Fertilization is the process by which an egg is fertilized by a sperm to produce a new organism. It begins with the collision of sperm and egg and ends with the formation of a single-celled zygote. Fertilization usually occurs in the fallopian tubes. The zygote then undergoes cell division called cleavage over the next few days as it is transported down the fallopian tube through the uterus and into the womb. By the fourth day, it develops into a hollow ball of cells called a blastocyst which implants in the uterine wall, where it continues to develop into an embryo.

1. SUJATHA.A
MSC NURSING
TOPIC ON FERTILIZATION

3. Anatomy of Sperm

4. AnatomyofOva

5. Corona radiata
Zona pellucida
(ZP-1, -2, and -3)
Cortical granules
The egg (and corona radiata) at ovulation

6. Follicle Maturation and Ovulation
Oocytes
~2 million at birth
~40,000 at puberty
~400 ovulated over lifetime
Leutinizing Hormone surge
(from pituitary gland)
causes changes in tissues
and within follicle:
• Swelling within follicle due to
increased hyaluronan
• Matrix metalloproteinases
degrade surrounding tissue
causing rupture of follicle
Egg and surrounding cells
(corona radiata) ejected into
peritoneum
Corona radiata provides bulk to
facilitate capture of egg.

7. FERTILIZATION
 Fertilization is the process of fusion of the
spermatazoon with the mature ovum.It begins
with sperm and egg collision and ends with the
production of mononucleated single cell called
ZYGOTE.
 Almost always fertilization occurs in
AMPULLARY portion of fallopian tubes.

10. APPROXIMATION OF GAMETES
 The ovum immediately following ovulation is
picked up by the tubal fimbriae which partly
envelope the ovary, specially at the time of
ovulation .
 The pick up action might be muscular or by a kind
of suction by ciliary action or by a postivie
chemotaxis exerted by the tubal secretion.

11. Transport through the oviduct
At around the midpoint of the
menstrual cycle (~day 14), a
single egg is ovulated and swept
into the oviduct.
Fertilization usually occurs in the
ampulla of the oviduct within 24
hrs. of ovulation.
Series of cleavage and
differentiation events results in
the formation of a blastocyst by
the 4th embryonic day.
Inner cell mass generates
embryonic tissues
Outer trophectoderm generates
placental tissues
Implantation into the uterine wall
occurs ~6th embryonic day (day 20
of the menstrual cycle)

13.  The ovum is rapidly transported to the
ampullary part.
 Fertilisable life span of oocyte ranges from 12
to 24 hours where as that of sperm is 48 to 72
hours.
 Out of hundred of millions of sperms
deposited in the vagina at single ejaculation.
Only thousands capacitated spermataozoa
enter the uterine tube while only 300-500
reach the ovum.
 The tubal transport is facilitated by muscular
contraction and aspiration action of the
uterine tube.

14. CONTACT AND FUSION OF THE
GAMETES
 Complete dissolution of the cells of the corona
radiata occurs by the chemical action of the
HYALURONIDASE liberated from theAcrosomal
cap of the hundreds of sperm present at the site.
 Penetration of Zona pellucida is facilitated by
Hyaluronidase from the acrosomal cap.
 Soon after the Sperm fusion ,penetration ofother
sperm is prevented by ZONA REACTION and
OLEMA BLOCK.

15. Embryologists
Fertilization age: moment of fertilization is dO
Division of pregnancy corresponding to development:
0-3 weeks –early development
3-8 weeks –embryonic period (organogenesis)
8 wks-term –fetal period
Total gestation time = 38 weeks
Clinicians
Menstrual age: last menses is dO
Division of pregnancy into trimesters
Total gestation time = 40 weeks
Timing of
pregnancy

16. 1. Acrosome Rx
sperm bind to ZP proteins in the
zona pellucida; this initiates the
release of enzymes from the sperm
allowing it to burrow through the
zona pellucida.
2. Zona Rx
binding of the sperm and egg
plasma membranes initiates Ca+
influx into the egg and release of
cortical granules from the egg that
block other sperm from fertilizing
the egg.
Fertilization is a multi-step process whereby multiple sperm bind to the
corona radiata, but only a single sperm usually fertilizes the egg

17. This so-called cortical reaction prevents other
sperm from fertilizing the egg (aka
“polyspermy”)
Cortical granule
enzymes digest ZP
proteins so other sperm
can no longer bind.
Hyaluronic acid and
other proteoglycans are
also released that
become hydrated and
swell, thus pushing the
other sperm away.

18. Meiosis II complete
Formation of male and
female pronuclei
Decondensation of male
chromosomes
Fusion of pronuclei
Zygote
Fertilization

19. Week 1:
days 1-6
1
Cleavage, day 2-3
Compaction, day 3
Formation of blastocyst, day 4
Ends with implantation, day 6Fertilization,
day

20. Fertilized egg
2 polar bodies
2 pronuclei
Day 1
0.1 mm
Fertilized egg (zygote)

21. Cleavage
Cleavage = cell division
Goals: grow unicellular
zygote to multicellular embryo.
Divisions are slow: 12 - 24h ea
No growth of the embryo-
stays at ~100 um in diameter
Divisions are not synchronous
Cleavage begins about 24h after
pronuclear fusion

22. 2 Cell Stage
Individual cells = blastomeres
Mitotic divisions maintain
2N (diploid) complement
Cells become smaller
Blastomeres are equivalent (aka totipotent).

