This document summarizes Heera KC Parajuli's seminar on conception and fetal development. It begins with an overview of gametogenesis - the processes of sperm and egg formation. It then discusses ovulation, explaining how a mature ovarian follicle ruptures monthly to release an egg. The next section covers fertilization, describing the steps where a sperm and egg fuse together in the fallopian tube to form a zygote. The document concludes with an introduction to the development of the zygote and early embryo.

1. Mrs. Heera KC Parajuli
M.Sc. Nursing 1st year 2016 batch
B. P. KOIRALA INSTITUTE OF HEALTH SCIENCES, DHARAN
Seminar entitled

2. Contents of seminar
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Gametogenesis
Ovulation
Menstrual cycle
Fertilization
Fetal development
References
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3. GAMETOGENESIS
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Male Female
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Spermatozoon Ovum
Maturation process

4. GAMETOGENESIS Cont...
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• Both mitosis and meiosis play a role in
gametogenesis.
• Mitosis provides the precursor cells.
• Meiosis brings about the reduction
divisions that result in gametes.
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Cell
cycle

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GAMETOGENESIS
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OOGENESIS
GAMETOGENESIS Cont...
Oogenesis Spermatogenesis

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OOGENESIS
Oogenesis
The process involved in the development of a mature ovum is called
oogenesis.

8. Fully matured ovum
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Fully mature ovum :
largest cell in the body,
130 micron in diameter.
Cytoplasm
23 chromosomes(23 x)

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Spermatogenesis
The process involved in the development of spermatids
from the primordial male germ cells and their
differentiation into spermatozoa is called spermatogenesis.
Primary spermatocytes
16 days.
61 days

10. Structure of a mature spermatozoon
• It has got two parts, a head
and a tail.
• The head
 Condensed nucleus and
acrosomal cap.
 Acrosome is rich in
enzymes.
• The tail divided into four
zones — the neck, the
middle piece, the principal
piece and the end piece.
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11. Difference between spermatogenesis and
oogenesis.
Spermatogenesis differs from oogenesis in three
ways
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1. All four products of meiosis develop into sperm while
only one of the four becomes an egg.
2. Spermatogenesis occurs throughout adolescence and
adulthood.
3. Sperm are produced continuously without the prolonged
interruptions like in oogenesis.

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13. Ovulation
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Ovulation is
a process
whereby a
secondary
oocyte is
released
from the
ovary
following
rupure
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of a mature Graafian follicle and becomes available
for conception.
Day 12
Day 1
Day 14Day 20
Day 28

14. In each ovarian cycle, Only one secondary oocyte is
likely to rupture.
It starts at puberty and ends in menopause.
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Ovulation cont...

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Ovulation cont...
Mechanism
Changes in follicle
Changes in oocyte
Preovulatory
changes

16. • Preovulatory enlargement of the Graafian follicle
due to accumulation of follicular fluid.
• Measures about 20 mm in diameter.
• The cumulus oophorus separates from the rest of the
granulosa cells.
• The follicular wall near the ovarian surface becomes
thinner.
Changes in the follicle
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17. • ovarian cycle
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Schematic diagram showing: (A) Mature Graafian follicle on the verge of ovulation (B)
Ovulation with discharge of secondary oocyte surrounded by cumulus oophorus (C) Formation of
corpus luteum (D) Secondary oocyte after first maturation division with formation of first polar
body and (E) Microscopic structure of corpus luteum

18. Significant changes occur just prior to ovulation
(few hours).
Cytoplasmic volume is increased.
Completion of the arrested first meiotic division
occurs with extrusion of first polar body, each
containing haploid number of chromosomes (23, X).
Changes in the oocyte
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19. CAUSES
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 2.Stretching factor(a passive stretching process
of granulosa cells.)
Contraction of the micromuscles in the theca
externa and ovarian stroma due to increased
prostaglandin secretion.
 Endocrinal
•LH surge
•FSH rise

20. Endocrinal
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LH surge: Sustained peak level of estrogen for 24–36
hours in the late follicular phase →
LH surge occurs from the anterior pituitary.
•Ovulation approximately occurs 16–24 hours after the
LH surge.
•LH peak persists for about 24 hours.

