By: Dr. Saeed Vohra
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Fertilization
(takes 24 hrs)
Fusion of male & female gametes
• Is a complex sequence of coordinated
molecular events
• Begins with a contact between sperm
and ovum
• Ends up with intermingling of maternal
and paternal chromosomes
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Site
• Usual site is the ampulla of uterine tubes
• Ampulla is the longest and widest part of the
uterine tubes
• Fertilization may occur in other parts of uterine
tubes
• Does not occur in the uterus
• Chemical signals from oocyte attract the sperms
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Phases of Fertilization
• Penetration of corona
radiata
• Penetration of zona
pellucida
• Fusion of the plasma
membrane of the oocyte and
sperm
• Completion of second
meiotic division & formation
of female pronucleus
• Formation of male
pronucleus
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Early Pregnancy Factor
• Is an immunosuppressant protein
• Secreted by trophoblast
• Appears in maternal serum within 24-48 hrs
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Chromosome
• Zygote is genetically unique
• Half of chromosomes from father and half from
mother
• New combination is formed which is different
from either of parents
• This mechanism forms biparental inheritance
and variation of human species
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Sex of the Embryo
• Embryo's chromosomal sex is
determined at fertilization
• Sex is determined by the type of sperm
(X or Y) fertilizes the oocyte
• It is father whose gamete decides the sex
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Results of Fertilization
• Stimulates the penetrated oocyte to
complete 2nd meiotic division
• Restores the normal diploid no. of
chromosomes
• Determines chromosomal sex of embryo
• Initiates cleavage (cell division) of zygote
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Cleavage of Zygote
• Consists of repeated mitotic divisions of
zygote
• Rapid increase in the number of cells
• These smaller embryonic cells are called
Blastomeres
• Normally occurs in the uterine tube
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Cleavage of Zygote
• Zygote divides first into 2 then 4 & 8
• Zygote is within the thick zona pellucida during
cleavage
• Zona pellucida is translucent under light
microscope
• Begins about 30 hrs after fertilization
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Compaction
• Blastomere change their shape after 9 cell stage
• Tightly align themselves against each other
• Form a compact ball of cells
• Probably mediated by cell surface adhesion
glycoprotein
• Permits greater cell to cell interaction
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Morula
• When there are 12-32 blastomeres the developing
human is called MORULA
• Enters the uterus at this stage
• Spherical morula forms about 3 days after fertilization
• Has outer and inner cell layer
• Resembles mulberry or blackberry
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Implantation
• As morula enters the uterus a fluid filled
space appears inside the morula called
Blastocystic cavity
• This fluid separates the blastomeres into 2
parts:
• Thin & outer cell layer Trophoblast
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• Inner
cell mass Embryoblast
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Blastogenesis
• At this stage the embryo is called
Blastocyst
• Embryoblast now projects into the
Blastocystic cavity
• Trophoblast forms the wall of the
blastocyst
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• It floats in the uterine secretions for 2 days
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Blastogenesis
• Zona pellucida gradually degenerates
• It allows blastocyst to increase in size
rapidly
• It derives nourishment from uterine glands
• Blastocyst attaches to the endometrium 6
days after fertilization
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Trophoblast
As soon the blastocyst attaches to the
endometrium the Trophoblast
differentiates into 2 layers:
• Inner Cytotrophoblast
• Outer Syncytiotrophoblast
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Implantation
• On 6th day syncytiotrophoblast extends
through the endometrium by finger like
processes and invade connective tissue
• By the end of 1st week blastocyst
superficially implants in compact layer of
endometrium
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Sequence of Implantation
• Blastocyst gets nourishment from eroded
maternal tissues
• Syncytiotrophoblast expands quickly in
area adjacent to embryoblast called
Embryonic pole
• Syncytiotrophoblast produces enzymes
that erode the maternal tissue
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Sequence of Implantation
• Blastocyst burrows into the endometrium
• At about 7 days a layer of cells Hypoblast
(primary endoderm) appear on the
surface of embryoblast
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