23. 4 cell; second cleavage
4 equivalent blastomeres
Still in zona pellucida

24. 8 Cell; third
cleavage
Blastomeres still
equivalent

25. Embryo undergoes compaction after 8-cell stage:
first differentiation of embryonic lineages
CCells aused by increased cell-cell adhesion
that are forced to the outside of the morula are destined to
become trophoblast--cells that will form placenta
The inner cells will form the embryo proper and are called
the inner cell mass (ICM).

26. Formation of the blastocyst
Sodium channels appear on the surface of the outer trophoblast cells;
sodium and water are pumped into the forming blastocoele. Note that
the embryo is still contained in the zona pellucida.

28. Early blastocyst
Day 3
Later blastocyst
Day 5
blastocoele
inner cell mass

29. Monozygotic twinning typically occurs
during cleavage/blastocyst stages

30. “Hatching” of the blastocyst:
preparation for implantation
Hatching of the embryo from the zona pellucida occurs just
prior to implantation. Occasionally, the inability to hatch
results in infertility, and premature hatching can result in abnormal implantation in the
uterine tube.

31. IMPLANTATION/NIDATION/NESTING
 Implantation occurs in the endometrium of the
anterior or posterior wall of the body near the
fundus on the 6th day which corresponds,to the 20th
day of regular menstural cycle.
 IMPLANTATION occurs through four stages.
 Apposition
 Adhesion
 Penetration
 Invasion

32.  The implantation process is completed by 10th
or 11th day.
 This type of deeper penetration of human
blastocyst is called as IMPLANTATION.
 Now the blastocyst is covered all sides by the
endometrium (DECIDUA).

33. Implantation and placentation (day 8)
Trophoblast further differentiates and invades maternal tissues
– Cytotrophoblast: stem cell population
– Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast
– Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood
Inner cell mass divides into epiblast and hypoblast:
– Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity
– Hypoblast contributes to forming the underlying yolk sac.

34. Implantation and placentation (day 9)
Trophoblast further differentiates and invades maternal tissues
– Cytotrophoblast: stem cell population
– Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast
– Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood
Inner cell mass divides into epiblast and hypoblast:
– Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity
– Hypoblast contributes to forming the underlying yolk sac.

35. Implantation and placentation (day 12)
Trophoblast further differentiates and invades maternal tissues
– Cytotrophoblast: stem cell population
– Syncytiotrophoblast: invasive fused cells (syncytium) derived from cytotrophoblast
– Breaks maternal capillaries, trophoblastic lacunae fill with maternal blood
Inner cell mass divides into epiblast and hypoblast:
– Epiblast contributes to forming the overlying amniotic membrane and amniotic cavity
– Hypoblast contributes to forming the underlying yolk sac.

36. Implantation and placentation (day 13)
Trophoblast further
differentiates and invades
maternal tissues
– Cytotrophoblast: stem cell
population
– Syncytiotrophoblast: invasive
fused cells (syncytium) derived
from cytotrophoblast
– Breaks maternal capillaries,
trophoblastic lacunae fill with
maternal blood
Inner cell mass divides into
epiblast and hypoblast:
– Epiblast contributes to forming
the overlying amniotic
membrane and amniotic cavity
– Hypoblast contributes to
forming the underlying yolk sac.

37. Week 3:
Days 14-
21
Two layer germ disc
Primitive streak forms
Gastrulation forms tri-laminar embryo
Neural induction
Left-right asymmetry
0.4mm - 2.0mm

39. Day 15
The
primitive
streak can
be seen on
the left side
Day 17
The primitive streak can still
be seen, and the opposite end
of the embryo is starting to
fold up.
The Blastocyst begins to
collapse

40. Day 19
The neural tube is seen along with somites
on either side of it.
Somites - zipper-like motion of the neural tube closing together,
three pairs of small bumps form on either side of the closure.
they will form the skeleton and the major muscles of the body.
Thirty-eight pairs of somites will line the neural tube within 2
weeks.

41. Day 22
Day 24

42. Day 26
Day 28

43. 5 Week Embryo
5 Week, 4 day Embryo

44. Four Weeks Embryo
After the cluster of cells attaches to
the womb it is called an embryo.
The embryo is between 1/100 and
4/100 inch long at this time.
The embryo continues rapid
growth.

45. * 8 Weeks
•The embryo is about ½ inch
long.
•The heart now has four
chambers.
•Fingers and toes begin to
form.
•Reflex activities begin as the
brain and nervous system
develop.
•Cells begin to form the eyes,
ears, jaws, lungs, stomach,
intestines and liver.

46. 10 Weeks
The embryo, is about 1 to 1¼
inches long (the head is about
half the length) and weighs less
than ½ ounce.
The beginnings of all key body
parts are present, but they are
not completed.
Structures that will form eyes,
ears, arms and legs can be seen.
Muscles and skeleton are
developing and the nervous
system becomes more
responsive.

