1. Fertilization involves the fusion of a sperm and egg nuclei to form a zygote. 2. It restores the diploid number of chromosomes and determines the sex of the embryo. 3. A series of events must occur for fertilization to be successful, including capacitation of sperm, penetration of the zona pellucida, fusion of membranes, and formation of male and female pronuclei.

1. BIO 415
LECTURE 2
FERTILIZATION
‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬

2. Fertilization
 Fertilization is part of mating or copulation between male
and female.
 Fertilization is the fusion of the sperm nucleus with the ova
nucleus to produce a zygote (fertilized egg).
 Fertilization brings the haploid nuclei of sperm and egg
together, forming a diploid zygote.
 The sperm’s contact with the egg’s surface initiates
metabolic reactions in the egg that trigger the onset of
embryonic development.

4. Human somatic cells (any cell other than a gamete) have 23
pairs of chromosomes. A diploid cell (2n) has two sets of
chromosomes, Where the diploid number is 46 (2n = 46).
Chromosomes in Human

5. •The sex chromosomes in mammals are called X and Y.
•Human females have a homologous pair of X chromosomes
(XX)
•Human males have one X and one Y chromosome.
•A gamete (sperm or egg) contains a single set of
chromosomes, and is haploid (n).
•For humans, the haploid number is 23 (n = 23)
•Each set of 23 consists of 22 autosomes and a single sex
chromosome
•In an unfertilized egg (ovum), the sex chromosome is X
•In a sperm cell, the sex chromosome may be either X or Y
Sex Determination

6. Fertilization in animal kingdom
There are two types of fertilization (external and
internal).
External fertilization
 Occurs mostly in wet
environments .
 It’s occur outside the body of
female ,where requires both
the male and the female to
release their gametes into
their surroundings (usually
water)
 It’s produce a large number
of offspring.
Ex) fish and amphibians
External fertilization in amphibians
and fish

7. Internal fertilization
 It’s occur in most of terrestrial
animals .
 It’s occur inside the body of
female, to
protect the developing egg.
 It’s produce less number of
offspring.
Ex) mammals, reptiles, birds
Fertilization in animal kingdom

8. Internal fertilization in mammals
• Fertilization in mammals occurs in the oviduct.
• The ova is viable for approximately 24 hours after ovulation.

9.  What are the factors that affect the sperm’s potential
for successfully fertilizing the egg?
1- Capacitation of sperm.
2- Structures surrounding the egg – barriers.

10. 1- Capacitation of sperm
• It is the process that allows the acrosome reaction to occur,
so the sperm can penetrate the zona pellucida of the oocyte.
• The physiological changes that confer on the sperm the
ability to fertilize are called capacitation.
• Capacitation includes multiple physiological and biochemical
modifications.
• occurs in the female’s vagina. Vaginal secretions cause a
molecular change in the sperm plasma lemma (removal of
decapacitating factor - semen proteins, results in increased
membrane fluidity,).
• Takes 4-5 hr. in humans, 1 hr. in mice, 6 hr. in rabbits.

11. 1- Capacitation of sperm
• The biochemical changes associated with the capacitation
process include:
1. an efflux of cholesterol from the plasma membrane leading
to an increase in membrane fluidity and permeability to
bicarbonate and calcium ions.
2. hyperpolarization of the plasma membrane, changes in
protein phosphorylation and protein kinase activity
3. increases in bicarbonate (HCO3) concentration and
intracellular pH, Calcium and cyclic adenosine
monophosphate (cAMP) levels.
4. Galactosyltransferase enzyme is a protein in the sperm
plasma lemma that binds to the zona pellucida 3 protein
(ZP3) receptor around the ova and initiates the acrosome
reaction.

12. 1- Capacitation of sperm

13.  Increased rate of metabolism.
 Flagellum beats more rapidly; Result: Sperm are more
motile.
 Changes in sperm plasma lemma proteins allow sperm-
egg binding and occurrence of the acrosome reaction.
 Pro-Acrosin (inactive) is converted to acrosin (active).
 Sperm become capable of chemotaxis.
Effects of Capacitation on Sperm

14. 2- Structures surrounding the egg - barriers
1. Follicle cells.
2. Zona pellucida (ZP)in
mammals.
3. (vitelline membrane in non-
mammals).
4. Oolemma (plasma membrane
of ova).

15.  Ovum is a larger nutritive cell.
Like an animal cell it also contains
all cell organelles.
 Its size varies in different animals.
Ovum is generally round/spherical
in shape.
 It is covered by a thin membrane
known as plasma membrane.
Beside this, it is also covered by
other covering membranes known
as egg membranes.
2- Structure of Ovum (egg)

16. These are divided into three-
1. Primary egg membrane- Secreted by ovum itself.( viteline .m)
2. Secondary egg membrane- Secreted by ovarian tissue like
follicular membrane.
3. Tertiary egg membrane- Secreted or generated by various
regions of uterus or oviduct.
Importance of the egg membranes:-
These membranes provide nutrition and protection for developing
embryo.
Egg membranes

17. Fertilization Stage
1. The acrosome reaction.
2. The Penetration of the zona pellucida.
3. Fusion of plasma membranes of oocyte and sperm.
4. 2nd meiotic division of oocyte is completed
5. Formation of male and female pronuclei.
6. Membranes of the pronuclei break down, chromosomes
condense and arrange themselves for mitotic cell division.

