Fetal development is the crucial part of human life.

1. Fetal
Development
By- Isha Thapa Magar
MN 1st Year

2. Objective
Know the process of fertilization and fetal
development, placenta development

3. Contents
 Fertilization
 Development of Zygote
 Pre-embryonic period
 Embryonic period
 Fetal period
 Fetal System Development
 Placenta Development
 Summarization

5. Fertilization
Human fertilization, known as conception, is the
fusion of genetic material from the haploid sperm
cell and secondary oocyte, to form zygote.
Process takes approximately 12-24 hours.
Normally occurs in ampulla of uterine tube.

6. Pre-embryonic Period
Blastomere
 During first week zygote travels along uterine
tube towards uterus.
 Zygote undergoes mitotic cellular division
called cleavage, and formation of blastomere
in about 30 hours after fertilization.

7. Contd..
 Blastomere continue to divide by binary division
into 2, 4, 8, 16 cell stage.
 8-16 cell stage of blastomere is called morula.
 On 4 days morula enters uterine cavity.
 Zona pellucida becomes stretched, thinned and
started to disappear.

8. Blastocyst
 Morula continue to divide, creating inner mass
cells and outer cells, called blastocyst.
 Process of development of blastocyst from morula
before implantation is called blastulation.

9.  Trophoblast surrounds inner cell mass and a
fluid-filled blastocyst cavity known as
blastocoele or blastocyst cavity.
 Blastocyst comprises 70-100 cells.
 Blastocyst possess
1. Inner cell mass (Embryoblast)
2. Outer layer of cells ( trophoblast)

11.  Zona pellucida disappears completely before
implantation.
 Blastocyst reaches uterus around 5 day, and implant
into uterine wall on about day 6.
 Trophoblast becomes placenta and chorion,
 Embryoblast becomes embryo, amnion and
umbilical cord.

12.  Implantation of blastocyst
 Attachment/ embedding of developing blastocyst
with wall of uterus is called implantation.
 Implantation start on 6th day after fertilization.
 Implantation is usually to upper posterior wall.
 At implantation stage, zona pellucida will have
totally disappeared.

13. Hatching of blastocyst
Implantation of trophoblast layer into
endometrium, is called decidua..
During week 2, trophoblast proliferates and
differentiates into two layers:
1. Outer syncytio-trophoblast
2. Inner cytotrophoblast

14. Contd..
 Syncytiotrophoblast layer invades decidua by
forming finger-like projections called villi that
make their way into decidua & spaces called
lacunae that fill up with mother’s blood.
 Syncytiotrophoblast & cytotrophoblast cells
produce hCG, a hormone that prevents menstrual
& maintains pregnancy by sustaining function of
corpus luteum.

15.  The process of implementation is completed by
10th or 11th day of fertilization .
 This type of implantation achieved is called
haemochorial.

17. Embryonic stage
 15 days - 8 weeks
 Organogenesis period
 Is the most critical period in development of
organs systems and main external features.
 By end of embryonic stage, all of organ system and
major structures are present, and embryo is fully
recognizable as human in form.

18. Contd..
 During embryonic period, cells of embryo
multiply and tissues begin to assume specific
functions, a process known as
differentiation.
 While trophoblast is developing into
placenta, which will nourish fetus, inner
cells mass is forming fetus.

19.  After implantation, embryo continues
developing.
 Cells of embryoblast differentiate into two types
of cells:
1. Epiblast (closest to trophoblast)
2. Hydroblast (closest to blastocyst cavity).
 These two layers of cells form a flat disc known
as bilaminar embryonic disc.

22. Contd…
 Two cavities appear on either side of
bilaminar embryonic disc.
1. Amniotic Cavity: Is between ectoderm and
trophoblastic layers lined by mesenchyme
2. Yolk Sac: Is a Cavity lined by primitive
mesenchyme on outside and endodermal layer of
germ cell disc on inside

25. Contd..
 A process of gastrulation turns bilaminar
disc into a tri-laminar embryonic disc
(three layers).
1. Ectoderm
2. Mesoderm
3. Endoderm

29. Contd..
 These three primary germ layers are main
embryonic tissues from which various
structures and organs will develop.
 The first appearance, of these layers,
collectively known as primitive streak, is
around day 15.

31. Fetal Period
 From 9th week to birth.
 During fetal stage, additional growth and maturation
of organs and body systems occur.
 Initially development of placenta and fetal
membranes occurs far more rapidly then
development of fetus itself.

32. Contd..
 During first 2 to 3 weeks after implantation of
blastocyst, fetus almost macroscopic, but
thereafter, length weight of fetus increases almost
in proportion to age.
 Within 1 month after fertilization of ovum, gross
characteristics of all different organs of fetus have
already begun to develop, and during next 2 to 3
months, most of details of different organs are
established.

33. Contd..
 Beyond month 4, organs of fetus are grossly same as
those of neonate.
 However, cellular development in each organ is usually
far from complete and requires full remaining 5 months
of pregnancy for complete development.
 Even at birth, certain structures, particularly in nervous
system, kidneys and liver, lack full development.

36.  
Development of
fetal System

37. Cardiovascular System
 Cardiovascular system is first system to function in
embryo.
 Heart and vascular system commences
development in 3rd weeks, and by 4th week a
primitive heart is visible and is beginning to
function, beating at around 22 day.

38.  Development continues to include remodeling and
septation while heart continue to beat.
 Blood is pumped around vessels from the 4th week,
by which time three major vascular system
developed.

