This document summarizes the process of fetal development from fertilization through the fetal periods. It describes the key stages and events, including fertilization and development of the zygote, pre-embryonic and embryonic periods where the blastocyst implants and the three germ layers form. It then covers the fetal period and development of major organ systems like the cardiovascular, respiratory, renal and others. It concludes with a section on placenta development from the chorionic villi fusing with the decidua.

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.