Fertilization, Cleavage and Implantation

FERTILIZATION, CLEAVAGE
AND IMPLANTATION
BY
DR. ELIOBA J. RAIMON
RESIDENT – OBSTETRICS & GYNECOLOGY
17/10/2023 1
Fertilization by Dr. Elioba

INTRODUCTION
Although life is a continuous process, fertilization is a critical
landmark because, under ordinary circumstances, a new, genetically
distinct human organism is thereby formed
The combination of 23 chromosomes present in each pronucleus
results in 46 chromosomes in the zygote.
Thus the diploid number is restored and the embryonic genome is
formed.
The embryo now exists as a genetic unit.
17/10/2023 Fertilization by Dr. Elioba 2

INTRODUCTION CONT’D
Fertilization is the process of fusion of male and female gametes
(pronuclei) to form a zygote.
It takes place within 12-24 hours of ovulation, at the ampulla of the
fallopian tube
For a proper understanding, fertilization is described under the
following headings:
Review of gametogenesis
Factors necessary for fertilization
Approximation of gametes
Fusion of gametes
Results of fertilization
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17/10/2023 4

GAMETOGENESIS
Gametogenesis occurs when a haploid cell (n) is formed from a
diploid cell (2n) through meiosis.
We call gametogenesis in the male spermatogenesis and it
produces spermatozoa.
In the female, we call it oogenesis. It results in the formation of
ova.

SPERMATOGENESIS
Males start producing sperm when they reach puberty, which is
usually from 10-16 years old.
Biological males continually produce sperm in large quantities
(~200 million a day).
This maximises the likelihood of sperm reaching the egg following
ejaculation.
Sperm production occurs in the testes of the male, specifically in
the seminiferous tubules.
In the testicles, a blood-testis barrier forms to keep the tubules
separate from the systemic circulation.

SPERMATOGENESIS CONT’D
Sertoli cells form the blood-testis barrier. This is important in
preventing substances found in blood from affecting the developing
sperm.
It is also important as it prevents the immune system of the male
from recognising the sperm as foreign.
Spermatogenesis takes approximately 70 days.
Therefore in order for sperm production to be continuous and not
intermittent, multiple spermatogenic processes are occurring
simultaneously within the same seminiferous tubule.
Spermatogonia are the initial pool of diploid cells that divide by
mitosis to give two identical cells (Type A and B spermatogonia)

Type A spermatogonia is used to replenish the pool of spermatogonia and this
replenishment of spermatogonia means that males are fertile throughout their adult
life.
The other cell, type B spermatogonium will eventually form mature sperm.
Type B spermatogonia replicate by mitosis several times to form identical diploid
cells known as primary spermatocytes.
Primary spermatocytes then undergo meiosis.
1. Meiosis I - Produces two haploid cells, known as secondary spermatocytes.
2. Meiosis II - Produces four haploid cells, known as spermatids
The spermatids are released into the lumen of the seminiferous tubule a process
called spermiation.
The spermatids undergo spermiogenesis (remodelling and differentiation into
mature spermatozoa) as they travel along the seminiferous tubules until they reach
the epididymis.

STRUCTURE OF MATURE SPERM

MOVEMENT OF THE SPERM
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OOGENESIS
Oogenesis differs from spermatogenesis in that it begins in the foetus prior
to birth.
Primordial germ cells (which originate in the yolk sac of the embryo) move
to colonise the cortex of the primordial gonad.
Replication by mitosis peaks at approximately 7-8 million by mid-gestation
(~20 weeks).
Cell death occurs after this peak to leave 2 million cells.
Meiosis I begins before birth and forms primary oocytes. There is therefore a
finite supply of ova.
Primary oocytes are arranged in the gonads as clusters. They have flattened
epithelial cells surrounding them, and this is called the primary follicle.

During childhood, further atresia (cell death) occurs, leaving ~40,000 eggs at
puberty.
The primary oocyte is arrested at prophase of Meiosis I until puberty.
Once puberty begins, a number of primary oocytes (15-20) begin to mature each
month, although only one of these reaches full maturation to become an oocyte.
The LH surge prior to ovulation induces meiosis I to be completed.
Inside the follicle, 2 unequally sized haploid cells form. One of the daughter cells
receives far less cytoplasm than the other and forms the first polar body, which will
not go on to form an ovum.
Another haploid cell is also formed, known as the secondary oocyte. Both daughter
cells then undergo meiosis II.
An initial polar body will replicate to give two polar bodies but the secondary oocyte
arrests in metaphase of meiosis II. This happens 3 hours prior to ovulation.
Meiosis II only become completed during fertilization when the sperm enter the
cytoplasm of the ovum
OOGENESIS CONT’D