39.  There are three phases of red blood cell formation:
o Yolk sac period: between week 3 to 13 weeks.
o Hepatic/liver period: between 5 to 36 weeks, and
o Bone marrow period: from 10th week throughout life.
 Fetal erythrocytes have a life span of 90 days, shorter
than adult erythrocytes.
 Genes passes from both parents determine fetal blood
group and Rhesus factor.

40. Respiratory System
 Development of respiratory system begins in 3rd
week.
 The lower respiratory tract and lungs develop
simultaneously.
 Lung development occurs on several levels and
continue after birth until about 8 years of age when the
full number of bronchioles and alveoli will have
developed.
 Development of type II alveolar cells begins around 20
weeks of fetal life, and are responsible for production
of surfactant.

41.  Amount of surfactant increases until alveoli
mature between 36 weeks and birth.
 At term, lungs contain about 100 ml of lung
fluid. About one-third of this is expelled during
birth and rest is absorbed and transported by
lymphatic and blood vessels as air takes its place.
 Babies born before 24 weeks of pregnancy have a
reduced chance of survival owing to the
immaturity of the capillary system in lungs and
lack of surfactant.

42. The urinary and renal system
 Kidney develop from 4th week of fetal life and
produce small amounts of urine between the 6th
and 10th week.
 They become more functional around the 15th
week when more urine is produced.
 The urine forms much of the amniotic fluid and
production increases with fetal maturity.

43. Genital/Reproductive system
 Sex of embryo is determined at fertilization: either
two X chromosomes or one X and one Y
chromosome are inherited.
 Gonads develop from 5th week from intermediate
mesoderm.

44.  Differentiation occurs from 7th week, but female
gonad development occurs slowly under influence
of pro-ovarian genes and the ovaries may not be
identifiable until the 10th week.
 External genitalia in both sexes develop in 9th
week, but male and females are not distinguishable
until about 12th week.

45. Endocrine System
 Adrenal glands develop from 6th week of
fetal life, and grow to 10-20 times larger
than adult adrenals.
 They produce precursors of placental
formation of oestriols and influence
maturation of lungs, liver and epithelium of
digestive tract.

46.  Pituitary gland develops and takes on its
characteristic shape form between 9th and 17th week
of fetal life.
 Fetal pituitary produces gonadotropins i.e.
luteinizing hormone (LH) and follicle stimulating
hormone (FHS) from weeks 13-14, and human
growth hormone (hGH) is present by weeks 19-20 .

47. Digestive system
 Primitive gut develops from endodermal layer
of yolk sac in 4th week of fetal life.
 By 5th week, foregut (esophagus, stomach and
duodenum) is visible.
 Liver, gallbladder, and pancreas from gut tube
around 4th to 5th week of fetal life.

48.  Midgut i.e. small intestine, caecum and vermiform
appendix, ascending colon and transverse colon
under take much its development in 6th weeks.
 Hindgut i.e. rectum and anal canal completes its
development in 7th week of fetal life.
 Around 12th weeks, digestive tract is well formed
and lumen is patent.
 Meconium is normally retained in gut until after
when it is passed as first stool of baby.

49. Nervous System
 Brain begins to develop from around day 19
and three structures are visible: forebrain,
midbrain, and hindbrain.
 By 5th week, there is differentiation between
thalamus and hypothalamus.

50.  Neural tube is derived form ectoderm and
closure of it takes place by 26 days.
 Development of eyes started from 22 days,
and completed formed by 20 weeks but
eyelids are fused until 24 weeks.

51. Integumentary, skeletal and muscular
systems
 By end of 4 weeks, a thin outer layer of
flattened cells cover embryo and further
development continues until 24 weeks.
 Brown adipose tissue (BAT) develops from
18 weeks.
 From 18 weeks, fetus is covered with a
white, creamy substance called vernix
caeseosa.

52.  Hair begins to develop between 9th and 12the
week.
 By 20th weeks fetus is covered with a fine downy
hair called lanugo and shed from 36 weeks and by
term there is little left.
 Fingernails develop from about 10 weeks but
toenails do not form until about 18 weeks.
 Skull develops during 4th week from mesenchyme
surrounding developing brain.

53. Placenta Development
Placenta is the organ which develops
from fusion of fetal components i.e.
choronion frondosum and meternal
component i.e. decidua basalis.

55. When interstitial implantation is completed
on 11th day, blastocyst is surrounded on all
sides by lacunar spaces around cords of
syncytial cells called trabeculae.
On 13th day, stem villi develops from
trabeculae which connect chorionic plate with
basal plate.

56. By the beginning of the 3rd week, trophoblast
produces figure like buds consisting of an outer layer
of syncytiotrophoblast and an inner core of
cytotrophoblast, these are called primary stem villi.
After the appearance of primitive mesenchyme and
development of chorion, the primary stem villi are
named chorionic villi.

58. Primary mesoderm invades central core of
primary chorionic villi and transforms them into
secondary villi by 10th days.
 Later mesodermal cells in the core of villi
differentiate to form blood vessels, thus
establishing a vascular villous system. The
vascularized villi are called tertiary villi and formed
by day 21.

60. Primary, secondary and tertiary villi are
successively developed from stem villi.
Anterio-capillary venous system in mesenchymal
core of each villus is completed on 21st day.
This ultimately makes connection with
intraembryonic vascular system through body stalk.

62. Discrete placenta is develop from chorionic
frondosum and decidua basalis begins at 6th week
and is completed by 12th week.
Until the end of 16th week, placenta grow both in
thickness and circumference and Subsequently, there
is little increase in thickness but it increases
circumferentially till term.