The document provides a comprehensive overview of human embryology, detailing the stages of development from fertilization to the fetus's growth over approximately 40 weeks. Key processes such as fertilization, cleavage, gastrulation, organogenesis, and maturation are explained, along with the timeline of embryonic and fetal development. Additionally, it discusses the formation of major structures such as the placenta, amniotic cavity, and germ layers and their functions throughout the early stages of pregnancy.

1. Embryology/Development
of the Human Embryo
By
Girish
BAMS
1st
year

2. What is embryology?
The study of developmental events
that occur during the prenatal
stage .
The branch of biology and
medicine concerned with the study
of embryos and their development.

3. Embryonic period vs. Fetal period
 Embryonic – first 8 weeks Development of the
three primary germ layers give rise to all structures
and Basic body plan takes shape
 Fetal period – remaining 30 weeks. Structures
and organs continue to grow and develop.
 8 weeks

4. Developmental History of a Human

5. Stages of Development
1. Fertilization
2. Cleavage
3. Gastrulation Embryogenesis
4.Organogenesis
5.Maturation

6. Fertilization
 The process of fusion or union of the
spermatozoon with the mature ovum is
known as conception /
fertilizaiton/impregnantation.
 Which produced the fertilized single mono-
nucleated cell called the zygote.

7. Embryogenesis: The formation
and development of an embryo.
1. Cleavage is a series of rapid mitotic
divisions (without cell growth)

8. 2. Gastrulation : is a phase early in the
embryonic development of most
animals/human being, during which the
single-layered blastula is reorganized into a
trilaminar ("three-layered") structure known
as the gastrula. These three germ layers are
known as the ectoderm, mesoderm, and
endoderm.
3. Organogenesis: The production and
development of the organs of an animal or
plant.

9. How fertilization occurs????
 Following ovulation, the ovum, which is about 0.15 mm in
diameter is picked up by the tubal fimbriae and is moved
along by the cilia and by peristaltic movement of the tube.
 At the time the cervix under the influence of estrogen,
secretes a flow of alkaline mucus that deposited in the
vagina, only thousands capacitated spermatozoa enter the
uterine tube while 300-500 reach the ovum, and remainder
are destroyed by the acid medium of the vagina.
 It takes about 1 hour for sperm to reach the site.

10.  The sperm release the enzyme,
Hylluronidase which allows penetration of
the zona pellucida and the cell membrane
surrounding the ovum.
 Many sperm are needed for this to take place
but only one will enter the ovum.
 After this the membrane is sealed to prevent
entry of any further sperm and the nucluei of
the two cell fuse.

11.  The sperm and ovum contribute half
the complement of chromosomes to
make a total of 46.
 The sperm and ovum is known as the
male and female gametes and the
fertilized ovum as the zygote

12. . Fertilization

13. Normal site for Conception
 The most common site of conception is
the ampullary part (Ampulla ) of the
fallopian tube which is the widest part
located closed to the ovary

14. Usual Time for Conception
 Neither sperm nor ovum can survive longer than 2-
3 days and the fertilization is most likely to occur
when intercourse takes place not more than 48
hours before and 24 hours after ovulation.
 So the conception will take place about 14 days
before the next period is due.
 The sex of the new individual at the time of
conception is determined by sex chromosomes.

15.  Every human cell contains 46 chromosomes,
which are made up of 44 autosome
chromosome and 2 sex chromosomes.
 The sex chromosome are X and Y .
 Woman have no Y chromosome and male has
Y chromosome (male 44+X+Y) (female 44+X+X).

 Therefor e, sex of child is always determined
by father

16. Development of the
Fertilized Ovum

17. Morula
1. After fertilization, the Zygote divides into 2
cells (blastomere) (mitosis division)in about
30 hours after fertilization.
2. The blastomeres continue to divide by
binary division through 4, 8, and 16 cell
stage until a cluster of cells is formed–
Morula, resemblibg a mulberry

18.  The morula after spending about 3 days(72
hours) in the uterine tube enters the uterine
cavity through the narrow uterine ostium
(1mm) on the 4th
day .

19. Morula
Morula

20. Morula
72 hours post-
fertilization
entering uterus

21. 4. Blastocyst
 Morula, once entering the uterine cavity, floats
freely(next 2 days) and is covered by endometrial
fluid and mucus.
 This fluid is absorbed through the canaliculi of the
zona pellucida and Morula begins to accumulate
fluid and forms a cavity between its cells.
 Once cavity appears, it is now called a blastocyst.

