The document describes gastrulation and the formation of the three germ layers - ectoderm, mesoderm, and endoderm - in the third week of human embryonic development. It discusses how cells migrate through the primitive streak and invaginate to form the germ layers. It specifically describes the formation of the notochord and how it induces the overlying ectoderm to form the neural plate. The document also summarizes the derivatives and tissues formed from each germ layer, including the central nervous system from ectoderm, muscles and connective tissues from mesoderm, and the gastrointestinal tract from endoderm. It discusses some congenital defects that can result from issues during gastrulation.

1. OJORA K .A
Third Week of Development:
Trilaminar Germ Disc
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2. Third Week
cytotrophoblast
Extraembryonic
Somatopleuric
Mesoderm
Extraembryonic
Splanchnopleuric
Mesoderm
Primitive
Yolk
Sac
Epiblast
Hypoblast
Extraembryonic
Coelom
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3. GASTRULATION
 The process that establishes all three germ layers
(ectoderm, mesoderm, and endoderm) in the embryo.
 with formation of the primitive streak on the surface of
the epiblast.
 The cephalic end of the streak, the primitive node,
consists of a slightly elevated area surrounding the small
primitive pit.
 Cells of the epiblast migrate toward the primitive streak.
 Upon arrival in the region of the streak, they become
flask-shaped, detach from the epiblast, and slip beneath
it(INVAGINATION)
 Once the cells have invaginated, some displace the
hypoblast, creating the embryonic endoderm, and
others come to lie between the epiblast and newly
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4. GASTRULATION
 As more and more cells move between the epiblast and
hypoblast layers, they begin to spread laterally and
cranially.
 Gradually, they migrate beyond the margin of the disc
and establish contact with the extraembryonic mesoderm
covering the yolk sac and amnion.
 The oropharyngeal membrane at the cranial end of the
disc consists of a small region of tightly adherent
ectoderm and endoderm cells that represents the future
opening of the oral cavity.
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5. 5