47. 12 Weeks
The fetus is about 2½ inches
long and weighs about ½ ounce.
Fingers and toes are distinct
and have nails.
Hair begins to develop, but
won't be seen until later in the
pregnancy.
The fetus begins small, random
movements, too slight to be felt.
The fetal heartbeat can be detected with a heart monitor.
All major external body features have appeared.
Muscles continue to develop.

48. 14 Weeks
The fetus is about 3½ inches long
and weighs about 1½ ounces.
The fetus begins to swallow, the
kidneys make urine, and blood
begins to form in the bone marrow.
Joints and muscles allow full body
movement.
There are eyelids and the nose is
developing a bridge.
External genitals are developing.

49. 18 Weeks
The fetus is about 5½ inches long
and weighs about 7 ounces.
The skin is pink and transparent
and the ears are clearly visible.
All the body and facial features are
now recognizable.
The fetus can grasp and move its
mouth.
Nails begin to grow.
The fetus has begun to kick. Some
women feel this movement.

50. 20 Weeks
The fetus is about 6¼
inches long and weighs
about 11½ ounces.
All organs and structures
are formed
Skin is wrinkled and pink to reddish in
color - thin and close to the blood vessels.
Protective skin coating, (vernix) begins to develop.
Respiratory movements occur - lungs have not developed enough to
permit survival outside the uterus.
By this time, mothers usually feel the fetus moving.
At this time an ultrasound can often identify the sex of the fetus.

51. 22 Weeks
The fetus is about 7½ inches
long weighs about one pound.
.
It has fingerprints and some
head and body hair.
It may suck its thumb and is more active.
The brain is growing very rapidly.
The fetal heartbeat can be easily heard.
The kidneys start to work.
At 23 weeks, approximately 31% of babies born survive.
Babies born at this age require intensive care and usually
have lifelong disabilities and chronic health conditions.

52. 24 Weeks
The fetus is about 8¼ inches long and
weighs about 1¼ pounds.
Bones of the ears harden making sound
conduction possible. The fetus hears
mother’s sounds such as breathing,
heartbeat and voice.
The first layers of fat are beginning to
form.
This is the beginning of substantial
weight gain for the fetus.
Lungs continue developing
At 25 weeks, approximately 68% of
babies born survive. Babies born at this
age require intensive care and usually
have life-long disabilities and chronic
health conditions.

53. 26 Weeks
The fetus is about 9 inches long and
weighs about 2 pounds.
The fetus can respond to sound from
both inside and outside the womb.
Reflex movements continue to
develop and body movements are
stronger.
Lungs continue to develop.
The fetus now wakes and sleeps.
The skin is slightly wrinkled.
At 27 weeks, approximately 87% of
babies born survive. Babies born at
this age require intensive care and
have an increased risk of
developmental delays and chronic
health conditions.

54. 28 Weeks
The fetus is about 10 inches
long and weighs about 2
pounds, 3 ounces.
Mouth and lips show more
sensitivity.
The eyes are partially open
and can perceive light.
More than 90% of babies born
at this age will survive. Some
survivors have developmental
delays and chronic health
conditions.

55. 30 Weeks
The fetus is about 10½ inches long
and weighs about 3 pounds.
The lungs that are capable of
breathing air, although medical
help may be needed.
The fetus can open and close its
eyes, suck its thumb, cry and
respond to sound.
The skin is smooth.
Rhythmic breathing and body
temperature are now controlled by
the brain.
Most babies born at this age will
survive.

56. 32 Weeks
The fetus is about 11
inches long and weighs
about 3 pounds, 12
ounces.
The connections between
the nerve cells in the brain
increase.
Fetal development now
centers on growth.
Almost all babies born at
this age will survive.

57. 34 Weeks
The fetus is about
12 inches long and
weighs about 4½
pounds.
Ears begin to hold
shape.
Eyes open during
alert times and
close during sleep.
Almost all babies
born at this age
will survive.

58. 36 Weeks
The fetus is about 12 to 13
inches long and weighs
about 5½ to 6 pounds.
Scalp hair is silky and lies
against the head.
Muscle tone has developed
and the fetus can turn and
lift its head.
Almost all babies born at
this age will survive.

59. 38 Weeks
The fetus is about 13½
to 14 inches long and
weighs about 6½
pounds.
Lungs are usually
mature.
The fetus can grasp
firmly.
The fetus turns toward
light sources.
Almost all babies born
at this age will survive.

60. 40 Weeks
The fetus is about 18
to 20 inches long
and may weigh
about 7½ pounds.
At the time of birth,
a baby has more
than 70 reflex
behaviors, which are
automatic behaviors
necessary for
survival.
The baby is full-term
and ready to be born.
Photo andText
Courtesy of
Georgia Department of Human Resources,
Division of Public Health

62. Fetal Development
 Objectives:
 Understand the growth of a
human fetus.
 Follow the development
from conception to birth
 Introduction to the birth
process
Photo andText
Courtesy of
Georgia Department of Human Resources,
Division of Public Health