fertilization & embryology

1. SHRI VINOBA BHAVE
COLLEGE OF NURSING
SUBJECT: OBSTETRIC AND GYNECOLOGICAL NURSING
TOPIC: FERTILIZATION AND EMBRYONIC DEVELOPMENT
PRESENTED BY: KHAN NAJBUN V.
I YEAR M.SC (N)
ROLL NO: 06

2. Introduction
 Human fertilization, known as conception, is the fusion of the
sperm with the secondary oocyte , to form the zygote. The process
takes approximately 24 hours and normally occurs in the ampulla of
the uterine tube.

3. Fertilization
“fertilization is the process of
fusion of the spermatozoon with
the mature ovum.”
fertilization begins with sperm
egg collision and ends with
production of a mononucleated
single cell called the zygote.

4. Objectives
 To initiate the embryonic development of the egg.
 To restore the chromosome no. of the species.
Almost , always fertilization occurs in the ampullary part of the
uterine tube.

5. Approximation of the gametes
 The ovum, immediately following ovulation is picked up by the tubal
fimbriae which partly envelop the ovary, especially at the time of
ovulation.
 The pick up action might be muscular or by a kind of suction or by a
positive chemotaxis exerted by the tubal secretion. The ovum is rapidly
transported to the ampullary part.
 Fertilizable life span of oocyte ranges from 12 to 24 hours whereas that of
sperm is 48 to 72 hours.
 Out of hundreds of millions of sperm deposited in the vagina at single
ejaculation, only thousands capacitated spermatozoa enter the uterine tube
while only 300-500reach the ovum. The tubal transport is facilitated by
muscular contraction and aspiration action of the uterine tube. It takes
only few minutes for the sperm to reach the fallopian tube.

6. 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 the acrosomal cap
of the hundreds of sperm present at the site.
 Penetration of the zona pellucida is facilitated by the release of
hyaluronidase from the acrosomal cap. More than one sperm may
penetrate the zona pellucida.
 Out of the many sperms, one touches the oolemma. Soon after the
sperms fusion, penetration of other sperm is prevented by zona
reaction and oolemma block. This is due to release of cortical granules
by exocytosis from the oocyte.
 Completion of the second meiotic division of the oocyte immediately
follows, each containing haploid numbers of chromosomes. The
biggest one is called the female pronucleus and the smaller one is
called second polar body which is pushed to the perivitelline space.

8.  In the human, both the head and tail of the spermatozoon enter the
cytoplasm of the oocyte but the plasma membrane is left behind on
the oocyte surface. Head and neck of the spermatozoon become
male pronucleus containing haploid number of chromosomes.
 The male and the female, pronuclei unite at the centre with
restoration of the diploid number of chromosomes which is constant
for the species.
 The zygote, thus formed, contains both the paternal and maternal
genetic materials.
 Sex of the child is determined by the pattern of the sex chromosome
supplied by the spermatozoon. If the spermatozoon contains ‘x’
chromosome, a female embryo (46,xx) is formed, if it contains a ‘y’
chromosome, a male embryo (46, xy) is formed.

10. Morula
 After the zygote formation, typical miotic division of the nucleus occurs
by producing two blastomeres.
 The two cell stage is reached approximately 30 hours after fertilization.
Each contains equal cytoplasmic volume and chromosome numbers. 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, resembling a
mulberry. As the total volume of the cell mass is not increased and the
zona pellucida remains intact, the morula after spending about 3 days in
the uterine tube enters the uterine cavity through the normal uterine
ostium (1mm) on the 4th day in the 16-64 cell stage.
 The transport is a slow process and is controlled by muscular contraction
and movement of the cilia. The central cell of the morula is known as
inner cell mass which forms the embryo proper and the peripheral cells
are called out cell mass which will form protective and nutritive
membranes of the embryo.