6. A. Dorsal side of the germ disc from a 16-day embryo indicating the
movement of surface epiblast cells (solid black lines) through the primitive
streak and node and the subsequent migration of cells between the
hypoblast and epiblast (broken lines).
B. Cross section through the cranial region of the streak at 15 days showing
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7. Dorsal view of an embryo showing the primitive node and streak and a cross
section through the streak.
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8. FORMATION OF THE
NOTOCHORD
 Some mesenchymal cells that have ingressed through
the streak migrate cranially from the primitive node and
pit, forming a median cellular cord, the notochordal
process.
 This process soon acquires a lumen, the notochordal
canal.
 It grows cranially between the ectoderm and endoderm
until it reaches the prechordal plate.
 The prechordal plate is the primordium of the
oropharyngeal membrane, located at the future site of
the oral cavity.
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9. FORMATION OF THE
NOTOCHORD
 Caudal to the primitive streak there is a circular area-the
cloacal membrane, which indicates the future site of the
anus.
 By the middle of the third week, intraembryonic
mesoderm separates the ectoderm and endoderm
everywhere except
1. At the oropharyngeal membrane cranially
2. In the median plane cranial to the primitive node,
where the notochordal process is located
3. At the cloacal membrane caudally
 The notochord
1. Defines the primordial longitudinal axis of the embryo
and gives it some rigidity
2. Provides signals that are necessary for the
development of axial musculoskeletal structures and
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10. A. Drawing of a sagittal section through a 17-day embryo. The most cranial
portion of the definitive notochord has formed, while prenotochordal
cells caudal to this region are intercalated into the endoderm as the
notochordal plate. Note that some cells migrate ahead of the notochord.
These mesoderm cells form the prechordal plate that will assist in
forebrain induction.
B. Schematic cross section through the region of the notochordal plate.
Soon, the notochordal plate will detach from the endoderm to form the
defi nitive notochord.
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11. Functions of the notochord
1.It forms the basis of the
axial skeleton (bones of the
head and vertebral column).
2. It induces the overlying
ectoderm to thicken and
form the neural plate; the
primordium of the central
nervous system (Notochord
is the organizer for nervous
system formation) .
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12. Functions of the notochord
3. The notochord degenerates
and disappears as the
bodies of the vertebrae
form. Its remnant is the
nucleus pulposus of the
intervertebral discs.
4. It functions as the primary
inducer in the early embryo
i.e. it is a prime mover in a
series of signal-calling
episodes that ultimately
transform unspecialized
embryonic cells into
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13. Remnants of Notochordal Tissue
 Both benign and malignant tumors (chordomas)
may form from vestigial remnants of notochordal
tissue.
 Approximately one third of chordomas occur at
the base of the cranium and extend to the
nasopharynx.
 Chordomas grow slowly and malignant forms
infiltrate bone.
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14. Teratogenesis associated with
gastrulation
 Holoprosencephaly- forebrain is small, lateral
ventricle fuses, the eyes are close together.
 Causes by high intake of alcohol which at this stage
kill cells in the anterior midline of the germ disc,
producing a deficiency of the middle in craniofacial
structures.
 Caudal dysgenesis (sirenomelia) – insufficient
mesoderm is formed in the caudal most region of the
embryo. This mesoderm contributes to the formation
of the lower limbs, urogenital system and lumbosacral
vertebrate.
 Features- fusion of lower limb, vertebral
abnormalities, renal agenesis, imperforate anus, and
anomalies of the genital organs.
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15. Teratogenesis associated with
gastrulation
 Holoprosencephaly- forebrain is small, lateral
ventricle fuses, the eyes are close together.
 Causes by high intake of alcohol which at this stage
kill cells in the anterior midline of the germ disc,
producing a deficiency of the middle in craniofacial
structures.
 Caudal dysgenesis (sirenomelia) – insufficient
mesoderm is formed in the caudal most region of the
embryo. This mesoderm contributes to the formation
of the lower limbs, urogenital system and lumbosacral
vertebrate.
 Features- fusion of lower limb, vertebral
abnormalities, renal agenesis, imperforate anus, and
anomalies of the genital organs.
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16. Two examples of
sirenomelia
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17. Tumors Associated With Gastrulation
 Remnants of the primitive streak may persist and give
rise to a sacrococcygeal teratoma.
 Because they are derived from pluripotent primitive
streak cells, these tumors contain tissues derived
from all three germ layers in incomplete stages of
differentiation.
 Sacrococcygeal teratomas are the most common
tumor in newborns and have an incidence of
approximately one in 35,000; most affected infants
(80%) are female.
 Sacrococcygeal teratomas are usually diagnosed on
routine antenatal ultrasonography, and most tumors
are benign.
 These tumors are usually surgically excised promptly,
and the prognosis is good
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18. Female infant with a large sacrococcygeal teratoma that developed from
remnants of the primitive streak.
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19. DERIVATIVES OF THE
ECTODERMAL GERM LAYER
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20. Neurulation
 The embryonic ectoderm
over the developing
notochord thickens to
form a neural plate .
 The ectoderm of the plate
is called neuroectoderm
and eventually gives rise
to the central nervous
system (brain and spinal
cord).
 Neuroectodermal cells at
the lateral edge of the
neural plate -do not
become part of the tube
 By the end of week 3, the
neural folds move
together, fuse, and
convert the neural plate
into the neural tube.
 Closure begins in the
middle of the embryo and
progresses toward both
cephalic and caudal ends.
It begins on day 21.
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21. Neurulation
 Until fusion is complete, the cephalic and caudal ends
of the neural tube communicate with the amniotic
cavity by way of the anterior (cranial) and posterior
(caudal) neuropores, respectively.
 Closure of the cranial neuropore occurs at
approximately day 25, whereas the posterior
neuropore closes at day 28.
 Neurulation is then complete, and the central nervous
system is represented by a closed tubular structure
with a narrow caudal portion, the spinal cord, and a
much broader cephalic portion characterized by a
number of dilations, the brain vesicles.
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23. 23

