The Ovum
Like any ordinary cell in its structure.
 It is large oval cell which varies from 117-142
µ in diameters.
 Coverings:
 a.has 2 membrane: an inner thin called
vitelline membrane &
an outer thick transparent
membrane named zona pellucida.
 b. Corona radiata: 2 or 3 layers of cells which
surround the zona pellucida when the ovum is
shed from follicle.

Its cytoplasm is called ooplasm,
 it is nucleus, the germinal vesicle,
 its nucleolus is called the germinal spot.
 The cytoplasm contains a nutritive which
nourishes the embryo in the early stages of
development.
 Transport of Ovum:
 In the uterine tube the ovum passes along
by:
 a. Activity of the cilia of tube.
 b. Muscular contraction of the tube which
increases during ovulation.
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Types of Ova
A.
According to the amount of yolk:

1. Oligolecithal eggs: small-sized eggs,
contain a very small amount of yolk. found
in amphioxus and eutherian mammals.
2. Mesolecithal eggs:
eggs contain
moderate amount of yolk, found in
amphibian (frogs).
3. polylecithal eggs: eggs contain enormous
amount of yolk and are found in insects,
reptiles and birds.
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B. According to the distribution of yolk:
1. Isolecithal eggs:
 the amount of yolk is found regularly
distributed throughout the egg cytoplasm. As
in oligolecithal eggs, that have a little amount
of yolk.
2. Telolecithal egg:
 have a polarized distribution of yolk in the
cytoplasm and are found in the mesolecithal
and polylecithal eggs.
 the yolk due to its gravity is concentrated more
in one hemisphere than in the other.

may be:
a. Moderately telolecithal (eggs of amphibian).
b. Heavy telolecithal (eggs of reptiles & birds).
3. Centrolecithal eggs: in insects, the yolk is
concentrated in the center of the egg and the active
cytoplasm forms a thin peripheral layer around the
yolk.
Functions of Yolk
Yolk (reserve material) that formed of lipid droplets
and glycogen granules) is used for two purposes:
 a. Supply of energy.
 b. Synthesis of the products required for elaboration
of the embryonic body.
 Also,
 the yolk influence on the size of egg, differentiation
of ooplasm, patterns of cleavages and on the
morphogenetic movements of blastomeres during
gastrulation.
Egg Membranes
membranes produced outside the plasma
membrane of egg.
 vary in different animal groups.
 There are several ways to classify these
membranes, but the simplest way is to group
them according to the origin as follows:

1. Primary Membranes:
Formed in the ovary
 between the egg plasma membrane and follicle cells.
 They are formed either by the egg or follicle cells as:
- Vitelline membrane: has been given different names
in different animals. Ex, in amphibian and birds it is
in close contact with ooplasmic surface until the egg
is fertilized at which time it separates from this
surface, forming a tough fertilization membrane.
- In fishes is called chorion.
- In reptiles and mammals named zona pellucida.

2. Secondary egg membranes:
 Is secreted outside the primary egg membrane
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by a layer of follicle cell that surrounds the oocyte
(by the ovarian tissue) before the egg leave the ovary
It occurs in the form of a chitinous shell
surrounding the egg in insects and called chorion, in
this case it have a micropyle for sperm entry.
It may secretes female sex hormones. As in mammals
like membrana granulosa & corona radiata.
3. Tertiary egg membranes:
Formed during passage of the egg in the oviduct or
during the presence of ovum in the uterus.
a. As the egg of the amphibian, three uniform layers of
albumen (jelly) are deposited around it. These jelly
envelopes hold the egg together in masses, protect
the eggs from infection and make them unappetizing
to predators.
b. In reptiles, birds tertiary membranes make up the
envelopes of the egg. External to vitelline membrane
are added the white of egg (albumen), two shell
membranes and a porous calcareous shell.

Significance of Egg Membranes:
1. Provide the protection to the contents of eggs
of developing embryos from different
ecological hazards (variable ph, temperature,
radiations, pollutions).
2. prevent polyspermy i.e. fertilization by more
than one sperm.
3. Help in sperm adhesion.
4. For maintaining normal cleavage of the egg.
Fertilization
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Fertilization, the process by which male and female
gametes nuclei fuses together to produce diploid
zygote.
Types of Fertilization
1. External:
Eggs are librated in water.
Occurs outside the female genital system.
Female laid a large number of eggs, them the male
pour its sperms in the same region in water
e.g. in fish and amphibian.
2. Internal:
 Occurs in animals that have a well
developed reproductive system, animals
may be:
 a) Oviparous: zygote develops in a shell e.g.
birds.
 b) Viviparous: zygote develops inside uterus
e.g. mammals.
 The intrauterine life is about 21 days in the
rat, 70 days in the in the Guinea pig while
its about 280 days in human.

