Gametogenesis
Once upon a time….
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
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 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
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
Stage 10 embryo
Cellular blastoderm embryo
Sense 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
homologues
• Synapsis – tetrad
(bivalent) formation
• Synapsis requires the
synaptonemal complex
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
• Stages of Meiosis • Chiasmata visible at points of
crossing over
• Prophase I
– Leptotene
– Zygotene
– Pachytene
– Diplotene
• Synaptonemal complex
completely broken down and
homologous chromosomes
separate
• Transcription occurs –
“lampbrush” chromosomal
appearance
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
1o spermatocytes
undergo meiosis I
2o 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
Oogenesis
1st polar body
Primary
oocyte
arrested in
diplotene of
prophase I
Completion
of meiosis I
entry into
meiosis II
Secondary
oocyte
arrested in
metaphase II
1st polar body
Completion
of meiosis II
2nd polar body
Activation or
Fertilization
Mature egg
Oogenesis
Formation of polar bodies
Mammalian Secondary Oocyte
1st Polar body
Cell Cycle Regulation During
Oogenesis
Prophase I