18. Fertilization
1- Acrosome reaction
• The acrosome reaction must be completed before the sperm
can fuse with the secondary oocyte.
• Occurs when sperms come into contact with the corona
radiata of the oocyte
• Perforations develop in the acrosome
• Point fusions of the sperm plasma membrane and the
external acrosomal membrane occur
• The acrosome reaction is associated with the release of
acrosome enzymes that facilitate fertilization
• Passage of sperm through the corona radiata depends on
enzyme action:
• Hyaluronidase released from sperm acrosome
• Tubal mucosal enzymes
• Flagella action also aids corona radiata penetration

19. Acrosome Reaction

20. Ovum and Sperms: (in vitro)
Advanced Fertility Center of Chicago
http://www.advancedfertility.com/
From this photograph, it should be
clear that the heads of human
sperm are less than 1/20 the
diameter of human eggs.
 Arrows point to sperm heads

21. Fertilization
2- penetration of the zona pellucida
• Penetration of the zona pellucida around the oocyte:
• Acrosomal enzymes: esterases, acrosin, and neuraminidase
cause lysis of the zona pellucida.
• Once sperm penetrates zona pellucida, the zona reaction
occurs: fertilization membrane will form.
• This reaction makes the zona pellucida impermeable to
other sperms. Block to poly spermy.
• When more than one sperm manages to enter the ovum
(dispermy = 2; triploidy = 3 the called poly spermy), the
fetus nearly always aborts.

22. Diagram of fertilization steps in mammals

23. Fertilization
3- fusion of plasma membranes of oocyte and
sperm
• Fusion of plasma membranes of oocyte and sperm.
• Head of a sperm enter the cytoplasm of the oocyte, but the
sperm plasma membrane remains behind.

24. Ova cortical granules reaction
• Once the sperm cell membrane fused the ova membrane,
the ova cortical granules fused with ova membrane and
released its content to the out side of the ova in the
perivitelline space.
• The released materials from the cortical granule will form a
hyaline layer between the ova membrane and the vitelline
membrane.
• This layer will prevent the entry of any other sperm to the
ova and considered as the second bloke to polyspermy.

25. DIAGRAM OF FERTILIZATION STEPS

26. Fertilization
4- second meiotic division of oocyte is completed
• 2nd meiotic division of oocyte is completed.
• The secondary oocyte was previously arrested in
metaphase of the 2nd meiotic division, and now forms
the mature ovum and another polar body.

27. Fertilization
5- formation of male and female pronuclei
• Formation of male and female pronuclei:
• Chromosomal material of the sperm decondensates
and enlarges
• Chromosomal material of the ovum decondensates
following the completion of meiosis
• At this stage, the male and female pronuclei are
indistinguishable.
• As they grow, the pronuclei replicate their dna  still 1n
(haploid)-( 23 chromosomes, each in chromatid pairs in
human).

28. • The male and female pronuclei
are indistinguishable from one
another.
• The second polar body can be
seen (blue arrow).
• The plasma membranes of the
two pronuclei are dissolving
and one diploid nucleus will
remain.
Fusion of the pronuclei
(in vitro)
Advanced Fertility Center of Chicago
http://www.advancedfertility.com/

29. Fertilization
6- membranes of the pronuclei break down
• Membranes of the pronuclei break down, chromosomes
condense and arrange themselves for mitotic cell division
• On membrane dissolution, there is 1 cell with 46
chromosomes = diploid (2N)
• The first cleavage follows shortly, leaving 2 cells, each
with 46 chromosomes in human.
• Mitosis in the new zygote uses centrioles derived
from the sperm.
• The oocyte has no centrioles.

31. Fertilization facts
• Completed within 24 hours of ovulation in human
• Approximately 400 to 600 MILLION sperms are deposited at
cervical opening during ejaculation.
• Some sperm are held up by the folds of the cervix and
are gradually released into the cervical canal; this
gradual release increases the chances of fertilization.
• Most human sperms do not survive longer than 48 hours
in the female genital tract.
• Only about 200 sperms reach the fertilization site; most
degenerate and are absorbed by the female genital tract.

32. The results of fertilization
• Stimulates the secondary oocyte to complete meiosis.
• Restores the normal diploid number of chromosomes (46 in
human).
• Results in variation of human species as maternal and
paternal chromosomes intermingle.
• The embryo contains only maternal mitochondria because the
sperm mitochondria are dispersed into the egg cytoplasm and
discarded.
• Determines the sex of the embryo.
• The sex chromosome (y or x) carried by the successful sperm
determines embryonic sex.

33. Summary of biochemical interaction during
fertilization

34. After fertilization, embryonic development proceeds
through cleavage, gastrulation, and organogenesis
• Important events regulating development occur during
fertilization and the three stages that build the animal’s
body
• Cleavage: cell division creates a hollow ball of cells
called a blastula
• Gastrulation: cells are rearranged into a three-layered
gastrula
• Organogenesis: the three layers interact and move to
give rise to organs