Gametogenesis and Fertilization

1
Gametogenesis and Fertilization
2
Gametogenesis
Gametogenesis - the process by which gametes are
produced
Oogenesis
Spermatogenesis
3
Gametogenesis
Gametogenesis - all gametogenesis follows the
same general sequence
Germ cell
2N
Mitosis to produce more germ cells
Onset of meiosis I
Primary gametocyte (meiosis I prophase)
2N
Complete meiosis I
N
N
Secondary gametocyte
Meiosis II
Gamete
N
N
N
N
4
Oogenesis
Oogonium
2N
Mitosis to produce more germ cells
Onset of meiosis I
Primary oocyte (meiosis I prophase - arrested -
present at birth)
2N
Complete meiosis I
First polar body
N
Secondary oocyte (arrested at metaphase of
meiosis II)
Ovulation
Ovum
N
Second polar body
5
Spermatogenesis
Spermatogonium
2N
Mitosis to produce more germ cells
Onset of meiosis I
Primary spermatocyte (meiosis I prophase)
2N
Complete meiosis I
N
N
Secondary spermatocyte
Meiosis II
N
N
N
N
Early spermatids
Differentiation
Spermatozoa
6
Spermatogenesis and oogenesis
Where does it occur??
spermatozoa
Spermatogenesis
spermatids
secondary spermatocyte
primary spermatocyte
spermatogonia
7
Spermatogenesis and oogenesis
Where does it occur??
Oogenesis
8
Fertilization
Sperm Structure
From drawing done in 1694
9
Fertilization
Sperm Structure
actin
nucleus
mitochondria
acrosome
tail
10
Fertilization - Egg structure
Jelly coat
Cortical granules
Egg cytoplasm
Plasma membrane
Vitelline layer
Sperm binding receptor
11
Events in Fertilization
4
5
3
2
6
1

• Head of the sperm contacts the jelly coat of the
  egg
• - triggers the release of hydrolytic enzymes
  which dissolve the jelly coat and starts the
  acrosomal reaction

12
Events in Fertilization
4
5
3
2
6
1
2. The Acrosomal Reaction
-hydrolytic enzymes dissolve a hole in the jelly
coat
-actin fibres begin elongate and form the
acrosomal process
13
Animation of Acrosome Reaction
14
Events in Fertilization
4
5
3
2
6
1
3. The Acrosomal Reaction
- the acrosomal process elongates further and
binds with the sperm binding receptors
-this binding causes the vitelline membrane to
begin to break down
15
Events in Fertilization
4
5
3
2
6
1
4. Membrane fusion
- membranes of the sperm and egg fuse
- fast block to polyspermy
- causes depolarization of the membrane which
turns off the sperm binding receptors
16
Events in Fertilization
4
5
3
2
6
1
5. Sperm nucleus and beginning of cortical
reaction
- fusion of membranes causes an increase in Ca
release
- Ca release causes cortical granules to fuse
with eggs plasma membrane and empty contents
into perivitelline space
- slow block to polyspermy
- sperm nucleus enters egg cytoplasm
17
Calcium (Ca) Distribution
18
Events in Fertilization
4
5
3
2
6
1
6. Continuation of Cortical Reaction and
Activation of Egg
- conversion of vitelline layer into
fertilization membrane

• increase in cellular respiration and protein
  synthesis in egg
• (egg activation)

19
Events in Fertilization in Mammals
follicle cells
zona pellucida
Differences - zona pellucida hardens - entire
sperm (head tail) enters egg
20
Summary of Events in Fertilization in Sea Urchins
Sperm approaches contacts eggs jelly coat
Hydrolytic enzymes released from sperms acrosome
vesicle
Hydrolytic enzymes dissolve jelly coat and allow
sperm entry
Actin fibres elongate and form acrosomal process
Acrosomal process binds with sperm binding
receptors
Binding causes the vitelline membrane to break
down
Membranes of sperm and egg fuse
Membrane depolarization turns off sperm binding
receptors
Vitelline membrane becomes fertilization membrane
Increase in Ca
Cortical granules fuse with plasma membrane and
empty contents into perivitelline space
Egg activation
Sperm and egg nuclei fuse
Sperm nucleus enters egg cytoplasm
21
Summary of Events in Fertilization in Sea Urchins
Sperm approaches contacts eggs jelly coat
Hydrolytic enzymes released from sperms acrosome
vesicle
Hydrolytic enzymes dissolve jelly coat and allow
sperm entry
Actin fibres elongate and form acrosomal process
Acrosomal Reaction
Acrosomal process binds with sperm binding
receptors
Binding causes the vitelline membrane to break
down
Membranes of sperm and egg fuse
Membrane depolarization turns off sperm binding
receptors
Vitelline membrane becomes fertilization membrane
Increase in Ca
Cortical granules fuse with plasma membrane and
empty contents into perivitelline space
Egg activation
Sperm and egg nuclei fuse
Sperm nucleus enters egg cytoplasm
22
Summary of Events in Fertilization in Sea Urchins
Sperm approaches contacts eggs jelly coat
Hydrolytic enzymes released from sperms acrosome
vesicle
Hydrolytic enzymes dissolve jelly coat and allow
sperm entry
Actin fibres elongate and form acrosomal process
Acrosomal Reaction
Acrosomal process binds with sperm binding
receptors
Binding causes the vitelline membrane to break
down
Cortical Reaction
Membranes of sperm and egg fuse
Membrane depolarization turns off sperm binding
receptors
Vitelline membrane becomes fertilization membrane
Increase in Ca
Cortical granules fuse with plasma membrane and
empty contents into perivitelline space
Egg activation
Sperm and egg nuclei fuse
Sperm nucleus enters egg cytoplasm
23
Timing in Fertilization
Fertilization envelope complete egg
activation (1 min)
Ca increases (10 sec)
Cortical reaction (20 sec)
Binding of sperm and egg (1 sec)
Acrosomal reaction (2 sec)
Fertilization envelope complete egg
activation (1 min)
Fusion of sperm and egg nuclei (20 min)
Onset of DNA synthesis (40 min)
First cell division (90 min) (Beginning of
development)
24
Some good websites
Calcium release and raising fertilization envelope
http//worms.zoology.wisc.edu/urchins/SUfert_slowb
lock2.html
Animations of all aspects of fertilization and
development
http//www.stanford.edu/group/Urchin/contents.html