12. Blastocyst
 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.
 Zona hatching is the next step so that trophectoderm cells interact with
endometrial cells and implantation occurs.
 Due to blastocyst enlargement the zona pellucida becomes stretched, thinned
and gradually disappears. Lysis of zona and escape of embryo is called zona
hatching. The cells on the outer side of the morula become trophectoderm &
the inner cell become inner cell mass by the mediation of epithelial cadherin.
 Trophectoderm differentiates into chorion (placenta) and the inner cell mass
into the embryo.
 Completely undifferentiated cells are called the pluripotent embryonic stem
(ES) cells. ES cells are able to produce mature somatic cells of any germ
layer.

14. Morula

15. Implantation ( Nidation)
 Implantation occurs in the endometrium of the anterior or posterior wall of
the body near the fundus on the 6th day which corresponds for the 20th day
of a regular menstrual cycle.
 The blastocyst possesses an inner cell mass or embryoblast and outer cell
mass or trophoblast.
 Trophoblast : becomes the placenta and chorion.
 Embryoblast : become embryo, amnion and umbilical cord.

17. Trophoblast
 Small projections begins to appear all over the surface of the trophoblast
becoming most abundant at the area of contact. These trophoblastic cells
differentiate into 3 layers.
 Outer syncytiotrophoblast
 Inner cytotrophoblast
 Primitive mesenchyme or mesoderm.
The syncytiotrophoblast erodes the walls of the blood vessels of the
decidua, to make the nutrients in the maternal blood accessible to the
developing embryo.
The cytotrophoblast, which is a well- defined single layer of cells,
produces a hormone known as human chorionic gonadotrophin (HCG).
This hormone is responsible for informing the corpus luteum that
pregnancy has begun.

18. As a result corpus luteum continues to produce estrogen &
progesterone. Progesterone maintains the integrity of the decidua
so that menstruation does not occur. The high level of estrogen
suppresses the production of FSH stopping further follicle
development in the ovary.
The mesenchyme develops to chorionic vesicle with its
membrane called chorion. It condenses about the embryo to form
the body stalk that connects the embryo to the nutrient chorion
and forms the umbilical cord later on.

19. The inner cell mass
 While the trophoblast develops to form the placenta, which will
nourish the fetus, the inner cell mass develops to form the fetus
itself. The cells differentiate into 3 layers.
1. Ectoderm
2. Endoderm
3. Mesoderm
each of which will form particular parts of the fetus.

20.  The ectoderm develops into the central & peripheral nervous system
and the epidermis with its appendages.
 The endoderm forms the dermis, the skeleton, the connective tissue,
vascular and the urogenital systems & most skeletal and smooth
muscles.
 The mesoderm forms certain internal organs such as the heart and blood
vessels, liver pancreas and also the bones and muscles.
 The 3 layers together are termed as the embryonic plate.
 As the development continues, two cavities appear in the inner cell
mass, one on either side of the embryonic plate. These are termed as the
amniotic cavity and the yolk sac.

21.  The amniotic cavity is lies on the side of the ectoderm. It is filled
with fluid and gradually enlarges to develop embryo. The amnion
forms from its lining.
 The yolk sac is lies on the side of the ectoderm and provide
nourishment for the embryo until the trophoblast is sufficiently
developed to take over. Part of it forms the primitive gut and
remainder atrophies under the amnion on the surface of the placenta.

23. Important events following fertilization
0 hours Fertilization (day-15 from LMP)
30 hours 2 cell stage (blastomeres)
40-50 hours 4 cell stage
72 hours 12 cell stage
96 hours 16 cell stage.morula enters the uterine cavity
5th day Blastocyst
4-5th day Zona pellucida disappears
5-6th day Blastocysts attachment to endometrial surface
6-7th day Differentiation of cyto & syncytiotrophoblastic layer
10th day Synthesis of hcG by syncytiotrophoblast
9-10th day Lacunar network forms
10-11th day Trophoblasts invade endometrial sinusoids establishing
uteropacental circulation
13th day Primary villi
16th day Secondary villi
21st day Tertiary villi
21st – 22nd day Fetal heart. Fetoplacental circulation.

