GASTRULATION,
NEURULATION AND
FOLDING
Dr Rania Gabr
OBJECTIVES
By the end of this lecture , the student should
be able to:
 Define Gastrulation
 Describe the formation of the primitive streak
 Describe the formation of the intraembryonic
mesoderm and the Trilaminar disc
 Explain the formation , function and fate of the
notochord
 Define Neurulation
 Describe the formation of the neural plate,
groove, fold , crest and canal
 Understand the process of folding its timing and
results
The Third Week
The significant event of third week is
Gastrulation
Gastrulation: is the process of
formation of the 3 germ layers
(ectoderm, mesoderm & endoderm).
GASTRULATION
 The
process by which the bilaminar disc is
converted into a trilaminar disc
 It
is the beginning of morphogenesis
(formation of body form)
 Consists
of formation of the primitive
streak, the three germ layers & the
notochord
 Embryo
is referred to as a Gastrula
PRIMITIVE STREAK



The primitive streak results
from proliferation of the
epiblastic cells in the
median plane, in the caudal
half of the epiblast, and lies
along the cranio-caudal axis.
Its cranial end forms the
primitive node
A groove, primitive groove,
appears in the primitive
streak, which continues with a
small depression, primitive
pit, in the primitive node.
A
circular thickening
appears in the hypoblast
near the cranial end, in
the midline, to form the
prechordal plate (
oropharyngeal
membrane), that marks
the future site of mouth
A
circular thickening
appears in the hypoblast
caudal to primitive streak
in the midline to form the
cloacal membrane, the
future site of the anus
FORMATION OF INTRAEMBRYONIC
MESODERM


The epiblastic cells from
the primitive streak
(groove) proliferate to
form mesenchymal
tissue
The newly formed cells
invaginate and migrate
ventrally, laterally &
cranially between the
epiblast and hypoblast &
organize to form the
intraembryonic mesoderm
FORMATION OF INTRAEMBRYONIC
MESODERM CONT’D
 Intraembryonic
mesoderm
merges with the extraembryonic mesoderm at the
periphery of the embryonic
disc
 By
the end of 3rd week,
mesoderm lies between
embryonic ectoderm and
endoderm everywhere
EXCEPT in the region of
prechordal plate and
cloacal membrane, as the
embryonic ectoderm &
endoderm are fused at these
regions
TRILAMINAR EMBRYONIC DISC
FORMATION OF INTRAEMBRYONIC
MESODERM CONT’D
 Some
mesenchymal cells
displace the hypoblasts
forming the embryonic
endoderm
 Cells
remaining in the
epiblast form the
embryonic ectoderm
Thus the EPIBLAST gives rise to all
three germ layers, Ectoderm,
Mesoderm, Endoderm in the embryo
EACH OF THE THREE GERM LAYERS GIVES
RISE TO SPECIFIC TISSUES AND ORGANS
FATE OF PRIMITIVE STREAK
 Actively
forms mesoderm
until the early part of 4th
week
 Then
it starts regressing and
becomes an insignificant
structure in the
sacrocooccygeal regions
 Normally
it degenerates and
disappears by the end of 4th
week
 Remnants
may persist and
give rise to a large tumor
called Sacrococcygeal
Teratomas
NOTOCHORD
A
rod of mesenchymal
cells located cranially,
in the midline,
extending between the
primitive node and the
prechordal plate
FORMATION OF NOTOCHORD
 Mesenchymal
cells
migrate cranially from
the primitive pit towards
the prechordal plate, and
form a rod like
notochordal process
 The
notochordal process
becomes canalized
forming a hollow tube,
the notochordal canal,
communicating with the
primitive pit.
FORMATION OF NOTOCHORD CONT’D
 The
floor of the tube and
the underlying
endoderm break down,
forming a notochordal
plate
 The
notochordal plate
becomes continuous with
the endodermal layer.
FORMATION OF NOTOCHORD CONT’D
A
temporary
communication is
established
between the
amniotic cavity
and the yolk sac,
termed the
neurenteric
canal.
NOTOCHORDAL PLATE FOLDS TO FORM THE
NOTOCHORD.
FUNCTIONS OF NOTOCHORD
 Defines
primordial axis of the embryo
 Provides rigidity to the embryo
 Serves as a basis for the development of the
axial skeleton
 Indicates the future site of the vertebral
bodies/column
 Regulates differentiation of surrounding
structures including the overlying ectoderm
(neural plate) and mesoderm (somites).
FATE OF NOTOCHORD
 Degenerates
and
disappears as the bodies
of the vertebrae develop,
but it persists as the
nucleus pulposus of each
intervertebral disc
 Remnants
of
notochordal tissue give
rise to tumors called
Chordomas
ECTODERMAL DERIVATIVES
THE NEURULATION
 It

is the process by which the neural tube is formed.
The stages of neurulation include the formation of:
Neural plate
Neural groove
Neural folds & their fusion
Neural crest cells
Neural tube

 Begins
during early part of the 4th week (22-23
days)
 Ends by the end of 4th week (27 days)
 Is induced by the notochord
THE NEURULATION
 Under
the
inducing effect of
the developing
notochord, the
overlying
ectodermal cells
thickens to form
the neural plate
 The
neural plate
first appears:
 Cranial to the
primitive node
and
 Dorsal to the
developing
notochord & the
mesoderm
adjacent to it
 As
the notochord forms
& elongates:
 The embryonic disc
elongates and becomes
club-shaped
 The neural plate
broadens and extends
cranially as far as the
buccopharyngeal
membrane, and later
on grows beyond it
 On
18th day: the neural
plate invaginates to
form neural groove &
neural folds
Neural fold
 Some
neuroectodermal cells along the
crest of the neural fold differentiate as the
neural crest cells.
Neural crest cells
Neural fold
 By
the end of 3rd week,
the neural folds move
to the midline and fuse
to form the neural tube
 The
fusion begins in
the future cervical
region and then
extends both in cranial
and caudal direction
The
neural tube separates from the
surface ectoderm, lies in the midline,
dorsal to the notochord
 Neural
tube is open
at both ends,
communicating
freely with the
amniotic cavity.
 The cranial opening,
the rostral
neuropore closes at
about 25th day & the
caudal neuropore
closes at about the
27th day
The
cranial ⅓ of
the neural tube
represent the
future brain
The
caudal ⅔
represents the
future spinal cord
FOLDING OF EMBRYO
-
Folding means
conversion of the
flat trilaminar
embryonic disc into
a cylindrical
embryo.
Time:
Folding of the embryo
begins by the end of
the 3rd week. It is
completed by the 4th
week.
 Folding
of the
embryo is due to
rapid growth of the
embryo specially the
nervous system.
 The
head folds first
then the tail . At the
same time, side to
side folding occurs.