22. Fertilization and the Events of the
First 6 Days of Development

23. Blastocyst
 The zona pellucida
becomes stretched, thinned
and gradually disappear
soon prior to implantation.
 The cell of the outer cell
mass forms the wall of the
blastocyst and is known as
trophoblast.
 The inner cell mass is
concerned with the
development of the
embryo.

24. Two Distinct Cell Types
1. Trophoblasts – will form the
invading placenta
2. Inner cell mass – will form the
embryo
Trophoblasts

25. Blastocyst
Trophoblast Inner cell mass
Placenta Chorion Fetus Amnion umbilical
cord

26. hCG is produced
 hCG is human chorionic gonadotropin
hormone
 It is produced by the trophoblasts starting on
day 6
 hCG causes endometrium of uterus to grow
and proliferate
 hCG prevents the menstrual cycle from
occuring
 This is why a female misses her periods when
she is pregnant

27. Implantation (Week 2)

28.  Implantation occurs about 6th
day after
fertilization (approx the 20th
day of a regular
menstrual cycle).
 As the zona pellucida surrounding the blastocyst
disappear, there is increased adhesiveness of the
trophoblastic cells to the endometrium.
 The trophoblastic cells invade the stromal cells
lying between the secretary endometrial glands
by histolytic action of the blastocyst bringing
about deeper penetration in the decidua.

29.  The syncytial cells ( multinucleate cell) ultimately
penetrate the endothelial coat of the maternal
decidual capillaries establishing communication
with the syncytial lacunar system
 Thus maternal blood circulates through the
syncytial lacunae providing nourishment to the
blastocyst.
Maternal blood vassels

30.  Further penetration is arrested by the maternal
immunological blocking factor. The point of
entry of the blastocyst into the decidual is sealed
by a fibrin coat and later by epithelium
 The process of implantation is completed by the
10th
or 11th
day of fertilization.

31.  This type of deeper penetration of the human
blastocyst is called interstitial implantation and
the blastocyst is covered on all sides by the
endometrium (decidual).
 Occasionally, there may be increased blood flow
into the lacunar space and cause bleeding. It is
called implantation bleeding and This
corresponds approx to 13th
day after fertilization
(at about the expected day of period).
 This may produce confusion in determination of
the EDD

32. The Early Development Of
The Embryo

33.  The outer cell mass of the blastocyst form the
trophoblast layer.
 The inner cell mass on the 8th
day differentiates
into bilaminar germ disc consisting of a dorsal
ectodermal layer of the tall columnar cells and a
ventral endodermal layer of flat polyhydral cells.
 The bilaminar germ disc is connected to the
trophoblast by a mesenchymal connecting stalk
called the body stalk which later forms the
umblical cord.

34. Formation of Amniotic cavity and Yolk sac
 Two cavities appear on either side of the
bilaminar embryonic disc.
 The dorsal cavity between the ectoderm and
trophoblastic layers lined by
mesenchyme(connective tissue) is called the
amniotic cavity.
 The cavity on the ventral aspect lined by the
primitive mesenchyme on the outside and the
endodermal layer of the germ cell disc on the

35. Week 2
 Inner cell mass divides into
epiblast and hypoblast
 2 fluid filled sacs
 Amniotic sac from epiblast
within which the embryo and
later the fetus develop until birth
 Yolk sac from hypoblast which is
one of the structures through
which the mother supplies
nutrients to the early embryo
 Bilaminar embryonic disc:
area of contact
(gives rise to the whole body)

36.  Bilaminar to trilaminar disc
 Three primary “germ” layers: all body tissues
develop from these
 Ectoderm
 Endoderm
 Mesoderm
Week 3

37. Formation of the 3 “germ” layers
 Primitive streak (groove)
on dorsal surface of
epiblast
 Grastrulation:
invagination of epiblast
cells
 Days 14-15: they replace
hypoblast becoming
endoderm
 Day 16: mesoderm (a
new third layer) formed
in between
 Epiblast cells remaining
on surface: ectoderm