24. Embryonic and fetal development
 from conception to 8 week: intrauterine life is referred as “embryo”
 9th week to till birth : intrauterine is referred as “fetus”

26. Embryoblast
 Develops the embryo, and differentiate into two types of cells-
• Epiblast : epiblast have three layers, which forms the particular parts
of the embryo. The first appearance of these layers, collectively
known as the primitive strek is around 15 days.
• Hypoblast : the hypoblast cell migrate along with inner
cytotrophoblastic lining secreting extracellular tissue which
becomes the yolk sac.

27. Development by weeks of gestation
 Pre- embryonic period
 Week 1-2 : no developments occurs since fertilization hasn’t
actually occurred.
 Week 3: from 15-21 days, embryonic 5-7 days.
• Fertilization occurs and form zygote.
• The embryo hatches from its protein shell and perform implantation.
(5-6 days).

28.  Week 4th : days 22 -28 from LMP
embryonic age 2 weeks
Events:
 Trophoblast cells surrounding the embryonic cells proliferate and
invade deeper into uterine lining.
 Eventually from placenta and embryonic membrane.
 Formation of yolk sac.
 Primitive streak develop at 13 days.
 Primary stem villi appears at 13 days

29.  Week -5:
 Gestational age : 4 weeks
 Embryonic age : week no 3
 A notochord forms in the centre of the embryonic disk. (day 16 of
fertilization)
 Gastrulation commences. (day 16 of fertilization)
 A nueral groove (future spinal cord) forms over the notochord with a
brain bulge at one end. Neuromeres appear. ( day 18 after
fertilization)
 Sometimes, the division of the future vertebrae, form(day 20 of
fert.)
 Primitive heart tube is forming. Vasculature begins to develop in
embryonic disk.( day 20 of fert.)

30.  Week -6 : gestational age: 5 week
embryonic age: 4 weeks
Events
 Embryo measures 4 mm
 The heart bulge, & begins to beat in a regular rhythm.
 The nueral tube closes.
 Arm buds & tails are visible.
 Pulmonary premordium appear.
 Hepatic plate appear.
 Buccopharyngeal membrane ruptures. This form the future mouth.
 Anterior & posterior horns differentiate in the spinal cord.

31.  Week 7: embryonic age 5 week
Events-
 Length is 9 mm
 Lens pits and optic curve develops
 Nasal pits form
 Brain divides into 5 vesicles including the early telencephalon.
 Leg buds form.
 The metanephros, precursor of kidney start to develop.
 Stomach differentiation begins.

32.  Week-8
 The embryo measures 13 mm in length.
 Lungs begin to form.
 The brain continues to develop.
 Arms and legs have lengthened with foot 7 hand areas
distinguishable.
 The feet & hand have digits, but may still be webbed.
 The gonadal ridge begins to be perceptible.
 The lymphatic system begins to develop.
 Main development of external genitalia starts.

33.  Week 9
 the embryo measures 18mm in length.
 Fetal heart tone (the sound of the heart beat) can be heard using
Doppler.
 Nipples and hair follicles begin to form.
 Location of the elbows and toes are visible.
 Spontaneous limb movements may be detected by ultrasound.
 All essential organs have at least begun.
 The vitelline duct normally closes

35.  Week 10 -12
 Embryo measures 30-80 mm (1.2-3.2 inches) in length.
 Intestine rotates
 Facial features continue to developed
 The eyelids are more developed.
 The external features of the ear begin to take their final shape
 The face is well formed
 The eyelids close and will not reopen until about the 28th week
 Tooth buds appear
 The fetus can make a fist with its fingers.
 Genitals appear well differentiated.
 RBCs are produced in the liver.

36.  Week 13- 16:
 The fetus reaches a length of about 15 cm (6 inches)
 Lenugo developed in the head.
 Fetal skin is almost transparent.
 More muscle tissue & bones have been developed, & the bones
become harder.
 Sucking motion are made with the mouth
 Muconium is made in the intestinal tract
 At 15 week development of external genitalia is finished.
 Abdominal wall closes.

37.  Week 17 -21
 The fetus reaches a length of 20 cm (8 inches)
 Lanugo covers entire body.
 Eyebrows & eyelashes appear.
 Nails appear on finger and toes.
 The fetus is more active with increased muscle development.
 “Quickening” usually occurs
 The fetal heart beat can be heard with a stethoscope.

