Germ Cells and Fertilization
Lecture 30
BSCI 420, 421
Nov 11, 2002
“The final aim of all love intrigues, be they comic or tragic,
is the composition of the next generation”
- A. Schopenhauer
1.
2.
3.
4.
5.
Sexual reproduction
Meiosis
Oogenesis
Spermatogenesis
Fertilization
1. Sexual reproduction
Has an advantage over asexual reproduction:
Increases diversity by producing offspring that are a mixture
of the traits of the parents.
Is diversity an advantage?
If so,
Why is diversity an advantage?
1. Meiosis
A pair of nuclear divisions that produce four
different haploid cells, usually but not always gametes.
Gametes are haploid cells capable of fusing to produce
A diploid cell called a Zygote.
Gametes can be of identical size and shape (isogamy).
Or they can be different in size and or shape (anisogamy).
Meiosis
vs mitosis
3. Oogenesis
Is gamatogenesis of the larger gamete called the egg or ovum.
Oogenesis
Mammalian (rabbit) oocytes (fig 24)
4. Spermatogenesis
Spermatozoon is a small,
highly streamlined cell,
specialized to deliver a
nucleus to an egg far
away, and to activate
development of the egg.
Spermatogenesis
5. Fertilization
Involves:
a. Activation of the sperm and the acrosomal reaction
b. Activation of the egg
1. The fast block to polyspermy
2. The slow block to polyspermy
3. Initiation of protein, RNA, & DNA synthesis
4. Fusion of the egg and sperm and male and female
pronuclei
a. Activation of the sperm and the acrosome reaction
Activation of the sperm:
Contact w
Zona
Pellucida
Aggregates
Receptors
Opens Na+
Channels
V change opens
VS Ca2+
Channels
Ca2+
influx
Exocytosis
of Acrosomal
vesicle
Release of
lytic enzymes
Exposure
of fertilins
b. Egg Activation:
1. Fast block to polyspermy
Sperm binding to
Egg cell membrane
By fertilin and its receptor on egg
Receptor linked Na+ channels open
Na+ influx
Membrane potential change
0
Fast block to polyspermy
(Discovered by
E.E. Just
Afro-Amer Dev.Biologist)
-70
mV
1
2
time, min
contact
Starfish egg injected with the dye aequorin that fluoresces
When the Ca2+ conc increases.
At time 0, the egg was fertilized, and a wave of Ca2+ sweeps
Over the egg in about 1 min. initiating cortical granule
Exocytosis and the slow block to polyspermy.
Slow Block to
polyspermy
2. Slow block to polyspermy
Binding of fertilin to receptor on egg
Tyrosine kinase activation
Phospholipase C-g activation
DAG production
3.
IP3
PKC activation
Stimulation of
protein syn
RNA syn
DNA synthesis
(both IP3 gated
& Ca-Induced)
Ca2+ release
Cortical granule
exocytosis
Slow block to
polyspermy
4. Fusion of
pronuclei
(MTs green
Fl-aT;
DNA blue)
Fig 35
The merging of mammalian pronuclei after fertilization