21. Endocrinal
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The LH surge stimulates completion of reduction
division of the oocyte and initiates luteinization of the
granulosa cells, synthesis of progesterone and
prostaglandins
FSH rise: Preovulatory rise of progesterone
facilitates the positive feedback action of estrogen to
induce FSH surge → increase in plasminogen
activator → plasminogen → plasmin → helps lysis
of the wall of the follicle.

22. The follicle is changed
into corpus luteum.
EFFECT OF OVULATION
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Menstruation is unrelated with ovulation.
The ovum is picked up into the Fallopian tube, and
undergoes either degeneration or further
maturation,if fertilization is to occur.

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Reproductive
cycle
A. Ovarian cycle-
B. Anterior
pituitary cycle
C. Ovarian
hormones
D. Uterine cycle
Proliferative phasephase
Secretory phase

24. • Fertilization is the process of fusion of the
spermatozoon with the mature ovum.
• It begins with sperm egg collision and ends with
production of a mononucleated single cell called the
zygote.
Definition
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25. FERTILIZATION Cont...
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(1) To initiate the embryonic development of the
egg and
(2) To restore the chromosome number of the
species.
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Objectives

26. FERTILIZATION Cont...
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Almost always, fertilization occurs in the
ampullary part of the uterine tube.
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Normal site for conception/ Fertilization

27. APPROXIMATION OF THE GAMETES
• Fertilizable life span of oocyte is 12 to 24
hours and sperm is 48 to 72 hours.
• Out of hundreds of millions of sperms
deposited in the vagina at single ejaculation,
only thousands capacitated spermatozoa enter
the uterine tube while only 300–500 reach the
ovum.
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28. APPROXIMATION OF THE GAMETES
Tubal transport is
facilitated by muscular
contraction and
aspiration action of the
uterine tube.
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It takes only few minutes for the sperm to reach
the Fallopian tube

29. CONTACT AND FUSION OF THE GAMETES
• Capacitation
• Acrosomal Reaction
• Cortical Reaction
• Development of zygote
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30. Capacitation
• The process of undertaking in the sperm when
inside the female genital tract influenced by the
secretions of the uterine tube is known as
capacitation.
• Sperm must be in the female genital tract 4-6
hours before they can fertilize an ovum.
• The sperm undergoes changes in the removal of
the glycoprotein coat.
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31. Acrosomal Reaction
• The acrosomal
layer of the sperm
becomes reactive
and releases the
enzyme
hyaluronidase
known as the
acrosome
reaction .
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It disperses the corona radiata
(outer layer of ovum) allowing
access to the zona pellucida .

32. Cortical Reaction
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• Penetration of the zona pellucida occurs with
the aid of several enzymes processed by the
sperm which breaks down the proteins of the
zona layer.
• Upon penetration a chemical reaction known
as the cortical reaction occurs which makes it
impermeable to other sperms.
The first sperm that reaches the zona
pellucida penetrates it.

33. • The plasma membrane of the sperm and oocyte
fuse.
• The oocyte at this stage completes the second
meiotic division and becomes mature, the
pronuclei has 23 haploid chromosomes.
• Male and female pronuclei fuses to form a new
nucleus that is a combination of the genetic
material from both the sperm and the oocyte
diploid cell.
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Cortical Reaction

34. DEVELOPMENT OF ZYGOTE
• The male and
female gametes
each contribute
half the
complement of
chromosomes to
make a total of
46. The new cell
is called zygote.
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35. CONTACT AND FUSION OF THE GAMETES
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Schematic diagram showing sequence of changes during fertilization :
(A) Sperm (acrosome intact) in between the corona radiata cells → attachment with zona →
acrosome reacted sperm penetrating the zona → acrosome reacted sperm in the perivitelline
space → incorporated sperm with vesiculating head. (B) Formation of male and female
pronuclei with completion of second polar body
A B

36. Zygote cont...
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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.

