Oogenesis
Behind every
successful
embryo stands
a hard-working
mother.
Eggs are
enormous cells
carrying nutrients for
the developing embryo
Why? Mass of egg must equal
the mass of the embryo
that emerges!
Even human eggs
are huge cells
Eggs are
enormous cells
carrying nutrients for
the developing embryo
• yolk=large stores of nutrients
• large stores of macromolecules
• most eggs surrounded by a coat or shell
The germ line is set aside early in
development as a separate cell lineage
"P granule"
This can sometimes be visualized by the
segregation of putative "determinants”
as in C. elegans
HOW are P-granules
segregated?
PAR proteins & microfilaments
see p. 246 and Fig. 8.43
Fertilized egg
P1
mother cell
of germline
= P4
C. elegans:
P granules
are segregated
at each division
and make the
cell that
inherits them
the germline
Figure 8.44
cell nuclei
P-granules
Similar mechanisms lead to
very early segregation
of the germline in the
fruit fly Drosophila
A
P
Germ cell
progenitors
The Germline is also set aside in Humans
Oocytes arrest
in prophase of
meiosis I for up
to 50 years!
Figure 19.23
How does a single egg cell
make all the stuff needed to
start development?
Fly Oogenesis
Nurse cells are mitotic sisters of the oocyte
Figure 19.4
Nurse cells synthesize macromolecules
and pump them into the oocyte
through the cytoplasmic bridges
follicle
cells
oocyte
nurse
cells
The Drosophila ovary is composed of somatic follicle cells,
germ line nurse cells, and the oocyte
Most of the instructions for forming
organisms are already in the egg!
Asymmetric distribution of
egg contents influences
embryonic development
Molecules synthesized by the nurse cells and
specifically localized in the oocyte determine
the anterior/posterior axis of the embryo
Oocyte
Anterior
Determinant
(bicoid)
Specific mRNAs
are bound by
proteins and
transported
along
microtubules by
motor proteins to
their destinations
Posterior
Determinant
(nanos)
Ventral follicle cells build a signal into the egg shell that
determines the dorsal/ventral axis of the embryo
Dorsal
See Fig. 9.11
for more details
Torpedo receptor
gurken mRNA
Anterior
nucleus
binds Gurken; signal
inhibits Pipe in dorsal
follicle cells
Posterior
Pipe protein
Oocyte signals follicle cells,
follicle cells signal back to oocyte
cytoplasm
Ventral
Ventral follicle cells
Pipe intiates a signaling pathway for activation of Dorsal
transcription factor in ventral cells only
nucleus
Ventral
signal
initiated
by
Pipe
Transmembrane
receptor
(Toll)
Signaling
pathway
Transcription factor (Dorsal)
relocalized to ventral nuclei
of embryo activating new genes
Localized mRNAs also influence
vertebrate development
Vg1 mRNA
See also Figure 5.38
Human oogenesis:
An example of hormonal control
of reproduction
Meiosis I
2 million present at birth
ovulation
Cyclic changes
in hormonal
signals regulate the
timing of female
reproduction
Release into the
Fallopian tube
Most mammalsonce/year
Humans- monthly
(Turns off
FSH and
LH!)
Cyclic changes
in hormonal
signals regulate the
timing of female
reproduction
RU486- blocks progesterone
receptors
Plan B-progestin (interferes
with ovulation, fertilization,
implantation)
Progesterone and estrogen
birth control pillsprevent maturation of new ova
Release into the
Fallopian tube
(Turns off
FSH and
LH!)
Anton van Leeuwenhoek
WNYC’s Radiolab clip from “Sperm” (12/2/08)
By Jad Abumrad & Robert Krulwich
Figure 7.1 The
Human Infant
Preformed in
the Sperm, as
Depicted by
Nicolas
Hartsoeker
(1694)
The sperm: a stripped down
machine for delivering DNA
Figure 7.2
Chemical cues from the egg attract sperm
In sea urchins, the cue is resact
0 sec
20 sec
40 sec
Figure 7.9 Sperm chemotaxis
Inject resact
90 sec
Step one:
Contact between
the sperm
and the eggshell
Step two:
Contact between
the sperm
and the egg’s
plasma
membrane
(zona pellucida=egg shell)
Fertilization:
a multistep
process
Sperm-eggshell contact triggers
the acrosome reaction
Example 1-- the sea urchin
Species specificity
(equivalent to
zona pellucida)
enzymes
attracts/activates sperm
Figure 7.8
Sperm-eggshell
contact triggers
the acrosome
reaction
Example 2:
Mammals
Eggshell includes
ZP3/Sperm have
ZP3 receptor
Figure 7.8
BLOCKING POLYSPERMY
1. Fast/transient: prevent
sperm fusion
2. Slow/permanent: removal
of other bound sperm
Slow block
20-60 seconds
permanent
Fast block
1-3 seconds
lasts 1 min
How do we know this?
We can measure these events directly!
(sperm can’t fuse with egg)
+20 mV
resting -70
(sperm can fuse with egg)
Figure 7.17 Membrane potential of sea urchin eggs before and
After fertilization.
Blocking polyspermy: SLOW BLOCK
Figure 7.6
Cortical
granules:
poised for
release
15,000 cortical granules in a sea urchin egg
• contain enzymes that clip the egg’s bindin receptor
and any attached sperm
• alter the vitelline envelope (zona pellucida in
mammals so that ZP3 receptor on sperm cannot bind)
Sperm-eggshell
contact triggers
the acrosome
reaction
Example 2:
Mammals
Sperm have
ZP3 receptor
A wave of increased calcium can be visualized
moving across the egg from the point of sperm entry
Figure 7.20
Slow block
20-60 seconds
permanent
Fast block
1-3 seconds
lasts 1 min
pH
1 0-5
7.4
Ca 2+
1 0-6
7.2
intracellular
calcium
intracellular pH
1 0-7
7.0
1 0-8
6.8
200
400
600
time (seconds)