9. PREEMBRYONIC PERIOD
(FIRST TO THIRD WEEK)
First week of life:
• Day 1: Fertilization
10. Day 2 and 3: Cleavage
• The original zygote divides about 30
hours after conception into two
daughter cells called blastomeres.
• Continued subdivisions of the
original cell result in increasing
numbers of blastomeres.
• During cell division the dividing cells
decrease in size. This type of cell
division is called cleavage.
• By the time the zygote is ready to
enter the uterus, it contains a solid
ball of 12 to 16 blastomeres called
the morula (from the latin word for
mulberry).
11. Day 4: Formation of the blastocyst
• Fluid within the intercellular spaces
of the morula gradually increases,
and spaces on one side of the inner
cell mass come together, forming a
single cavity, the blastocele.
• The outer layer organizes into the
trophoblasts, which give rise to the
placenta, and the inner layer of cells
form the embryo.
• The cavity of the blastocele fills with
fluid, and the conceptus is now
called the blastocyst.
12. Preparation of the endometrium
• Resting phase
• Proliferative phase
• Secretory phase
13. Attachment of the blastocyst
• The blastocyst attaches to the uterine
lining in the V-shaped.
• When the trophoblast (the outer cell
layer) attaches to the endometrium, it
proliferates and separates into an
inner cytotrophoblastic layer (fetal
side) and an outer
syncytiotrophoblastic (placental side).
• The outer layer develops finger like
projections that proliferate and
superficially attach the blastocyst to
the endometrium within 6 days after
conception.
14. MAJOR EVENTS OF FIRST WEEK
POSSIBLE ABNORMAL
NORMAL EVENTS EVENTS
Fertilization and Abnormal
formation of the zygote implantation
(30hours). Maternal infection
Cleavage of the zygote or a genetic defect
into 12 to 16 Hydatidiform mole
blastomeres- the
Abortion
morula (day 2 and 3).
Formation of the
Ectopic implantation
blastocyst ( day 5-8).
15. Second week of life
• A slitlike amniotic cavity appears about day 8,
and the yolk sac appears as a second cavity
on day 12. Bilaminar embryonic disc is
formed in between these two layers.
• The endodermal disc becomes thicker at it’s
cephalad end, forming the prochordal plate.
• During early development of the nervous
system, the function of the prochordal plate
is to indicate the site of the mouth and to
form the membranes of the mouth and
throat.
• The formation of the decidua, fetal
membranes, and placenta extends beyond
the second week, but their development
begins at this point.
16. THIRD WEEK OF LIFE
• During the third week of life, the conceptus
develops rapidly. This period also coincides with the
first missed menstrual cycle of the mother.
• The primitive streak is formed during the third
week, and three germ layers develop.
• This periods from approximately day 15 to day 21,
is called the “period of threes”; not only do the
three germ layers develop, but the primitive streak,
the notochord, and the neural tube are formed.
17. GASTRULATION
• Gastrulation is the process by
which the bilaminar embryo
becomes a trilaminar embryo.
• On about day 15, the
cytotrophoblast cells proliferate
into the blastocyst to form the
extraembryonic mesoderm, which
later become the extra-embryonic
coelom.
• The mesoderm lies between the
ectoderm and the endoderm,
completing the trilaminar disc of
the primitive streak. All tissues and
organs of the embryo are
developed from these three layers.
19. 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
20. 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.
21. • Closure of neural tube: begins at end of week 3; complete
by end of week 4.
• Extends cranially (eventually brain) and caudally (spinal
cord)
• Neural crest, lateral ectodermal cells, pulled along and form
sensory nerve cells and other structures
22. DIVISON INTO SOMITES
• 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 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.
• 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 about 1.0 mm
in length.
23. MAJOR EVENTS OF
third WEEK
POSSIBLE
NORMAL EVENTS ABNORMAL EVENTS
• Formation of blood vessels within • Monozygous
the chorionic villi ( day 13). twins result
• Gastrulation or conversion of the
bilaminar embryonic disc into the • Conjoined twins
three- layered trilaminar disc (day are formed.
14). • The heart is most
• Continued development of the susceptible to
chorion with formation of tertiary
teratogens
chorionic villi( day 15-20).
between the 19th
• Development of the neural tube
(day 18). and the 41st day.
• Formation of somites (day 21).
• Beginning of blood circulation (day
24).
24. fourth week of life
• The embryo grows
dramatically during the
fourth week.
• It more than triples its
length to 3.5mm and its
weight approximates 5
mg.
25. • Elongation of the embryo has
occurred, and it has become
curved pon 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.
• Somites formed in a craniocaudal
sequence as the neural tube
closes, can be observed through
the ectoderm.
26. • The pericardial sac around
the heart enlarges, causing
the head region to elevate.
• The larygotracheal 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.
27. • The intestinal system is formed
from the yolk sac, and
differentiation of the buds,
which will become the
oesophagus, stomach, liver and
pancreas, is progressing.
• The thyroid and thymus glands
are also developing.
• The primitive circulatory system
is established, and the heart is
beating.
• The budlike projections on the
surface of the embryo are the
beginning of the limbs.
28. FIFTH WEEKS OF LIFE
• 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.
29.
30. • The umbilical cord is
formed from the union of
the amnion, the yolk, and
the connecting stalk. It
now contains two umbilical
arteries and one umbilical
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.
31. SIXTH WEEK OF LIFE
• The head has become 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.
• The lower limbs are not developed to the
same degree.
• Changes are beginning to occur in the
genital region. (The CRL is 8 to 14 mm, the
weight 50 to 400mg).
32. SEVENTH WEEK OF LIFE
• 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 from early
because their function is vital to the maintenance and
survival of the embryo.
• As the embryo continues to grow, the umbilical cord
shrinks.
• The arm and hand of the upper limbs and the thigh, leg,
and foot segments of the lower limbs become apparent.
• The fingers develop, the 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.
33. EIGHTH WEEK OF LIFE
• 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.
• Eyelids folds develop. These will become
fused during the ninth week and remain
so until the seventh month.
34. • 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 30mm; the
weight 1000 to 3000 mg).
35. MAJOR EVENTS OF
fourth to eighth WEEK
POSSIBLE ABNORMAL
NORMAL EVENTS EVENTS
• Conversion of the flat trilaminar embryonic disc • Abnormalities of the
into a c- shaped cylindrical embryo. genes and
• Formation of the head, tail, and lateral folds. chromosomes.
• Formation of the lateral and ventral body walls. • Alterations of
• Acquisition of an epithelial covering by the maternal health,
umbilicus through the expansion of the amnion. such as infection
• Establishment of ventral position of the heart and from rubella or
development of the brain in the cranial region of herpes.
the embryo. • Ingestion of
• Differentiation of the three germ layers into teratogenic
various tissues and layers that will become substances.
established as the major organ systems. • The risk of mortality
• Appearance of the brain, limbs, ears, eyes, nose. is greater than at
any other time of
• Development of a human appearance by the life.
