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 Boveri observed chromosome diminution in somatic cells Hypothesized that the factors in cytoplasm of germ cells prevented diminution

Boveri’s Experiments on Germ Plasm in Parascis spp

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

Pole Cell Movements Fig 19.12

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

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

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

Germ Plasm in Xenopus

Mammalian Primordial Germ Cells

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

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

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

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

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

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

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

Diplotene Chromosomes

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

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

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

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

Sex Switch in C. elegans Hermaphrodite

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

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

Mammalian Spermatogenesis 1 type A = 16 type B = 64 spermatids Meiosis I Meiosis II

Mammalian Sperm

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

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

Oogenesis Formation of polar bodies

Mammalian Secondary Oocyte 1 st Polar body

Cell Cycle Regulation During Oogenesis Prophase I