Animal Reproduction

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