1) The sperm and egg undergo a series of changes to prepare for fertilization. The sperm undergoes capacitation in the female reproductive tract while the egg is swept into the fallopian tubes. 2) Upon contact, the acrosomal reaction occurs, releasing enzymes that allow the sperm to penetrate the egg's extracellular layers. 3) One sperm fuses with the egg, and their pronuclei also fuse to form a single diploid cell with 46 chromosomes. Rapid changes then prevent additional sperm from fertilizing the egg.

1. By
Dr. Priti D.Diwan
Assistant Professor
Department of Zoology
J.D.Patil sangludkar Mahavidyalay Daryapur
Daryapur

3. Anatomy of Sperm

5. Anatomy of Ova

6. Ovum and sperms: (In vitro)
The surfaces of unfertilized eggs are usually smooth in appearance. The mottled look
of this egg is not normally seen, but apparently all the ova from this woman had this
appearance.
From this photograph, it
should be clear that the
heads of human sperm
are less than 1/20 the
diameter of human eggs.
sperm heads

7. Site of fertilization

8. Pre- fertilization events
 Helping the sperm:
At ovulation, the cervical mucus increases in amount
and becomes less thick, allowing easier sperm
transport.
Passage of the sperm through the uterus and oviduct
occurs mainly due to muscular contractions of these
organs.
 Oocyte:
The ends of the oviducts come into close contact
with the ovary during ovulation.
Fimbriae of oviduct ends “sweep” the ovulated ovum
into the oviduct.
Peristaltic waves of oviduct musculature bring the
ovum into the ampulla of the oviduct.

9. Capacitation: readying the sperm
 Sperms cannot fertilize oocytes when they
are newly ejaculated.
 The process of capacitation takes 5-7 hours.
 Capacitated sperms are more active.
 Location: capacitation occurs in the uterus
and oviducts and is facilitated by substances
of the female genital tract.
 The acrosomal reaction cannot occur until
capacitation has occurred.

10. Process of Fertilization
(1) Chemo attraction
(2) Release of acrosomal enzymes
(3) Binding of sperm to extra cellular envelopes
(4) Passage through extra cellular envelopes
(5) Fusion of sperm and egg pronuclei

11. (1) Chemo attraction
 Species specific in many invertebrates, In sea
urchin a peptide resact is such chemo tactic
molecule. It is also sperm activating molecule.
 Egg jelly has other compounds as well, which
bind with specific receptors on sperm, which
inturn opens calcium ion channels, leading to
exocytosis of acrosome.

12. (2) Release of acrosomal enzymes
 In mammals, zona pellucida ZP-3 glycoprotein
which triggers Acrosomal reaction.
 Prerequisite for the fusion of nuclei. Steps are:

13. Cont.

14. (3) Binding of sperm to extra cellular envelopes
 Passage of sperm through the corona radiata depends
on enzyme action:
 Flagella action also aids corona radiata penetration.
 Penetration of the zona pellucida around the oocyte:
 Once sperm penetrates zona pellucida, the zona
reaction occurs:
This reaction makes the zona pellucida impermeable to
other sperms.
When more than one sperm manages to enter the ovum
(dispermy = 2; triploidy = 3), the fetus nearly always

15. Prevention of polyspermy
 In mammals, prevention of polyspermy is
mainly achieved by:
 Changing of the electric potential of egg
membrane by pumping high amount of Na+
outside the egg surface-
 The cortical reaction occurs in fertilisation when a
sperm cell unites with the egg's plasma
membrane, (zona reaction).This reaction leads to
a modification of the zona pellucida that blocks
polyspermy-

16. Fast Block of polyspermy

17. Slow Block of polyspermy

18. (4) Passage through extra cellular envelopes
Head and tail of a sperm enter the cytoplasm
of the oocyte, but the sperm plasma
membrane remains behind.
The secondary oocyte was previously
arrested in metaphase of the 2nd meiotic
division, and now forms the mature ovum

19. (5) Fusion of sperm and egg 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

20.  The male and female
pronuclei are
indistinguishable from one
another.
 The second polar body can
be seen (arrow).
 The plasma membranes of
the two pronuclei are
dissolving and one diploid
nucleus will remain.
Fusion of the pronuclei:

21. Cont.
 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.
Mitosis in the new zygote uses centrioles derived
from the sperm. The oocyte has no centrioles.

22. Fertilization facts:
 Completed within 24 hours of ovulation
 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.

23. The results of fertilization:
 Stimulates the secondary oocyte to complete
meiosis.
 Restores the normal diploid number of
chromosomes (46).
 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.