution in somatic cells • Hypothesized that the factors in cytoplasm of germ cells prevented diminution

  4. Boveri’s Experiments on Germ Plasm in Parascis spp

  5. Germ Plasm & Pole Cells of Drosophila • Developmental mutants in Drosophila • Pole cells are 1st group of nuclei to cellularize and separate from syncytium • Arise at posterior pole of embryo and are moved into interior by germ band extension

  6. Pole Cell Movements Fig 19.12

  7. Drosophila Pole Cells • Specification and proper formation of pole cells dissected through mutagenic screens • Several genes identified genetically • Nanos (nos), germ-cell-less (gcl), oskar (osk), vasa (vas) • Non-genetic factors – mtrRNA, Pgc • The combination of these factors makes the polar granules

  8. Localization of Pgc RNA to Pole Cell Granules Cellular blastoderm embryo Cleavage stage embryo Stage 10 embryo Sense Pgc probe control EM of Pgc in situ of polar granules

  9. Germ Plasm in Amphibians • Xenopus homologues of nos (Xcat2) and vas (XVLG1) have been identified • These mRNAs & proteins are localized to the vegetal-most region of the embryo

  10. Germ Plasm in Xenopus

  11. Mammalian Primordial Germ Cells

  12. Function of the Germ Plasm • Boveri’s hypothesis – prevents diminution • Diminution DOES NOT OCCUR in vertebrates (or most insects) • Blocks differentiation • Prevents transcription • Prevents translation • nos & vas– RNA binding proteins - block translation

  13. Gametogenesis • Process of producing gametes • Spermatogenesis • Oogenesis • Meiotic cell division • Packaging of material into oocytes • Removal of cytoplasm from sperm

  14. Stages of Meiosis I Prophase I Leptotene Zygotene Pachytene Diplotene Metaphase I Anaphase I Telophase I Stages of Meiosis II Prophase II Metaphase II AnaphaseII TelophaseII Meiosis: A Review

  15. Stages of Meiosis Prophase I Leptotene Zygotene Pachytene Diplotene DNA replication has finished Sister chromatids remain thin and closely associated Chromosomes just begin condensing Meiosis: A Review

  16. Stages of Meiosis Prophase I Leptotene Zygotene Pachytene Diplotene Synapsis – tetrad (bivalent) formation Synapsis requires the synaptonemal complex Meiosis: A Review homologues

  17. Stages of Meiosis Prophase I Leptotene Zygotene Pachytene Diplotene Condensation continues chromatids thicken Synaptonemal complex breaks down and crossing over occurs Meiosis: A Review

  18. Chiasmata visible at points of crossing over Synaptonemal complex completely broken down and homologous chromosomes separate Transcription occurs – “lampbrush” chromosomal appearance Stages of Meiosis Prophase I Leptotene Zygotene Pachytene Diplotene Meiosis: A Review

  19. Diplotene Chromosomes

  20. Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II Often oogenesis halts at the pro I –meta I boundary and/or during meta II Allows for the accumulation of proteins/mRNAs in oocyte Meiosis: A Review

  21. Mitosis or Meiosis? • Gametes arise from germ cells which are like stem cells and must be replenished by mitotic cell division • At some point cells must initiate meiotic cell division • Regulatory mechanisms must exist to determine which type of cell division occurs

  22. DTC expresses Lag-2 PGCs express Glp-1 C. elegans Gonad: Mitosis-Meiosis Switch

  23. Mitosis or Meiosis? • C. elegans gonad • Distal tip cell filipodia • expresses Lag-2 (Delta homologue) • Contact with PGCs prevents onset of meiosis • Express Glp-1 (Notch homologue) • As cells move away from DTC, meiosis begins

  24. Sex Switch in C. elegans Hermaphrodite

  25. Spermatogenesis • Two equal cytoplasmic meiotic divisions • Produces 4 equal sized gametes • Usually spermatozoa are motile

  26. Mammalian Spermatogenesis Spermatogonia divide mitotically 1o spermatocytes undergo meiosis I 2o spermatocytes undergo meiosis II Spermatids mature into sperm

  27. MammalianSpermatogenesis 1 type A = 16 type B = 64 spermatids Meiosis I Meiosis II

  28. Mammalian Sperm

  29. Oogenesis • More variable than spermatogenesis • Two major modes • Continuous oogenesis from a self-sustaining population of mitotically dividing oogonia • Basically like spermatogonia • Fish, amphibians, urchins, insects • A finite population of oogonia which begin meiosis, arrest in meiosis I and undergo maturation individually or in small groups • Mammals, reptiles, birds

  30. Oogenesis 1st polar body Primary oocyte arrested in diplotene of prophase I Secondary oocyte arrested in metaphase II Completion of meiosis I entry into meiosis II 1st polar body 2nd polar body Completion of meiosis II Mature egg Activation or Fertilization

  31. Oogenesis Formation of polar bodies

  32. Mammalian Secondary Oocyte 1st Polar body

  33. Cell Cycle Regulation During Oogenesis Prophase I

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