38. Major derivatives of the
embryonic germ layers

53. Trophoblast
 Small projection begins to appear all
over the surface of the blastocyst,
becoming most prolific at the area of
contact. These trophoblastic cells
differentiate into 3 layer.
 1. The outer syncytiotrophoblast
(syncytium): it erodes the walls of the
blood vessels of the decidua, making
the nutrient in the maternal blood

54.  2. The inner cytotrophoblast: it’s a single
layer cells which produces a hormone HCG. This is
responsible for informing the corpus luteum that
a pregnancy has begun and corpus luteum
continues to produce estrogen and progesterone,
which maintain the integrity of the decidua.
 3. In the inner aspect: a layer of mesoderm or
primitive mesenchyme, which consists of loose
connective tissue.
 There is similar tissue in the inner cell mass and
the two are continuous at the point where they
join in the body stalk. (fig 3.9 1st
picture from book
tuitui)

57.  The trophoblast is responsible for the
formation of the placenta and fetal
membrane and sub-serve the important
functions of attachment of the fetus to
maternal tissues, providing nutrition,
oxygenation, and clearing of the fetal
metabolic wastes and producing hormones,
thus ensuring growth and development of the
fetus.

58. Decidua
 The endometrial lining of the uterus is
called decidua during pregnancy and it
is shed after delivery.
 Progesterone from the corpus luteum
and placenta maintain decidua during
pregnancy. It is called the decidual
reaction.

59. Well developed Decidua consists of 3
layers.
1. Superficial compact layer: consists
of compact mass of decidual cells, gland,
ducts and dilated capillaries. The greater part
of the surface epithelium is either thinned
out or lost.
2. Intermediate spongy layer
(cavernous or functional layer): contains
dilated uterine glands, decidual cells and
blood vessesls. Placenta separation occurs in

60. 3. The basal layer: containing the basal
portion of the glands and is opposed to the
uterine muscle. Regeneration of the mucous
coat occurs from this layer following
parturition.
After the implantation of the blastocyst into the
compact layer of the decidua, the different
portions of the decidua are renamed as:

61. 1. Decidua basalis or
serotina: The portion
of the decidua in
contact with the base of
the blastocyst.
2. Decidua capsularies
or reflexa: The thin
superficial compact
layer covering the
blastocyst.
3. Decidua vera or
parietalis: the rest of
the decidua lining the
uterine cavity outside
the site of implantation.

62.  The ovum bulges into the
uterine cavity. The space
between the decidua
capsularies and decidua vera
is called the decidual space.
 It progressively diminishes
as the ovum enlarges in
pregnancy
 Until at 16 weeks, the space
is obliterate because of the
fusion of the decidua
capsularis with the decidua
vera.
 At term this fused layer is
gradually attenuated and its
constituent layers can not be
identified.
 The decidua basalis
becomes the maternal
portion of the
placenta.

63. Functions of Decidua
1. To provide place for implantation of the
fertilized ovum.
2. To provide nutrition to the growing ovum.
3. Provide a protective action.
4. Provides the basal plate of the placenta.

64. Important Events Following Fertilization
 “O” hour - Fertilization
 30 hours - 2 cell stage
 40-50 hours - 4 cell stage
 72 hours - 12 cell stage
 96 hours - 16 cell stage morula enter the
uterine cavity
 5th
day - blastocyst
 6-7th
day - zona pellucida disappear.
Interstitial implantation occurs.

65.  9th
day - lacunar period, endometrial
vessels tapped
 10-11th
day- implantation completed
 13th
day - primary villi seen
 16th
day - secondary villi
 21st
day - tertiary villi
 21st
-22nd
- fetal heart, fetal placental
circulation started

66. Fetal Development

67. The prenatal development of the
fetus can be divided into 3 phases.
1. Ovular period or germinal period
(It lasts from the stage of fertilization upto 2 weeks
after ovulation)
The embryonic period (first 8 weeks
Development of the three primary germ layers give
rise to all structures and Basic body plan takes
shape )
Fetal period (remaining 30 weeks.
Structures and organs continue to grow and
develop)

68. Fetal Length
 In the earlier weeks, it is expressed as
the measurement from the vertex to the
coccyx (crown rump length) while after
mid pregnancy (20 weeks onwards), the
measurement of the fetus is determined
from the vertex to the heel (crown heel
length).