OOGENESIS CONT’D

CONDITIONS NECESSARY FOR FERTILIZATION
MALE
Production of sperm in sufficient
number and motility
Patent vas deferens
Healthy spermatozoa should be
deposited high up in the vagina
Transport of sperms to the fallopian
tube
Sperm undergo capacitation and
acrosome reaction
FEMALE
Ovulation should take place
The egg must be picked up by the
tube
Patent fallopian tube
Receptive endometrium for
implantation
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EVENTS DURING FERTILIZATION
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APPROXIMATION OF GAMETES
During coitus (sexual intercourse), a male ejaculates about 200 -
300 million sperms into the female’s vagina
Only 200 - 500 sperms survive (i.e., only hundreds out of millions)
to contact the female gamete (the secondary oocyte in the uterine
tube)
The journey of sperms from the vagina to the ampulla of uterine
tube is promoted by prostaglandins present in the semen.
The prostaglandins cause powerful contraction of uterine muscle,
which is substantiated by oxytocin released by neurohypophysis.
Uterine contractions create a negative pressure in the uterine
cavity.
17/10/2023 17

Before fertilization, the sperm must gain the ability to disintegrate the
various oocyte barriers. These barriers of an oocyte are:
1. First barrier is formed by cells of corona radiata.
2. Second barrier is formed by zona pellucida made of
glycoproteins, ZP1, ZP2, and ZP3.
3. Third barrier is formed by vitelline membrane of oocyte itself.
This ability is achieved by two processes:
1. Capacitation
2. Acrosome reaction
APPROXIMATION OF GAMETES CONT’D
17/10/2023 18

CAPACITATION
It is a process of conditioning of sperm in the female genital tract;
it lasts about 7 hours.
During capacitation, the glycoprotein coat and seminal plasma
proteins covering the plasma membrane of sperm in the
acrosomal region are removed.
Only capacitated sperm undergoes acrosome reaction and
fertilizes the ovum
17/10/2023 19

ACROSOME REACTION
It occurs when capacitated sperm comes in contact with the zona
pellucida (i.e., acrosome reaction is induced by zona proteins).
The acrosomal reaction leads to release of enzymes from
acrosomal cap such as hyaluronidase and acrosin.
The antigenic coating of the sperm initiates an immunological
reaction between the oocyte’s fertilizin and the sperm’s
antifertilizin
17/10/2023 20

STEPS OF FERTILIZATION
A. Penetration of
corona radiata
The capacitated
sperm can freely
penetrate
through the
corona radiata to
reach the zone
pellucida by the
movements of its
tail.
17/10/2023 21

STEPS OF FERTILIZATION CONT’D
B. Penetration of zona pellucida
Acrosome reaction
Once the sperm penetrates
the zona pellucida a change in
the properties of zona
pellucida (zona reaction)
occurs that makes it
impermeable to other sperms.
17/10/2023 22

C. Fusion of sperm and oocyte cell membranes
The plasma membranes of sperm and oocyte
come in contact and breakdown at the site of
fusion.
The head and tail of sperm enter the cytoplasm
of the oocyte but its plasma membrane and
mitochondrial sheath are left behind on the
oocyte surface.
As soon as the sperm enters the oocyte a
calcium wave appears in the cytoplasm of
oocyte that makes oocyte membrane
impermeable to other sperms.
17/10/2023 23

D. Completion of second meiotic division
of oocyte and formation of female
pronucleus
The penetration of oocyte by the
sperm activates the oocyte to
complete its second meiotic
division.
The two cells are produced (one cell
containing all the cytoplasm called
mature oocyte and second cell
containing hardly any cytoplasm
called second polar body)
17/10/2023 24

E. Formation of male pronucleus
The sperms move forward
to come in close contact
with the female pronucleus.
Its nucleus becomes swollen
and forms the male
pronucleus.
The tail detaches and
degenerates.
Morphologically, the male
and female pronuclei are
indistinguishable.
17/10/2023 25

F. Formation of zygote
The male and female
pronuclei loose their cell
membrane and
chromosomes of two nuclei
(23 in each) mix together to
form diploid (i.e., 46
chromosomes).
17/10/2023 26

17/10/2023 27

RESULTS OF FERTILIZATION
When the oocyte is fertilized by the sperm a life of new individual
begins. The main results of fertilization are as follows.
1. Completion of second meiotic division of the female gamete
(i.e., secondary oocyte)
As soon as the sperm enters into the secondary oocyte the latter
completes its second meiotic division and extrudes the second polar
body into the perivitelline space.
2. Restoration of diploid number of chromosomes
The male and female pronuclei (both haploid) fuse with each other to
restore normal diploid number of chromosomes.
17/10/2023 28

