Fertilization:  The process of fusion or union of the spermatozoon with the mature ovum is known as conception /fertilizaiton/ impregnantation.

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

2. Fertilization
Chapter 3
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3. 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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5. 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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6. 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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7. 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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8. 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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9. 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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10. 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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11. 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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13. 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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14. Not in
mammals
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15. 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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16. • 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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17. 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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18. 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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19. 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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20. 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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21. 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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22. 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