It have 4 major steps:

Contact and recognition between sperm and egg.
(same species)

Regulation of sperm entry into the egg. (only one
and inhibiting the others)
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Fusion of the genetic material of sperm and egg.

Activation of egg metabolism to start
development.
Sperm Capacitation
(enhanced sperm function)
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1.
2.
Results from secretion in the female’s reproductive
tract.
Freshly ejaculated sperm are unable or poorly able
to fertilize.
Rather, they must first undergo a series of changes
(4 sets of molecular changes) known collectively as
capacitation.
The sperm cell membrane may be altered by
changing its lipid composition. (lowering its
cholesterol)
Particular proteins or carbohydrates on the sperm
surface are lost.
3. Certain proteins are phosphorylated. (in other
meaning activated)
4. The membrane potential of the sperm is
dramatically lowered ( from -30 mV to about 50mV)
Finally, and in brief

Capacitation appears to destabilize the sperm's
membrane to prepare it for the acrosome
reaction, only capacitated sperms can pass
through the corona cells and undergo the
acrosome reaction.
Recognition of Egg and Sperm:

1.
2.
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1.
2.
There are 2 problems:
How can sperms and eggs meet in a dilute
concentration .(water)??
What mechanisms prevents sperms of one
species from trying to fertilize eggs of other
species.???
Two mechanisms evolved to solve these
difficulities:
Species-specific attraction of sperm.
Species-specific sperm activation.
Sperm Activation (The Acrosomal Reaction)

1.
2.
The acrosomal reaction is the discharge of
hydrolytic enzymes from a vesicle in the
acrosome of a sperm cell. On 2 steps:
The fusion between the acrosomal vesicle
and sperm plasma membranes (an
exocytosis that results in the release of the
contents of the acrosomal vesicle).
The extension of the acrosomal process.( by
polymerization of globular actin molecules
into actin filaments)
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Initiated by soluble egg jelly or by contact with the
egg itself.
Can activated artificially by increasing the
calcium concentration.
First: Contact and Recognition
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The plasma membrane of the sea urchin egg is
surrounded by the vitelline layer and the thicker,
outer jelly coat (zona pellucida in mammals)
The acrosome reaction:
 The acrosome of the sperm releases proteolytic
enzymes.
 Acrosome digests a path through the external
coverings.
 The acrosomal process of the sperm contact the
vitelline envelope of the egg.
If sea urchin gametes are of the same species, a
protein called bindin on the acrosome adheres to a
specific receptor on the vitelline membrane.
Step 2: Sperm and Egg fuse
(Sea Urchins)
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The microvilli of the egg
membrane
form
a
fertilization cone
Sperm is drawn into the
cone.
Gamete plasma membranes
fuse and the sperm is
drawn into the egg cell
Preventing entry of more than one sperm
(Polyspermy)
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Fast block to polyspermy involves egg plasma
membrane action potential, prevents additional
sperm from entering. (-70µ to +10µ) also release of
Ca ions.
The slow block to polyspermy: In mammals, the zona
pellucida sperm receptors are modified to prevent
further entry of sperm
Some organisms do not block polyspermy.
 In salamanders, many sperm enter, only one
survives, the rest are degraded
Step 3: Activation of the Egg
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1.
2.
3.
4.
Prior to fertilization, the egg is in a quiescent
state, arrested in metaphase of the second
meiotic division.
Upon binding of a sperm, the egg rapidly
undergoes a number of metabolic and
physical changes that collectively are called
egg activation.
Aerobic respiration increases in the egg.
Enzyme systems become activated.
A burst of protein synthesis begins.
In most animals, the nucleus undergoes the
second division of meiosis.

In many eggs it is possible to physically
stimulate an egg to undergo activation, and
even division
Step 4: Pro-nuclear Fusion
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Once the second polar body is ejected, the female
pronucleus can fuse with the male pronucleus
This is the genetic beginning of a new organism
The haploid genetic complements of the two
pronuclei form a 2N nucleus, which prepares the
nucleus, and cell, for cleavage.
The Cortical Reaction
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The fusion of egg & sperm membranes stimulates
a series of changes in the egg’s cortex known as a
cortical reaction.
Chemical reactions change the egg’s cortical
granules.
Granules fuse with the plasma membrane releasing
enzymes separating the vitelline layer from the
plasma membrane.
Swelling “lifts” the vitelline membrane forming the
fertilization membrane.
Prevents penetration by other sperm
The Zona Reaction Animation
Acrosomal, Cortical Reactions
Thank you for your time.
Questions?
Dr. Abdelalim Gadallah