3. Determination of chromosomal sex of the new individual
The oocytes are only of one type, i.e., they contain only ‘X’ chromosomes
The sperms are of two types:
 ‘Y’-bearing sperms (androsperms) and
 ‘X’-bearing sperms (gynosperms)
If an oocyte (X) is fertilized by ‘Y’- bearing sperm the result will be a male
baby and if an oocyte is fertilized by an ‘X’-bearing sperm the result will
be a female baby
Therefore, it is the father who is responsible for determination of the sex
of the baby and not the mother.
4. Initiation of cleavage
After fertilization, the zygote undergoes a series of rapid mitotic divisions.
This is called cleavage.
RESULTS OF FERTILIZATION CONT’D
17/10/2023 29

CLEAVAGE - MORULA
After the zygote formation, mitotic division occur producing
blastomeres.
Starting with the two cell stage after 30 hours from fertilization.
The blastomeres continue to divide by binary division through 4, 8,
16 cell stage until a cluster of cells is formed and is called morula.
The morula spend about 3 days in the uterine tube and enters the
uterine cavity on the 4th day.
The central cell of the morula is known as inner cell mass which
forms the embryo proper and the peripheral cells are called outer
cell mass which will form protective and nutritive membranes of
the embryo
17/10/2023 30

CLEAVAGE CONT’D
17/10/2023 31

CLEAVAGE CONT’D

While the morula remains free in the uterine cavity on the 4th and
5th day, it is covered by a film of mucus.
The fluid passes through the canaliculi of the zona pellucida which
separates the cells of the morula and is now termed blastocyst.
Due to blastocyst enlargement the zona pellucida becomes stretched,
thinned and gradually disappears. Lysis of zona and escape of embryo
is called zona hatching
It is the blastocyst which become implanted in the uterine
endometrium
17/10/2023 33
CLEAVAGE – BLASTOCYST CONT’D

BLASTOCYST

IMPLANTATION
Implantation occurs in the endometrium of the superior posterior wall
of the body near the fundus mostly but can also occur in other parts of
the upper uterine lining.
Implantation in the lower uterine segment leads to placenta previa
It occur on the 6th day which corresponds to the 20th day of a regular
menstrual cycle.
Implantation occurs through four stages
1. Apposition
2. Adhesion
3. Penetration
4. Invasion
Implantation is interstitial in humans
17/10/2023 35

IMPLANTATION

SAMPLE MCQs
1. At the 32-cell stage of division, the embryo is known as a morula that
contains inner cell mass and outer cell mass. What do the inner and outer cell
masses eventually become?
A. Placenta / embryo and foetus
B. Embryo and foetus / placenta
C. Chorionic membrane / amniotic membrane
D. Amniotic membrane / chorionic membrane
2. Where is the normal location of initial implantation of the embryo?
A. Posterior superior wall of uterus
B. Anterior posterior wall of uterus
C. Lateral superior wall of uterus
D. Medial posterior wall of uterus
E. Ampulla of uterine tube
17/10/2023 38

3. Before a spermatozoan can fertilize an ovum
A. The cell membrane is removed from the spermatozoan head
B. The zona pellucida must be shed
C. The acrosomal membrane must perforate
D. The tailpiece is lost
E. The decidua reaction must occur
4. At fertilization
A. The haploid number of chromosomes is restored
B. The chromosomal sex is determined
C. Cleavage is inhibited
D. The inner cell mass becomes recognizable
E. The ovum becomes the blastocyst
17/10/2023 39
SAMPLE MCQs

5. The daughter cells formed as a result of cleavage of a zygote are called
A. Blastocysts
B. Blastula
C. Blastomeres
D. Blastocoel
6. Which layer of blastocyst get attached to the endometrium of
the uterus
A. Trophoblast
B. Embryoblast
C. Inner cell mass
D. Blastocoel
17/10/2023 40
SAMPLE MCQs

7. The cavity present in the blastocyst is called
A. Coelom
B. Fluid
C. Blastocele
D. Bastopore
8. All of the following are directly involved with the implantation process
except
A. Decidual reaction
B. Progesterone
C. Epiblast
D. Invasion
E. none of the above
17/10/2023 41
SAMPLE MCQs

9. The seven-day blastocyst
A. Has a single layer of trophoblast at the embryonic pole
B. Has an amniotic cavity
C. Is attached to the endometrial epithelium
D. Is surrounded by a degenerating zona pellucida
E. Is called the hypoblast
10. The most significant factors in sperm transport to the vicinity of the oocyte is
A. Sperm flagellum (tail)
B. Vaginal cilia
C. An intact zona pellucida
D. Smooth muscle contractions
E. Gravity
17/10/2023 42
SAMPLE MCQs

REFERENCE
1. Vishram singh, Textbook of Clinical Embryology, 2012
page 20 – 45
2. Williams textbook of Obstetrics, 25th edition
3. Obstetrics normal and problem pregnancy by Gabbe et
al
17/10/2023 43

THE END
AFTER FERTILIZATION HAS TAKEN
PLACE A NEW HUMAN BEING HAS
COME INTO BEING
17/10/2023 44

Fertilization, Cleavage and Implantation

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