37. DEVELOPMENT OF ZYGOTE
Divided into three periods:
–Pre-embryonic period: 1st two weeks after
fertilization.
–Embryonic period: 2- 8 weeks
–Fetal period: 8 weeks to birth.
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38. PRE-EMBRYONIC PERIOD
• This period includes the implantation of the
zygote into the endometrium.
• The embryo takes about 3 days to travel
through the fallopian tube to reach the uterus.
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39. PRE-EMBRYONIC PERIOD
• It takes another 3 days to get implanted,
usually in the dorsal wall of the uterus .
• Thus embryo is about 1 week old when it gets
implanted.
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40. PRE-EMBRYONIC cont...
• Zygote undergoes mitotic division and cellular
replication known as cleavage which results in the
formation of smaller cells known as blastomers.
• During this period a strong membrane of
glycoprotein called zona pellucida surrounds the
zygote.
• The zygote mainly gets nourishment (glycogen)
from the goblet cells of the uterine tubes and later
the secretory cells of the uterus.
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41. PRE-EMBRYONIC PERIOD cont...
The zygote divides
into
• 2 cells at day 1, (or
30 hours)
• 4 at 2 days,
• 8 by 2.5 days,
• 16 by 3 days, now
known as
morula,
Resembling a
mulberry. (16-64
cell stage)
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42. • Cells bind tightly together in a process known
as compactation.
• Next cavitation occurs whereby the outermost
cells secrete fluid into the morula and a fluid
filled cavity or blastocele appears.
• This results in the formation of the blastula or
blastocyst comprising 58 cells.
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PRE-EMBRYONIC PERIOD cont...

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Schematic representation of the mitotic division of the
zygote resulting in formation of: (A) Two-cell stage; (B)
Four-cell stage; (C) Morula and (D) Blastocyst
A B C D

44. • Development of the morula to
the blastocyst, has occurred by
day 4.
•
Blastulation
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Blastocyst
Trophoblast
Placenta
chorion
Inner cell
mass
Fetus
Amnion
Umbilical
cord

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Diagrammatic representation of the events — ovulation, fertilization and
implantation: (1) Secondary oocyte extruded at ovulation (2) Secondary
oocyte in tube (3) Fertilization with extrusion of second polar body (day
14–16) (4) Formation of zygote (5) Two-cell stage (6) Four-cell stage (7)
Early morula (day 17) (8) Late morula (day 18) (9) Early blastocyst stage
with disappearance of zona pellucida (day 19–20) (10) Early phase of
implantation (day 20–21)

46. IMPLANTATION
Schematic representation showing interstitial implantation of the blastocyst in stratum
compactum of the decidua
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47. • Embedding of the embryo to the walls of
uterus.
• Also called as Nidation.
• 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 a regular menstrual cycle.
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IMPLANTATION cont...

48. • Implantation occurs through four stages.
apposition, adhesion, penetration and invasion.
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IMPLANTATION cont...
APPOSITION: Occurs through pinopod formation.
Pinopods are long finger like projections (microvilli)
from the endometrial cell surface.
These pinopods absorb the endometrial fluid which is
secreted by the endometrial gland cells. This fluid, rich in
glycogen and mucin provides nutrition to the blastocyst
initially.

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IMPLANTATION cont...
Unless this fluid is absorbed, adhesion phase cannot occur.
Adhesion of blastocyst to the endometrium occurs through
the adhesion molecules like integrin, selectin and cadherin
(glycoproteins).
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.

50. THE DECIDUA
Endometrium
during
pregnancy is
known as the
decidua.
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Three layers are
found.

51. THE DECIDUA
1. The basal layer: Lies immediately above the
myometrium. Remains unchanged in itself but
regenerates the new endometrium during the
puerperium.
2. The functional layer: consists of tortuous glands
which are rich in secretions.
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It provides a secure anchorage for the placenta
and allows it access to nutrition and oxygen, but
as soon as the baby is born separation can occur.

52. 3.The compact layer: This layer forms the
surface of the decidua and is composed of
closely packed stroma cells and the neck of the
glands.
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THE DECIDUA
BASAL DECIDUA: underneath the blastocyst.
CAPSULAR DECIDUA: that which covers it.
PARIETAL DECIDUA: remainder of the decidua (true
decidua)

53. Functions
(1) It provides a good nidus for the implantation of the
blastocyst.
(2) It supplies nutrition to the early stage of the growing
ovum by its rich sources of glycogen and fat.
(3) Deeper penetration of the trophoblast is controlled
by local peptides, cytokines and integrins.
(4) Decidua basalis takes part in the formation of
basal plate of the placenta.
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54. GASTRULATION
Embryo Development
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1.Outer Syncytiotrophoblast (syncytium):
•Layer composed of nuclear protoplasm, capable of breaking
down the tissues in the process of embedding.
Function: Makes nutrients in maternal blood accessible to
the developing embryo.
During week 2nd, the
trophoblast proliferates and
differentiates into layers.

