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					         Intraembryonic Mesoderm
l   Origin: The epiblastic cells
    from the primitive streak
l   The newly formed cells
    migrate ventrally, laterally &
    cranially between the epiblast
    and hypoblast
l   At the margins of the
    embryonic disc, the
    intraembryonic mesoderm
    merges with the extra-
    embryonic mesoderm
l By  the end of 3rd week,
  mesoderm lies between
  embryonic ectoderm and
  endoderm everywhere,
  EXCEPT in the region of:
l Buccopharyngeal membrane
  (fused prechordal plate +
l Cloacal membrane, as the
  embryonic ectoderm &
  endoderm are fused at these
l Notochord is a rod of
 mesenchymal cells
 § located in the
 § extending cranially
   from the primitive
   node to the
Formation of Notochord
l   Origin: Primitive node/pit
l   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
l   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
l The floor of the tube
  and the underlying
  endoderm fuse and then
  break down, forming a
  notochordal plate
l The notochordal plate
  becomes continuous
  with the endodermal
Formation of Notochord cont’d
lA  temporary
 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
l Defines  primordial axis of the embryo
l Provides rigidity to the embryo
l Serves as a basis for the development of the axial
l Indicates the future site of the vertebral
l Regulates differentiation of surrounding
  structures including the overlying ectoderm and
  the mesoderm
              Fate of Notochord
l Degenerates   and
  disappears as the bodies of
  the vertebrae develop
l The part that lies between
  the vertebral bodies
  persists as the nucleus
  pulposus of each
  intervertebral disc
l Remnants of notochordal
  tissue give rise to tumors
  called Chordomas
          Differentiation of the
       Intraembryonic Mesoderm
l Induced  by the
l Differentiates into the:
   § Paraxial mesoderm
   § Intermediate cell
   § Lateral plate
Ectodermal Derivatives

    Dr. Zeenat Zaidi
                  The Neurulation
l Itis the process by which the neural tube is
  formed. The stages of neurulation include the
  formation of:
       l Neural plate
       l Neural groove

       l Neural folds & their fusion

       l Neural crest cells

       l Neural tube

l Begins   during early part of the 4th week (22-23
l Ends by the end of 4th week (27 days)
l Is induced by the notochord
              The Neurulation
l Under the
 inducing effect of
 the developing
 notochord, the
 ectodermal cells
 thickens to form
 the neural plate
l The   neural plate first
l Cranial to the
  primitive node and
l Dorsal to the
  notochord & the
  mesoderm adjacent
  to it
l As  the notochord forms &
l The embryonic disc
  elongates and becomes
l The neural plate broadens
  and extends cranially as
  far as the
  membrane, and later on
  grows beyond it
                            Neural fold

l On 18th day: the neural
 plate invaginates to
 form neural groove &
 neural folds
l Some  neuroectodermal cells along the
 crest of the neural fold differentiate as the
 neural crest cells.
                       Neural crest cells

                                            Neural fold
l By the end of 3rd week,
 the neural folds move
 to the midline and fuse
 to form the neural tube

l The fusion begins in
 the future cervical
 region and then
 extends both in cranial
 and caudal direction
l 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)
l Theneural tube separates from the surface
 ectoderm, lies in the midline, dorsal to the
l Neural  tube is open
  at both ends,
  freely with the
  amniotic cavity.
l The cranial opening,
  the rostral neuropore
  closes at about 25th
  day & the caudal
  neuropore closes at
  about the 27th day
l The  cranial ⅓ of the
  neural tube represent
  the future brain
l The caudal ⅔
  represents the future
  spinal cord
    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
l Surfaceectoderm
l Neuroectoderm
Surface Ectoderm Derivatives
 l   Epidermis of the skin
 l   Hair
 l   Nail
 l   Sweat & Sebaceous glands
 l   Mammary glands
 l   Enamel of the teeth
 l   Lens of eye
 l   Internal ear
 l   Anterior lobe of the pituitary gland
l Neural Tube
l Neural Crest Cells
Neural Tube Derivatives
l Central nervous system
l Peripheral nervous system
l Retina
l Sensory epithelia of nose & ear
l Pineal gland
l Posterior lobe of the pituitary gland
Neural Crest Cells Derivatives
l   Sensory ganglia (cranial & spinal)
l   Autonomic ganglia
l   Meninges (Pia mater & Arachnoid mater) of
    the brain & spinal cord
l   Schwann cells
l   Satellite cells
l   Melanoblasts
l   Suprarenal medulla (chromaffin cells)
l   Several skeletal & muscular components in
    the head (derived from pharyngeal arches)

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