38.  Week 23
 The fetus reaches a length of 28 cm (11.2 inches)
 The fetus weighs about 925g
 Eyebrows and eyelashes are well formed.
 All of the eye components are developed.
 The fetus has a hand and startle reflex.
 Footprints and fingerprints continue forming
 Alveoli (air sacs) are forming in lungs.

39.  Week 27
 The fetus reaches a length of 38 cm (15 inches).
 The fetus weighs about 1.2 kg.
 The brain develops rapidly.
 The nervous system develops enough to control some body
functions.
 The eyelids open and close.
 The respiratory system, while immature, has developed to the point
where gas exchange is possible.

40.  Week -31
 The fetus reaches a length of about 38-43 cm (15-17 inches).
 The fetus weighs about 1.5kg
 The amount of body fat rapidly increases.
 Rhythmic breathing movements occur, but lungs are not fully
mature.
 Thalamic brain connections, which mediate sensory input, form.
 Bones are fully developed, but are still soft and pliable.
 The fetus begins storing a lot of iron, calcium and phosphorus.

41.  Week 35
 The fetus reaches a length of about 40-48 cm (16-19 inches.)
 The fetus weighs about 2.5 to 3 kg.
 Lanugo begins to disappear.
 Body fat increases.
 Fingernails reach the end of the fingertips.
 A baby born at 36 weeks has a high chance of survival, but may
require medical interventions.

42.  Week 36 – 40
 The fetus is considered full-term at the end of the 39th week of
gestational age.
 It may be 48 to 53cm
 The lenugo is gone except on the upper arms and shoulders.
 Fingernails extend beyond fingertips.
 Small breast buds are present on both sexes.
 Haed hair is now coarse and thickest.

43. Principal events of embryonic & fetal development
 Day 14-21 post conception : -notochord develops
-ectoderm thickens to form nueral plate
and neural folds.
 Day 21-28 post conception: - neural folds fuse to form neural tube,
-four primitive cardiac chambers,
- first heart beats on D-21
 Weeks 4-6 post conception: -optic vesicles appear, complete neural
tube closure, limb buds appear ,
-formation of face
 Weeks 6-8 post conception :- all major structures form, complete
ventricular septum recognizably
human

44.  Weeks 8-12 post conception: -external genitalia develop
 Weeks -20 :- skin is covered with lanugo-vernix caseosa is
present.
 Weeks- 28 :- testes descend to the internal inguinal ring.
Baby is viable
 Weeks-36 :- one testicle usually descends into the scrotum.
Lanugo tends to disappear.
 Weeks- 40 :- both the testicles descend into the scrotum.
- Nails project beyond the finger tips.

45. Research article
 P. latha (2017) conducted descriptive study to assess the knowledge
regarding fetal development among antenatal mothers in selected
villages, Nellore, Andhra Pradesh, in this study 100 antenatal
mothers was selected by using non-probability convenience
sampling technique. Study revealed that, among 100 antenatal
mothers 76% had inadequate knowledge ,& 24% had moderately
adequate knowledge regarding fetal development.

47. e.g. apposition, adhesion, penetration and invasion.
 Apposition :
occurs through pinpods formation. Pinpods are long finger like projections from
the endometrial cell surface. These pinpods absorb the endometrial gland cells. This fluids, rich
in glycogen and mucin provides nutrition to the blastocyst initially.
 Adhesion :
adhesion of blastocyst to the endometrium occurs through the adhesion
molecules like integrin, selectin and cadherin.

48.  Penetration :
actual penetration and invasion occur through the stromal
cells in between the glands and is facilitated by the histolytic action of
the blastocyst.
with increasing lysis of the stromal cells, the blastocyst is
burrowed more and more inside the stratum compactum of the decidua.
further penetration is stopped probably by the maternal
immunological factors and the original point of entry is sealed by fibrin
clot and later by epithelium. The process is completed by 10th or 11th
day which corresponds to D 24-25 from LMP.
this type of deeper penetration of the hiamn blastocyst is
called interstitial implantation and the blastocyst is covered on all sides
by the endometrium.