55. Gastrulation cont...
2. Inner Cytotrophoblast:
A well defined single layer of cells which produce
a hormone HCG.
This hormone is responsible to inform the corpus
luteum that pregnancy has begun.
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3. A layer of Mesoderm or the primitive gut:
Layer having cells similar to inner cell mass.
Simultaneously to implantation, the embryo is
developing from the embryoblast.

56. Embryoblast differentiate into two
layers
a. Epiblast : closest to the trophoblast, gives rise
to cells of the embryo. Epiblast again gives rise
to three layers which are collectively known as
the primitive streak at around day 15.
b. Hypoblast: closest to the blastocyst cavity.
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epiblast

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Bilaminar disc differentiating into
trilaminar disc.(week 3)

61. GASTRULA LAYERS DEVELOP
1. The Ectoderm: skin and the nervous system.
2. The Mesoderm: bones, muscles, heart, blood
vessels and some visceral organs.
3. The Endoderm: mucous membranes and
glands
Two cavities appear on either side of the
embryonic plate.
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INTO…PRIMARY GERM LAYERS

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63. The amniotic
cavity
Epiblast separates
from the trophoblast
and forms a cavity that
lies on the side of the
ectoderm, derives from
the ectoderm layer.
Filled with fluid,
enlarges and folds
around the embryo
gradually.
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2. The yolk
sac
Cavity that lies on the
side of the endoderm
and develops from the
hypoblast.
Provides nourishment
for the embryo untill
the trophoblast is
sufficiently developed
to take over.

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Schematic
representation of
formation of amniotic
cavity, secondary yolk
sac, extraembryonic
coelom and body stalk:
(A) Enlargement of
extraembryonic
coelomic cavity (B)
The amniotic sac
enlarges and begins to
occupy the
extraembryonic coelom
(C) The amniotic sac
has surrounded the
embryo with almost
completely obliterating
theextraembryonic
coelom; formation of
body stalk completed

65. DEVELOPMENT OF PLACENTA
• By the end of 2nd month, trophoblast is
charactered by great no. of secondary & tertiary
villi.
• By four month cytotrophblastic cells &
connective tissue disappears, only syncytium &
endothelial vessel remains (tertiary villi).
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66. DEVELOPMENT OF PLACENTA
• In early stage, villi covers the entire surface of
chorion.
• As, the pergnancy advances, villi on embryonic
pole continues to grow & expand – chorion
frondosum (bushy chorion).
• Villi on an embryonic pole degenerate- chorion
levae. The fusion of amnion & chorion occurs
to form aminochorionic membrane.
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68. Placenta is an organ that connects the
developing fetus to the uterine wall to allow
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•Nutrient uptake
•Waste elimination
•Gas exchange via
the mother’s blood
supply
•Hormone
production
•Formation of a
barrier.
Function as a
fetomaternal organ
with 2 components-
Fetal placenta
Maternal placenta

69. THE “PLACENTAL BARRIER”
• Sugars, fats and oxygen diffuse from mother’s
blood to fetus
• Urea and CO2 diffuse from fetus to mother
• Maternal antibodies actively transported across
placenta
– Some resistance to disease (passive immunity)
• Most bacteria are blocked
• Many viruses can pass including rubella,
chickenpox, sometimes HIV
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70. THE “PLACENTAL BARRIER”
• Many drugs and toxins pass including
alcohol, heroin, mercury
• Placental secretion of hormones
–Progesterone and HCG (human
chorionic gonadotropin, the hormone
tested for pregnancy): maintain the uterus
–Estrogens and CRH (corticotropin
releasing hormone): promote labor
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72. STAGES OF DEVELOPMENT
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73. 1 Month
• First Trimester
At the end of four weeks:
• Baby is 1/4 inch in length.
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Placenta (sometimes called "afterbirth") begins to
develop.
The single fertilized egg is now 10,000 times larger
than size at conception
Heart, digestive system, backbone and spinal cord
begin to form.

74. Month 2
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FirstTrimester
At the end of 8 weeks:
• Baby is 1-1/8 inches
long
• Heart is functioning.
•Eyes, nose, lips, tongue, ears and teeth are forming
• Penis begins to appear in boys.
•Baby is moving, although the mother can not yet feel
movement.
•The limb buds are now clearly arms and legs, while
the fingers and toes are still developing.

