Fertilization is the process of fusion or union of the spermatozoon with the mature ovum is known as conception.

1. Fertilization
By: Zekarias B. (MSc. Clinical Anatomy)

2. Brain stroming questions
1.When does formation of primary spermatocytes begin?
(A) During week 4 of embryonic life
(B) During month 5 of fetal life
(C) At birth
(D) During month 5 of infancy
(E) At puberty
2. In the production of female gametes, which of the following cells can remain
dormant for 12–40 years?
(A) Primordial germ cell
(B) Primary oocyte
(C) Secondary oocyte
(D) First polar body
(E) Second polar body

3. 3. In the production of male gametes, which of the following cells remains dormant for 12
years?
(A) Primordial germ cell
(B) Primary spermatocyte
(C) Secondary spermatocyte
(D) Spermatid
(E) Sperm
4. Approximately how many sperm will be ejaculated by a normal fertile male during
sexual intercourse?
(A) 10 million
(B) 20 million
(C) 35 million
(D) 100 million
(E) 350 million

4. Fertilization
Chapter 3
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5. FERTILIZATION
Definition:
The process of fusion of two mature germ cells, an
ovum and a spermatozoon to form a single cell zygote.
Site of fertilization:
─The ampulla of fallopian tube which is the widest
portion.
─ Lateral third of fallopian tube.
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7. Abnormal sites:
─ Peritoneal cavity
─ Ovarian follicle
Pecularities:
Two highly specialized haploid gametes are fused resulting in the formation of most
unspecialized or undifferentiated diploid zygote.
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8. Phases of Fertilization
Passage of a sperm through the corona radiata.
Penetration of the zona pellucida.
Fusion of plasma membranes of the oocyte and sperm.
Completion of the second meiotic division of oocyte and formation of female
pronucleus.
Formation of the male pronucleus.
As the pronuclei fuse into a single diploid aggregation of chromosomes, the ootid becomes a zygote.
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9. Mechanism of fertilization is complex, for the descriptive purpose it
can be studied through following stages,
- Approximation of the gametes
- Contact & fusion of the gametes
- Effects of fertilization
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10. 1. Approximation of the gametes:
It involves the transport of the sperms and ova in the female genital tract to reach the
uterine tube.
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11. Cont..
During the ascent of spermatozoa through
female genital tract the spermatozoa are gradually
reduced in number by barriers provided by abrupt
constrictions at the cervix & uterine ostium of the
tube.
As a result remarkably reduced number of
sperms(about 300-500 sperms) appears at the site
of fertilization.
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12. Cont…
Most of the sperms die within 24hrs after
ejaculation.
Transport of the oocyte into the uterine tube is
encouraged by the ciliary beats & rhythmical
contractions of the musculature of uterine tube.
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13. 2.Contact & fusion of the gametes
A direct contact between the spermatozoa & zona pellucida of secondary
oocyte is necessary before actual fusion of the gametes.
Three barriers protect the female gamete before penetration by the
fertilizing sperm.
The barriers are provided by the following,
1)Corona radiata
2)Zona pellucida
3)Vetilline membrane
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15. Cont ..
Prior to penetration the spermatozoa undergo a process of capacitation, followed by acrosome
reaction in the female genital tract.
Capacitation:
Increase the activity of the sperm
Acrosome reaction:
• Enzyme contained in the Acrosome released and react with the zona pellucid
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16. Not in
mammals
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17. Cont..
The cytoplasm of mature ovum contains two
pronuclei.
1. The sperm head with its nuclear envelop.
It contains 1N haploid chromosomes is somewhat
swollen to form male pro nucleus.
2. In the same way the nucleus of mature ovum forms
female pronucleus.
At this stage both male & female pronuclei also
called ootids approach each other for fusion.
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18. • A. Scanning electron micrograph of
sperm binding to the zona pellucida.
• B. The three phases of oocyte
penetration.
• ► In phase 1,
spermatozoa pass through the
corona radiate barrier;
• ► In phase 2,
• one or more spermatozoa
penetrate the zona pellucida;
• ► In phase 3,
• one spermatozoon penetrates
the oocyte membrane while losing
its own plasma membrane.
• Inset shows normal
• spermatocyte with acrosomal head
cap.
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19. RESULT OF FERTILIZATION
1)Completion of second meiotic
division of the female gamete
Immediately after the entrance of a
spermatozoon into the secondary
oocyte, the latter completes the
second maturation division.
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20. Cont…
2)Restoration of diploid number of chromosomes in
the zygote
The head of the spermatozoon containing haploid
chromosomes swells and form male pronucleus
Both male and female pronuclei each containing 1N
DNA complement replicate their DNA to form DNA
duplex before their actual fusion.
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21. Cont..
The male and female pronuclei meet each other
near the centre of the ovum and the nuclear
membrane disappear.
This results in the nuclear fusion with the
restoration of the diploid number of chromosome
which is constant for the species.
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22. 3)Determination of chromosomal sex
Out of total sperm population half of them contain x
bearing chromosomes and the remaining half y
bearing chromosomes.
When an X bearing spermatozoon fertilize an ovum
the zygote contains 2X chromosomes plus 44
autosomes & a female child is formed.
If the ovum is fertilized by Y bearing spermatzoon the
sex of zygote will be male containing XY
chromosomes plus 44 autosomes.
Therefore the father decides the chromosomal sex of
the off spring.
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23. 4)Initiation of cleavage division
Fertilization provides energy for
the repeated mitotic division of
the single celled zygote.
It results in the formation of a
mass of cells called the morula.
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24. The line of entry of the spermatozoon probably decides the cephalo caudal
polarity of the zygote.
Also bilateral symmetry of the embryo.
6)Embryo contain only maternal mitochondria because
sperm mitochondria are dispersed in cytoplasm of ovum
and discarded.
RESULT OF FERTILIZTION
1) ON OVARY – the corpus luteum changes to corpus luteum of pregnancy.
2) ON UTERUS – the secretory changes starts in endometrium.
3) ON ZYGOTE – cleavage division starts.
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5)Determination of the polarity of the embryo