69. Age of the Fetus
 The length is more reliable criterion
than the weight to calculate the age of
the fetus.
 Haase’s rule is employed in calculating
the age of fetus from its length in
centimeters, divided by 5.

70. 1. First lunar month: Fertilization to 2weeks
of embryonic growth
 Implantation is complete.
 Primary chorionic villi forming
 Embryo develops into two cell layers
(bilaminar embryonic disks)
 Amniotic cavity appears.
 Note: lunar month is a period that is measured based on the
movement of the moon. It is roughly 28 days and it constitutes
a month. In that context, pregnancy is 10.

71. 2. Second lunar month: 3 to 6 weeks of
embryonic growth
 At the end of 6 weeks of growth, the embryo is
approximatey 1.2 cm long.
 Arms and leg buds are visible; arm buds are
more developed with finger ridges beginning to
appear.
 Rudiments of the eyes, ears and nose appear.
 Primitive intestinal tract is developing.
 Primitive cardiovascular system is functioning.
 Neural tube, which forms the brain and spinal
cord closes by the 4th
week.

72. 3. Third lunar month: 7 to 10 weeks of
growth
 The middle of this period (8 weeks)marks the end of the
embryonic period and beginning of the fetal period.
 At the end of 10 weeks of growth, the fetus is 6.1 cm
from crown to rump and weigh 14 gm.
 Appearance of external genitalia.
 By the middle of this month, all major organs, systems
have formed.
 The heart has formed four chambers (by 7th
week)
 The fetus assumes a human appearance
 Bone ossification begins
 Rudimentary kidney begins to secrete urine.

73. 4. Fourth lunar month: 11 to 14 weeks old
fetus
 At the end of 14 weeks of growth, the fetus
is 12 cm crown to rump length and 110 gm.
 Head erect; lower extremities well
developed.
 Hard palate and nasal septum have fused.
 External genitalia of male and female can
now be differentiated.
 Eyelids are sealed

74. 5.Fifth lunar month: 15 to 18 weeks old
fetus
 At the end of 18 weeks of growth, the fetus is
16 cm crown rump length and 320 gm.
 Ossification of fetal skeleton can be seen on
X-ray.
 Ears stand out from head.
 Fetus makes sucking motions and swallows
amniotic fluid.
 Fetal movement may be felt by the mother
(end of month)

75. 6. Sixth lunar month: 19-22 weeks old fetus
 At the end of 22 weeks of
growth, the fetus
is 21 cm crown rump length
and 630 gm.
 Vernix caseosa covers the
skin.
 Head and body hair (lanugo)
visible.
 Skin is wrinkled and red.
 Brown fat(adipose tissue), an
important site of heat
production, is present in neck
and sternal area.

76. 7. Seventh lunar months: 23-26 weeks old
fetus
 At the end of 26 weeks of
growth, the fetus is 25 cm
crown to rump length and
1000 gm.
 Fingernails present.
 Lean body
 Eyes partially open:
eyelashes present.
 Bronchioles are present:
primitive alveoli (terminal
sacs) are forming
 Skin begins to thicken
on hands and feet
 Startle reflex present,
grasp reflex is strong.

77. 8. Eighth lunar month: 27-30 weeks old
fetus
 At the end of 30 weeks
of growth, the fetus is
28 cm crown rump
length and 1,700 gm.
 Eyes open
 Ample hair on head:
lanugo begins to fade
 Skin slightly wrinkled
 Toe nails presents
 Testes in inguinal
canal, begin descent
to scrotal sac
 Surfactant coat in
much of the alveolar
epithelium

78. 9. Ninth lunar months: 31-34 weeks of old
fetus
 At the end of 34 weeks of growth, the
fetus is about 32 cm crown –rump
length and 2,500 gm
 Fingernails reach finger tips
 Skin pink and smooth
 Testes in scrotal sac

79. 10. Tenth lunar month: 35 to 38 weeks old
fetus
 End of 38 weeks of growth, fetus is about 36
cm crown –rump length and 3400 gm.
 Ample of subcutaneous fat present
 Lanugo is decreasing
 Toe nails reach upto toe tips
 Testes in scrotum
 Vernix caseosa mainly on the back
 Breasts are firm.

80. Weeks 3-4

81. Weeks 5-8
 Embryo will develop all structures that an adult
has by the end of week 8

82. Fetal Period
9-12
13-16
17-20
21-38

86. Doubts