Fertilization involves the fusion of an egg and sperm cell. It is a multi-stage process beginning with sperm penetration of the egg's outer layers and ending with the fusion of genetic material to form a single cell called a zygote. The zygote then undergoes cell division called cleavage as it travels through the fallopian tube. Around 6 days after fertilization, the blastocyst implants in the lining of the uterus, where it derives nourishment and establishes the beginnings of the placenta and embryonic development. Implantation normally occurs in the uterine wall, but can sometimes occur in abnormal locations leading to ectopic pregnancy.

1. FERTILIZATION & IMPLANTATION

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

3. 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
of maternal &
paternal
chromosomes.

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

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

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

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

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

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

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

11. 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.

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

13. 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.

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

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

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

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

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

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

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

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

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

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