02 Fertilization & Implantation.pptFertilization & Implantation

FERTILIZATION & IMPLANTATION
FERTILIZATION & IMPLANTATION

OBJECTIVES
OBJECTIVES

By the end of the lecture, the student should
By the end of the lecture, the student should
be able to
be able to:
:

Define the terms ‘
Define the terms ‘fertilization & implantation
fertilization & implantation’.
’.

Describe the phases of fertilization and its
Describe the phases of fertilization and its
outcome.
outcome.

Describe the cleavage and the stage at which
Describe the cleavage and the stage at which
implantation occurs.
implantation occurs.

Describe the process of implantation.
Describe the process of implantation.

Define the normal site of implantation.
Define the normal site of implantation.

Describe the
Describe the abnormal sites of implantation
sites of implantation
(ectopic pregnancy)
(ectopic pregnancy)

Fertilization
Fertilization
 It is the fusion
of male &
female
gametes.
 It is a complex
process, that
begins with
contact
between a
sperm & an
oocyte.
 It ends by the
intermingling
intermingling
of maternal &
paternal
chromosomes.

Viability of Gametes
Viability of Gametes 
Human
Human oocytes
oocytes
are usually
are usually
fertilized within
fertilized within
12 hours
12 hours after
after
ovulation.
ovulation.

Oocyte cannot be
Oocyte cannot be
fertilized after
fertilized after 24
24
hours
hours as it shortly
as it shortly
degenerates
degenerates
thereafter.
thereafter.

Human
Human sperms
sperms do
do
not survive for
not survive for
more than
more than 48
48
hours.
hours.

Sperm
Sperm
Capacitation
Capacitation
 Freshly ejaculated
Freshly ejaculated
sperms are unable
sperms are unable
to fertilize the 2ry
to fertilize the 2ry
oocyte.
oocyte.
 They must undergo
They must undergo
a series of changes
a series of changes
known as
known as
capacitation
capacitation.
.
 Capacitation
Capacitation
occurs in the
occurs in the female
reproductive tract.
.
 It takes about
It takes about 7-8
7-8
hours.
hours.
 Sperms that have
Sperms that have
undergone
undergone
capacitation
capacitation become
become
hyperactive & highly
hyperactive & highly
motile.
motile.

Site of Fertilization
Site of Fertilization

Usually occurs
Usually occurs
in
in ampulla
ampulla of
of
the uterine
the uterine
tube.
tube.

It may occur in
It may occur in
other parts of
other parts of
the tube but
the tube but
never in the
never in the
uterus.
uterus.

Chemical signals
Chemical signals
from oocyte
from oocyte
attract the
attract the
sperms, also
sperms, also
uterine tube
uterine tube
contraction helps
contraction helps
the sperm to
the sperm to
ascend.
ascend.

Stage 1
Stage 1:
:
Passage of sperm through Corona Radiata
Passage of sperm through Corona Radiata
It results from
It results from:
:

1- Action of an
1- Action of an
enzyme called
enzyme called
hyaluronidase
released from
released from
the
the acrosome
acrosome
of the sperm
of the sperm
which helps in
which helps in
dispersal of
dispersal of
corona radiata
corona radiata
cells.
cells.

2- Constant
2- Constant
propulsive
propulsive force
force
of the sperm's
of the sperm's
tail.
tail.

Stage 2:
Stage 2:
Penetration of Zona Pellucida
Penetration of Zona Pellucida

Constant
Constant
propulsive
propulsive force
force
from the sperm’s
from the sperm’s
flagellating tail.
flagellating tail.

acrosomal
acrosomal
enzymes
enzymes
(esterases,
(esterases,
acrosin &
acrosin &
neuraminidase)
neuraminidase)
allow the sperm
allow the sperm
to create a tract
to create a tract
through the zona
through the zona
pellucida
pellucida

Stage 3
Stage 3:
:
Fusion of plasma
Fusion of plasma
membranes of
membranes of
the oocyte & the
the oocyte & the
sperm.
sperm.
Entry of sperm
Entry of sperm
contents into the
contents into the
oocyte.
oocyte.
Stage 4
Stage 4:
:
Completion of
Completion of
second meiotic
second meiotic
division of the 2ry
division of the 2ry
oocyte and
oocyte and
formation of
formation of
female
female
pronucleus.
pronucleus.

Stage 5
Stage 5. Formation of
. Formation of
male pronucleus
male pronucleus
Stage 6.
. Fusion of pronuclei & formation of the
Fusion of pronuclei & formation of the
zygote and preparation of first mitotic division
zygote and preparation of first mitotic division

Results of Fertilization
• Stimulates the
penetrated 2ry
oocyte to complete
its 2nd
meiotic
division.
• Restores the diploid
number of
chromosomes in
the zygote (46).
• Determines the
chromosomal sex
of the embryo.
• Initiates cleavage
(cell division) of the
zygote.

Sex of the Embryo
• Embryo's
Embryo's
chromosomal
chromosomal
sex
sex is determined
is determined
at the time of
at the time of
fertilization.
fertilization.
• Sex is determined
Sex is determined
by the type of
by the type of
sperm (X or Y)
sperm (X or Y)
that fertilizes the
that fertilizes the
oocyte.
oocyte.
• So, it is the
So, it is the father
father
whose gamete
whose gamete
decides the sex of
decides the sex of
the embryo.
the embryo.

