This Presentation deals with the Development of a Human Embryo from conception till Week 8.
The content has been accumulated from various sources, including slideshare.
Content Credit to rightful owner.
Slides belong to this author.
Human Embryology and Development of Major Organ System.pptx
1. Embryology and Development
of Major Organ Systems
Presented by:
Dr. Nazia Anjum
Dept. of Ilmul Atfal (Paediatrics)
Nov-23
Dr. Nazia Anjum
2. What is Embryology?
The study of developmental events that
occur during the prenatal stage.
OR
The branch of biology and medicine
concerned with the study of embryos
and their development.
Nov-23
Dr. Nazia Anjum
3. Embryonic period vs Fetal period
Embryonic period :- first 8 weeks.
Development of the three primary germ
layers occur which give rise to all
structures and a basic body plan takes
place.
Fetal period :- 9th week until birth.
Structures and organs grow and
develop.
Nov-23
Dr. Nazia Anjum
4. Stages of Development
1. Fertilization
2. Cleavage
3. Gastrulation
4. Organogenesis
5. Maturation
Embryogenesis
Nov-23
Dr. Nazia Anjum
5. Week 1
Fertilization
The process of fusion
or union of the
spermatozoa with the
mature ovum is
known as Fertilization
or Conception.
Fertilization produces
zygote (a single
mono-nucleated cell).
Nov-23
Dr. Nazia Anjum
7. Normal site of conception-
The most common site of conception is the
ampullary part (Ampulla) of the fallopian
tube which is the widest part located closest
to the ovary.
Usual time for conception-
Neither sperm nor ovum can survive longer
than 2-3 days and the fertilization is most
likely to occur 48 hours before and 24 hours
after ovulation following intercourse.
So conception will take place 14 days before
the next period is due.
Nov-23
Dr. Nazia Anjum
8. The sex of the new individual is determined by
sex chromosome.
Every human cell contains 46 chromosomes,
which are made up of 44 autosomes and 2 sex
chromosomes.
The sex chromosomes are X and Y.
Males have Y chromosome whereas females
have no Y chromosome (male 44+X+Y) (female
44+X+X).
Therefore, sex of child is always determined by
the father.
Nov-23
Dr. Nazia Anjum
9. Embryogenesis
1. Cleavage: is a series of rapid mitotic cell
divisions (without cell growth).
2. Gastrulation: is a phase early in the embryonic
development during which a 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 a living
organism. Nov-23
Dr. Nazia Anjum
12. 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, resembling a
mulberry.
3. The morula after spending about
3 days (72 hours) in the uterine
tube enters the uterine cavity
through the narrow uterine
ostium (1 mm) on the 4th day. Nov-23
Dr. Nazia Anjum
13. 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.
The zona pellucida becomes stretched, thinned
and gradually disappear soon prior to
implantation.
Nov-23
Dr. Nazia Anjum
14. The cell of the outer cell mass forms the wall of
the blastocyst and is known as trophoblast. The
trophoblast eventually form the placenta.
The inner cell mass is concerned with the
development of the embryo.
Nov-23
Dr. Nazia Anjum
17. WEEK 2
Implantation- The Blastocyst implants itself into
the uterine wall.
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)
Nov-23
Dr. Nazia Anjum
20. WEEK 3
Bilaminar to
trilaminar disc
Three primary
“germ” layers:
responsible for the
development of all
body tissues-
Ectoderm
Mesoderm
Endoderm
Nov-23
Dr. Nazia Anjum
21. Formation of the 3 “germ” layers
Primitive streak (groove) on the dorsal
surface of epiblast.
Gastrulation: 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.
Nov-23
Dr. Nazia Anjum
24. Notochord
Days 16-18
Primitive node epiblast
cells invaginate and
migrate anteriorly with
some endoderm cells.
Rod defining the body
axis is formed.
Future site of the
vertebral column.
Nov-23
Dr. Nazia Anjum
26. Neurulation
Notochord signals overlying ectoderm.
The neural tube is developed from the closure
of the neural plate and the neural fold- a
process called neurulation- at about 21 to 26
days.
Closure of neural tube: begins at end of week
3; complete by the end of week 4.
Extends cranially (eventually brain) and
caudally (spinal cord).
Neural crest, lateral ectodermal cells are
pulled along and form sensory nerve cells and
other structures. Nov-23
Dr. Nazia Anjum
28. Division into Somites
At about day 20, the mesoderm divides into paired
bodies called somites.
Located on either side of the developing neural tube,
these paired bodies give rise to the skeleton and
muscle tissue.
