Primitive Streak, Notochord, Neural Tube, Neural Crest Cells

PRIMITIVE  STREAK 

• It is defined as a narrow median groove with raised margins at the caudal end of epiblast.
• It appears on the 15^th day after fertilization by invagination of pluripotent ectodermal cells.

Gastrulation :-

• Process of invagination and migration of ectodermal cells from primitive streak is known as gastrulation.
• Through this process intra-embryonic mesoderm is formed.
• Primitive streak acts as a primary organizer in inducing the gastrulation. 

Cells from the bottom of groove actively proliferate and invaginate into the space between the epiblast and hypoblast

Initially, migrating epiblast cells invade the hypoblast and displace the cells of primitive endoderm, to form a new layer of cells known as definitive endoderm

Later, epiblast cells migrate bilaterally and occupy the space between epiblast and definitive endoderm to from third germ layer, known as intra-embryonic mesoderm

Now the epiblast is known as definitive ectoderm

Thus, the trilaminar germ disc with definitive endoderm, mesoderm and ectoderm is formed from the primitive streak of pluripotent epiblast.

• Primitive streak regresses at the end of 3^rd week and disappears by 26^th day of development. 

Intra-embryonic Mesoderm :–

• It is continuous with the extra-embryonic mesoderm at the periphery of germ disc.
• It extends between the ectoderm and endoderm throughout the embryonic disc except at –
• Buccopharyngeal membrane cranially
• Cloacal membrane caudally
• Cephalic to the buccopharyngeal membrane intra-embryonic mesoderm occupies a horse-shoe shaped area known as pericardial bar.

NOTOCHORD

Formation of Notochord :-

At the cephalic end of primitive streak ectodermal cells proliferate to form a knob-like elevation called as Primitive node or Hensen’s node

The center of primitive node presents a depression known as primitive pit

Cells from primitive node proliferate to form a cord of cells which extends in cephalic direction in the median plane between ectoderm and endodermal layers

Such cellular cord is known as notochordal process

It is distinguishable on the 17^th or 18^th day of development

Notochordal process extends only till the bilaminar prechordal plate

Meanwhile, the depression from primitive pit extends into the notochordal process along its length, forming notochordal canal

Now the cells at the floor of notochordal canal fuse with the underlying endodermal cells and start degenerating

So that the notochordal canal communicates below with the yolk sac directly and above with the amniotic cavity through primitive pit.

Such communicating canal is known as neurenteric canal

Neurenteric canal provides nutrition to the rapidly differentiating ectodermal cells by diffusion of yolk sac fluid

At this stage, the margins of notochordal process are continuous with the endoderm forming notochordal plate

Later, the margins of notochordal plate starts folding and fuse with each other

 Endoderm proliferates to form a continuous layer

Notochordal canal is filled by proliferating cells and a solid flexible cord of cells is formed known as definitive notochord.

Fate of Notochord :-

• Notochordal cells persist as nucleus pulposus in the center of intervertebral disc.

 

NEURAL  TUBE

Formation of Neural Tube :-

Neuroectodermal cells overlying the notochord become thickened to form neural plate

Neural plate forms a longitudinal gutter in the midline known as neural groove

Neural groove extends from the primitive node to buccopharyngeal membrane

Neural groove is concave dorsally and bounded by raised lateral margins known as neural folds

Tips of neural folds are known as neural crests

Neural crests are continuous with surface ectoderm at the periphery

Neural folds start fusing dorsally and covert the neural plate into neural tube

The process of formation of neural tube from neural plate folding is known as neurulation

Fusion of neural folds takes place in the cervical region at first then extends cranially and caudally

Before the neural folds completely fuse with each other the neural tube shows two openings

Cranial / Anterior neuropore & Caudal / Posterior neuropore

Amniotic fluid flows through the neuropores and provides nutrition to the neural tube

Anterior neuropore closes by 25^th day and posterior neuropore closes by 28^th day of development

Once the completely closed neural tube is formed it gets detached from the surface ectoderm

In this process cells from the neural crest get separated and lie between the surface ectoderm and neural tube.