24. Neural tube formation and neural crest
cells migration
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25. Neural Crest Derivatives
1. Connective tissue and bones of the face and skull
2. Cranial nerve ganglia
3. C cells of the thyroid gland
4. Conotruncal septum in the heart
5. Odontoblasts
6. Dermis in face and neck
7. Spinal (dorsal root) ganglia
8. Sympathetic chain and preaortic ganglia
9. Parasympathetic ganglia of the gastrointestinal tract
10. Adrenal medulla
11. Schwann cells
12. Glial cells
13. Arachnoid and pia mater (leptomeninges)
14. Melanocytes
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26. Defects in the closure of neural pore
will give rise to
 Anencephaly – when
the anterior neuropore
does not close, the
brain will not develop
and the fetus will be
born with out brain
inside the cranial
cavity.
 Spina bifida – there
will be a wide open
gap at the lower end
of spinal cord and it
will be exposed out as
a vesicle, as the
posterior neuropore
doesn't close.
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27.  In general terms, the ectodermal germ layer gives rise
to organs and structures that maintain contact with
the outside world:
● The central nervous system;
● The peripheral nervous system;
● The sensory epithelium of the ear, nose, and eye; and
● The epidermis, including the hair and nails.
 In addition, it gives rise to:
● Subcutaneous glands,
● The mammary glands,
● The pituitary gland,
● And enamel of the teeth
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28. DERIVATIVES OF THE
MESODERMAL GERM LAYER
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29. Mesoderm:
It differentiates into:
1-Axial mesoderm (primitive
streak, primitive node,
notochord)
2-Paraxial mesoderm gives
final count of
approximately 35 pairs of
somites. These somites
further differentiate into 3
components:
a- Sclerotome (cartilage and
bone)
b- Myotome (muscles)
c- Dermatome (dermis of
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30. 4-Lateral plate of mesoderm
differentiates into:
a-somatopleure (gives the
voluntary muscles of chest
and abdomen and the
parietal layer of pleura and
peritoneum.
b- splanchnopleure (gives
the involuntary muscles of
heart, bronchial tree, and
gut and the visceral layer
of pleura and peritoneum).
 The cavity between these
two layers is the
intraembryonic coelom.
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31. 31

32. DERIVATIVES OF THE ENDODERMAL
GERM LAYER
 The gastrointestinal tract is the main organ system
derived from the endodermal germ layer.
 This germ layer covers the ventral surface of the
embryo and forms the roof of the yolk sac.
 As a result of cephalocaudal growth and closure of the
lateral body wall folds a continuously larger portion of
the endodermal germ layer is incorporated into the
body of the embryo to form the gut tube.
 The tube is divided into three regions: the foregut,
midgut, and hindgut.
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33.  At its cephalic end, the foregut is temporarily bounded
by an ectodermal–endodermal membrane called the
oropharyngeal membrane.
 In the fourth week, the oropharngeal membrane
ruptures, establishing an open connection between the
oral cavity and the primitive gut.
 The hindgut also terminates temporarily at an
ectodermal– endodermal membrane, the cloacal
membrane.
 The membrane breaks down in the seventh week to
create the opening for the anus.
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34. 34

35.  The role of the yolk sac is not clear.
 It may function as a nutritive organ during the earliest
stages of development prior to the establishment of
blood vessels.
 It also contributes some of the first blood cells,
although this role is very transitory.
 One of its main functions is to provide germ cells that
reside in its posterior wall and later migrate to the
gonads to form eggs and sperm.
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36.  Hence, the endodermal germ layer initially forms the
epithelial lining of the primitive gut and the
intraembryonic portions of the allantois and vitelline
duct.
 During further development, endoderm gives rise to:
1. The epithelial lining of the respiratory tract;
2. The parenchyma of the thyroid, parathyroids, liver,
and pancreas
3. The reticular stroma of the tonsils and the thymus;
4. The epithelial lining of the urinary bladder and the
urethra
5. The epithelial lining of the tympanic cavity and
auditory tube.
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38. ASSIGNMENT
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 The end