During the somite period, day 20 to day 30, 38 pairs
of somites develop.
Their total number eventually reaches 42 to 44 pairs
of which 4 are occipital, 8 cervical, 12 thoracic, 5
lumbar, 5 sacral and 8 to 10 coccygeal.
Nov-23
Dr. Nazia Anjum
29. Some of the somites- first occipital and the fifth
to seventh coccygeal- disappear, while the rest
form the axial skeleton.
By the end of the third week of life, the conceptus
is 1.0 mm in length.
Nov-23
Dr. Nazia Anjum
30. The embryo grows dramatically during the fourth
week.
It triples its length to 3.5 mm and it weighs
approximately 5 mg.
Elongation of the embryo occurs and it becomes
curved upon itself with the formation of a head
and tail fold.
Lateral body folds develop, making the embryo
tubular rather than flat and disc-shaped.
Closure of the neural tube begins in the area of
the occiput and proceeds upward and downward
from that point.
WEEK 4 (contd.)
Nov-23
Dr. Nazia Anjum
31. Somites form in a cranio-caudal sequence as the
neural tube closes, can be observed through the
ectoderm.
The pericardial sac around the heart enlarges, causing
the head region to elevate.
The laryngo-tracheal groove and lung buds, which will
become the respiratory system are present.
The mandible and maxilla of the jaw become distinct,
and rudimentary forms of the eyes, ears and nose are
present.
The intestinal system is formed from the yolk sac and
differentiation of the buds progress, which will
become the oesophagus, stomach, liver and
pancreas.
The thyroid and thymus glands also develop. Nov-23
Dr. Nazia Anjum
32. The primitive circulatory system is established and
the heart starts beating.
The budlike projections on the surface of the embryo
are the beginning of the limbs.
4th week Embryo Nov-23
Dr. Nazia Anjum
34. WEEK 5
As the embryo and then the fetus grow during the
first half of pregnancy, it is measured by its
crown-rump length (CRL).
The fetus is measured during the last half of
pregnancy by its crown heel length (CHL) or
standing height. The CRL grows from 4 to 8 mm
in this week, and exceeds the growth.
The growth of the head is rapid and exceeds the
growth of the body during this week. The embryo
lengthens and bends into a C shape, while an
additional 42 to 44 pairs of somites are added to
its caudal end.
Nov-23
Dr. Nazia Anjum
35. The umblical cord is
formed from the union
of the amnion, the yolk
and the connecting
stalk. It now contains
two umblical arteries
and one umblical vein.
The doubling of the
size of the heart makes
it prominent and its
atria and ventricles are
visible through the
ectoderm. The
embryo’s four limb
buds are most
vulnerable to
teratogens at this time.
Nov-23
Dr. Nazia Anjum
36. WEEK 6
The head is larger than the trunk
and is bent over the heart
prominence.
Elevations in the facial ectoderm are
evident and the position of the eyes,
nose, mouth and the groove that is
to become the external acoustic
meatus is established.
In the upper limbs, the elbows and
wrists are identifiable and the hand
plates develop ridges called finger
rays.
Changes are beginning to occur in
the genital region. (The CRL is 8 to
14 mm, the weight 50 to 400 mg). Nov-23
Dr. Nazia Anjum
37. WEEK 7
Cerebral hemispheres appear as the head enlarges rapidly.
The eyes move from a lateral to a more frontal position as
the face elongates.
Prominences appear over the ventral body wall because
their function is vital to the maintenance and survival of
the embryo.
As the embryo continues to grow, the umblical cord
shrinks.
The arm and hand of the upper limbs and the thigh, leg
and foot segments of the lower limbs becomes apparent.
The fingers develop, their growth is critical at this point
(40 to 50 days).
The CRL is 14 to 20 mm; the weight is 400 to 1000 mg.
Nov-23
Dr. Nazia Anjum
38. WEEK 8
During this final week of
the embryonic period,
the embryo exhibits
definite human
characteristics.
The cerebral
hemispheres have grown
so rapidly that the head
now makes up 50% of
the mass of the embryo.
The face occupies the
lower half of the head
and the eyes continue to
move to a more frontal
plane. Nov-23
Dr. Nazia Anjum
39. Eyelid folds develop. These will become
fused during the ninth week and remain so
until the seventh month.
The fingers lengthen and the toes are
distinct by the end of the eighth week.
The external ears are set low and are taking
on their final shape.
Sexual differences in the external genitalia
can now be seen by the trained eye.
The CRL is 21 to 30 mm; the weight 1000 to
3000 mg.
Nov-23
Dr. Nazia Anjum
