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 1 st 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 1 st 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 Cleavage stage embryo Cellular blastoderm embryo Stage 10 embryoSense 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. Meiosis: A Review 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

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

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

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

18. Meiosis: A Review 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

19. Diplotene Chromosomes

20. Meiosis: A Review 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

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

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 1 o spermatocytes undergo meiosis I 2 o spermatocytes undergo meiosis II Spermatids mature into sperm

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

31. Oogenesis Formation of polar bodies

32. Mammalian Secondary Oocyte 1 st Polar body

33. Cell Cycle Regulation During Oogenesis Prophase I