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Intraembryonic Mesoderm
Origin: The epiblastic cells
from the primitive streak
 The newly formed cells
migrate ventrally, laterally &
cranially between the epiblast
and hypoblast
 At the margins of the
embryonic disc, the
intraembryonic mesoderm
merges with the extraembryonic mesoderm
By the end of 3rd week,
mesoderm lies between
embryonic ectoderm and
endoderm everywhere,
EXCEPT in the region of:
 Buccopharyngeal membrane
(fused prechordal plate +
 Cloacal membrane, as the
embryonic ectoderm &
endoderm are fused at these
 Notochord
is a rod of
mesenchymal cells
 located in the
 extending cranially
from the primitive
node to the
Formation of Notochord
Origin: Primitive node/pit
 Like the primitive streak, the
primitive pit cells proliferate
and then migrate cranially in
the midline, toward the
buccopharyngeal membrane,
and form a rod like
notochordal process
 The notochordal process
becomes canalized forming a
hollow tube, the notochordal
canal, which communicates
with the amniotic cavity at
the primitive pit.
Formation of Notochord cont’d
The floor of the tube
and the underlying
endoderm fuse and then
break down, forming a
notochordal plate
 The notochordal plate
becomes continuous
with the endodermal
Formation of Notochord cont’d
communication is
between the
amniotic cavity
and the yolk sac,
termed the
neurenteric canal.
Notochordal plate folds to form the notochord,
which gets separated from the underlying
Functions of Notochord
Defines primordial axis of the embryo
 Provides rigidity to the embryo
 Serves as a basis for the development of the axial
 Indicates the future site of the vertebral
 Regulates differentiation of surrounding
structures including the overlying ectoderm and
the mesoderm
and of Notochord
disappears as the bodies of
the vertebrae develop
 The part that lies between
the vertebral bodies
persists as the nucleus
pulposus of each
intervertebral disc
 Remnants of notochordal
tissue give rise to tumors
called Chordomas
Differentiation of the
Intraembryonic Mesoderm
 Induced
by the
 Differentiates into the:
 Paraxial mesoderm
 Intermediate cell
 Lateral plate
Ectodermal Derivatives
Dr. Zeenat Zaidi
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
 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
ectodermal cells
thickens to form
the neural plate
The neural plate first
 Cranial to the
primitive node and
 Dorsal to the
notochord & the
mesoderm adjacent
to it
As the notochord forms &
 The embryonic disc
elongates and becomes
 The neural plate broadens
and extends cranially as
far as the
membrane, and later on
grows beyond it
Neural fold
On 18th day: the neural
plate invaginates to
form neural groove &
neural folds
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
 Following
fusion of
the neural folds, the
neural crest cells
get separated and
move laterally to
form the sensory
neurons of the
spinal (dorsal root)
 The
neural tube separates from the surface
ectoderm, lies in the midline, dorsal to the
 Neural
tube is open
at both ends,
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
cranial ⅓ of the
neural tube represent
the future brain
 The caudal ⅔
represents the future
spinal cord
 The
Congenital Anomalies of the Nervous
Disturbance of neurulation may result in severe
abnormalities of the brain and the spinal cord
Most defects are the result of non-closure or
defective closure of the neural tube:
• In the brain region (e.g. anencephaly)
• In the spinal cord regions (e.g. spina
High level of alpha-fetoprotein (AFP) in the
amniotic fluid is a strong sign of neural tube
Surface ectoderm
 Neuroectoderm
Surface Ectoderm Derivatives
Epidermis of the skin
Sweat & Sebaceous glands
Mammary glands
Enamel of the teeth
Lens of eye
Internal ear
Anterior lobe of the pituitary gland
 Neural
 Neural Crest Cells
Neural Tube Derivatives
Central nervous system
 Peripheral nervous system
 Retina
 Sensory epithelia of nose & ear
 Pineal gland
 Posterior lobe of the pituitary gland
Neural Crest Cells Derivatives
Sensory ganglia (cranial & spinal)
Autonomic ganglia
Meninges (Pia mater & Arachnoid mater) of
the brain & spinal cord
Schwann cells
Satellite cells
Suprarenal medulla (chromaffin cells)
Several skeletal & muscular components in
the head (derived from pharyngeal arches)