75. Month 3
(FirstTrimester)
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At the end of 12 weeks:
Baby is 2 1/2 to 3 inches
long (4.4cm)
Weight is about 1/2 to 1
ounce.
Baby develops recognizable form. Nails start to develop and
earlobes, arms, hands, fingers, legs, feet and toes are fully
formed.
Eyes are almost fully developed .
Baby's heart rate can be heard at 10 weeks with a special
instrument called a Doppler.
Baby has developed most of his/her organs and tissues .

76. Month 4
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At the end of 4 months:
• Baby is 6 1/2 to 7 inches long
• Weight is about 6 to 7 ounces
Developing reflexes, such as sucking and swallowing. may
begin sucking his/her thumb .Tooth buds are developing. Eyes
blink.
Sweat glands are forming on palms and soles, well defined
Fingers and toes . Sex is identifiable .
Bright pink, transparent skin covered with soft, downy hair .
Although recognizably human in appearance, the baby would
not be able to survive outside the mother's body.
Second Trimester

77. • Baby is 8 to 10 inches long
(13.2 cm)
• Weight is about 1 pound.
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At the end of 5 months
•Hair begins to grow on baby's head. Soft woolly hair called
lanugo will cover its body. Some may remain until a week
after birth, when it is shed.
•Mother begins to feel fetal movement.
•
•Internal organs are maturing.
•Eyebrows, eyelids and eyelashes appear.

78. At the end of 6 months
• Baby is 11 to 14 inches long.
• Weight is about 1 3/4 to 2
pounds .
• Eyelids begin to part and eyes
open sometimes for short
periods of time.
• Skin is covered with
protective coating called
vernix .
• Baby is able to hiccup.
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Month 6
•The baby's inner ear
canals are developed at 24
weeks, so researchers
speculate the baby can
sense its position in the
uterus.

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Month 7 • Third Trimester
At the end of 7
months:
• 14 to 16 inches
long.
• 2 1/2 to 3 1/2
pounds
• Taste buds
developed .
• Fat layers are
forming.
• Organs are
maturing .
•Skin still wrinkled and red.
• If born at this time, baby will
be considered a premature
baby and require special care.

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Month 8 (Third Trimester) At the end of 8 months:
• 16 1/2 to 18 inches long
• 4 to 6 pounds weight.
• Overall growth is rapid
this month .
• Tremendous brain growth
occurs at this time .
• Most body organs are now
developed with the
exception of the lungs
•Movements or "kicks" are
strong enough to be visible
from the outside .
•Kidneys are mature .
•Skin is less wrinkled.
•Fingernails now extend
beyond fingertips.

81. Third Trimester
At the end of 9 months:
• Baby is 19 to 20 inches
long
• Weight is about 7 to 7
1/2 pounds
• The lungs are mature
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Month
9
Baby is now fully developed and can survive outside the
mother's body .
Skin is pink and smooth
Baby settles down lower in the abdomen in preparation for
birth and may seem less active

82. FETAL MEMBRANES
• Amnion
Smooth, tough, translucent membrane derived from
inner cell mass.
Have role in the formation of amniotic fluid .
• Chorion
Thick, opaque, friable membrane, derived from
trophoblast.
Continuous with the chorionic plate which forms the
base of the placenta.
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83. REFERENCES
• Dutta D.C. Text book of gynaecology,calcutta 8th edition : New Central
Agency. 2005. P. 74-89
• Bennett Ruth V., Brown Linda K. Myles text book of midwives. 13th edition.
Harcourt Publishers; Churchill Livingstone. 1999. P. 963-979
• Bijlani RL. Understanding medical physiology: a text book for medical
students. Third edition. New Delhi; Emca house, Jaypee Brothers Medical
Publishers Pvt. Ltd. P . 577-583
• Leifer G. Introduction to maternity and pediatric nursing. Sixth edition.USA:
Elsevier Inc; Elsevier Saunders. 2011. P. 31-41
• Pillitteri Adele. Maternal and child health nursing. Fourth edition. Quebecor
Versailles prints: Lippincott William Wilkins. 2003. P.67 – 84
• Internet citation on title embryology (cited date:27/september 2016)
http://www.medicinenet.com/fetal_development_pictures_slideshow/article.htm l
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84. 10/21/2016 85
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