Chromosomes in zygote
• Zygote is genetically unique.
• Half of its chromosomes come from the father and
the other half comes from the mother.
• zygote contains 46 chromosomes (diploid).
• New combination is formed which is different from
either of the parents.
• This mechanism forms biparental inheritance and
leads to variation of the human species.

Cleavage
Cleavage

Repeated
Repeated mitotic
mitotic division
division
of the zygote.
of the zygote.

Begins about
Begins about 30
30 hours
hours
after fertilization.
after fertilization.

There is rapid increase in
There is rapid increase in
the number of cells.
the number of cells.

The cells which is called
The cells which is called
(
(blastomeres
blastomeres) become
) become
smaller with each division.
smaller with each division.

Normally occurs as the
Normally occurs as the
zygote passes along the
zygote passes along the
uterine tube to the uterus
uterine tube to the uterus

During cleavage, zygote is
During cleavage, zygote is
within the
within the zona pellucida.
zona pellucida.


After 8-cell
After 8-cell
stage, the
stage, the
cells become
cells become
compactly
compactly
arranged
arranged
compaction
compaction

16
16 cell stage
cell stage
is called
is called
morula
morula.
.

It is formed
It is formed
about
about 3 days
3 days
after
after
fertilization
fertilization
and enters
and enters
the uterus.
the uterus.

 Fluid filled space
Fluid filled space
called the
called the
blastocyst cavity
blastocyst cavity
(blastocele)
(blastocele)
appears inside the
appears inside the
morula.
morula.
 Now Blastomeres
Now Blastomeres
are separated into:
are separated into:
 Outer cell layer,
Outer cell layer,
the
the trophoblast
trophoblast,
,
which gives rise
which gives rise
to embryonic
to embryonic
part of placenta.
part of placenta.
 Inner cell mass
Inner cell mass
(embryoblasts)
(embryoblasts)
which gives rise
which gives rise
to the embryo.
to the embryo.

 At this stage, the
At this stage, the
conceptus is called
conceptus is called
Blastocyst
Blastocyst. It has
. It has
two poles:
two poles:
embryonic
embryonic &
&
abembryonic.
abembryonic.
 Zona pellucida
Zona pellucida
gradually
gradually
degenerates and
degenerates and
disappears.
disappears.
 Blastocyst takes its
Blastocyst takes its
nourishment from
nourishment from
uterine secretions.
uterine secretions.
 It enlarges in size.
It enlarges in size.
 It is ready to get
It is ready to get
attached and
attached and
implanted to the
implanted to the
uterine wall.
uterine wall.
Embryonic pole
Abembryonic pole

Implantation
Implantation

6 days after
6 days after
fertilization:
fertilization:

Blastocyst
Blastocyst
attaches to the
attaches to the
endometrial
endometrial
epithelium, at
epithelium, at
its
its embryonic
embryonic
pole.
pole.


Trophoblast
Trophoblast
proliferates rapidly
proliferates rapidly
and differentiates
and differentiates
into two layers:
into two layers:

Inner cellular
Inner cellular
cytotrophoblast
cytotrophoblast, and
, and

Outer mass
Outer mass of
of
syncytiotrophoblast
syncytiotrophoblast
(multinucleated
(multinucleated
protoplasm with no
protoplasm with no
cell boundaries).
cell boundaries).

Finger like processes
Finger like processes
of
of
syncytiotrophoblast
syncytiotrophoblast
extend through the
extend through the
endometrium and
endometrium and
invade the
invade the
endometrial
endometrial
connective tissue.
connective tissue.


By the end of
By the end of 7
7th
th
day
day, the
, the
blastocyst gets
blastocyst gets
implanted in the
implanted in the
superficial
superficial
compact layer of
compact layer of
the endometrium
the endometrium
and derives its
and derives its
nourishment
nourishment
from the eroded
from the eroded
endometrium.
endometrium.


The blastocyst
The blastocyst
gradually embed
gradually embed
deeper in the
deeper in the
endometrium
endometrium
and the defect in
and the defect in
the endometrial
the endometrial
epithelium is
epithelium is
filled by
filled by closing
closing
plug
plug (day 10).
(day 10).

 The defect
The defect
gradually
gradually
disappear as
disappear as
the endometrial
the endometrial
epithelium is
epithelium is
repaired
repaired (
(day 12
day 12
& 13)
& 13)
 Blood filled
Blood filled
lacunae appear
lacunae appear
in the
in the
syncytiotrophoblast
syncytiotrophoblast
which filled with
which filled with
maternal blood,
maternal blood,
establishing
establishing
primitive
primitive
uteroplacental
uteroplacental
circulation.
circulation.

Implantation Sites
Implantation Sites

Uterine:
Uterine:
 Usually occurs
Usually occurs
in the
in the posterior
posterior
wall
wall of the body
of the body
of uterus near
of uterus near
the fundus.
the fundus.
 Implantation in
Implantation in
the lower
the lower
segment leads
segment leads
to
to placenta
placenta
Praevia
Praevia

Extrauterine:
Extrauterine:
leading to ectopic
leading to ectopic
pregnancies:
pregnancies:
 Fallopian tube
Fallopian tube
 Ovary
Ovary
 Abdomen
Abdomen
 Cervical
Cervical

02 Fertilization & Implantation.pptFertilization & Implantation

02 Fertilization & Implantation.pptFertilization & Implantation

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