Derivatives of Neural Tube :-

• Cephalic part of the neural tube give rise to fore brain mid brain & hind brain.
• Caudal part of the neural tube give rise to spinal cord.

Neural Tube Defects (NTDs) :-

Open Neural Tube Defects –

Anencephaly –

• Failure of closure of anterior neuropore.
• Cranial vault is absent.
• Brain substance is exposed to the surface as a mass.
• Prominent eyes bulge forward.
• Chin is continuous with the chest due to absence of neck.
• Associated with hydramnios.
• Frequent in first-born child.
• Ante-natal diagnosis can be done by- Ultrasonography or amniocentesis.

Rachischisis –

• Failure of closure of posterior neuropore.
• Caudal part of neural groove is exposed to the surface.
• Affects the development of spinal cord known as myelocele.

Closed Neural Tube Defects –

Spina bifida –

• Failure of fusion of laminae of vertebra leads to complete bifid spinous process.
• It affects one or two vertebrae in lumbo-sacral region.

Spina bifida occulta-

• Skin over the affected area presents tuft of hair.

Meningocele-

• Spinal meninges protrude through the opening of bifid spine and form a cystic swelling.
• The swelling is covered with skin.

Meningo-myelocele-

• Spinal meninges along with the spinal cord and its nerves protrude through the opening of bifid spine as a cystic swelling.
• Peripheral part of sac is covered by skin.
• Summit of the sac is covered by a thin friable vascular membrane.

Syringo-myelocele-

• It is similar to meningo-myelocele with the distended central canal of spinal cord.

Hydrocephalus –

• Excessive accumulation of CSF within the brain.

Communicating hydrocephalus-

• CSF appears in ventricular system and subarachnoid space, but cannot reach the absorption site.

Non-communicating hydrocephalus-

• CSF appears in the ventricular system but cannot reach the subarachnoid space due to obstruction.
• CSF flow may be obstructed at foramen of Monro or aqueduct of Sylvius or foramen of Luschka or foramen of Magendie.
• Due to obstruction ventricular system is enormously distended with CSF.
• Cranial vault expands and cranial bones are separated at sutures.

Microcephaly –

• Unusually small brain due to cranio-stenosis.
• Mental retardation.

Meningocele –

• Squamous part of occipital bone fails to ossify forming a gap.
• Through the gap meninges of brain with CSF bulges outwards.

Meningoencephalocele –

• Squamous part of occipital bone fails to ossify forming a gap.
• Through the gap a portion of brain along with the meninges and CSF herniate.
• If the ventricle of brain also herniates such condition is called as meningo-hydroencephalocele. 

NEURAL  CREST  CELLS

Formation of Neural Crest Cells :-

• Ectodermal cells along the tips of neural folds are called as neural crest cells.
• When the neural tube gets detached from the surface ectoderm neural crest cells get separated and lie between the surface ectoderm and neural tube.
• Neural crest cells get divided into dorsal mass and ventral mass of cells.

Derivatives of Neural Crest Cells :-

Dorsal mass derivatives –

Neuroblast cells-

• Dorsal root ganglia
• Sensory ganglia of V, VII, IX and X cranial nerves
• Skeletal elements of pharyngeal arches
• Odontoblasts of teeth
• Parafollicular cells of thyroid gland

Spongioblast cells-

• Satellite cells in ganglia
• Schwann cells

Pleuripotent cells-

• Melanocytes

Ventral mass derivatives –

Sympatho-chromaffin organ

Sympathoblasts

• Neurons of sympathetic ganglia
• Neurons of parasympathetic ganglia

Chromaffin cells

• Chromaffin cells of adrenal medulla
• Para-aortic body
• Argentaffin cells in respiratory system
• Enterochromaffin cells in gut