Chapter 18 – CNS Development
1. Introduction to CNS Development
• Appearance of the CNS:
o The CNS begins forming during the third week of development as the neural plate,
a slipper-shaped thickened ectoderm located in the mid-dorsal region, just cranial to
the primitive node.
o The neural plate forms in response to signals from the notochord and prechordal
mesoderm.
• Neural Groove and Neural Folds:
o Lateral edges of the neural plate elevate to form neural folds, while the mid-region
invaginates to form the neural groove.
o As development proceeds, neural folds converge at the midline, eventually fusing to
form the neural tube.
• Neuropores:
o After fusion begins, the cranial and caudal ends remain open temporarily, forming
neuropores that communicate with the amniotic cavity.
o Cranial neuropore: Closure begins cranially at the cervical site and progresses
rostrally and caudally, closing fully by the 25th day (18–20 somite stage).
o Caudal neuropore: Closes approximately three days later (~28th day).
• Primary Vesicles:
o Following neural tube formation, the cephalic end develops three primary vesicles
(forebrain, midbrain, and hindbrain), while the caudal end forms the spinal cord.
2. Development of Brain Vesicles
• Primary Brain Vesicles:
o The cephalic end of the neural tube expands into three primary brain vesicles:
1. Prosencephalon (Forebrain):
§ The most anterior vesicle.
§ Gives rise to:
§ Telencephalon: Forms the cerebral hemispheres.
§ Diencephalon: Forms the thalamus, hypothalamus, and optic
vesicles.
2. Mesencephalon (Midbrain):
§ Forms the midbrain structures.
, § Cavity becomes the cerebral aqueduct connecting the third and
fourth ventricles.
3. Rhombencephalon (Hindbrain):
§ Divides into:
§ Metencephalon: Forms the pons and cerebellum.
§ Myelencephalon: Develops into the medulla oblongata.
• Brain Flexures:
o As the neural tube grows and elongates, flexures develop due to differential growth
rates:
1. Cephalic Flexure: Occurs in the midbrain, marking the anterior curvature.
2. Cervical Flexure: Forms between the hindbrain and spinal cord.
3. Pontine Flexure: Appears within the hindbrain during later stages of
development, dividing the myelencephalon and metencephalon.
3. Neural Tube Layers
• Histological Layers:
o The neural tube develops into three distinct layers:
1. Neuroepithelial Layer:
§ Initially forms the entire thickness of the neural tube wall.
§ Contains rapidly dividing pseudostratified neuroepithelial cells.
§ Neuroepithelial cells produce:
§ Neuroblasts: Primitive nerve cells.
§ Glioblasts: Supportive cells of the CNS.
2. Mantle Layer:
§ Located around the neuroepithelial layer.
§ Forms gray matter in the spinal cord and brain.
§ Contains migrating neuroblasts that differentiate into neurons.
3. Marginal Layer:
§ Outermost layer containing axonal processes of neuroblasts.
§ Forms white matter due to axonal myelination.
• Neuroblast Differentiation:
o Neuroblasts initially appear as apolar cells (lacking processes).
o Develop into bipolar neuroblasts, with two cytoplasmic processes on opposite sides.
o Eventually mature into multipolar neuroblasts, which form axons and dendrites.
1. Introduction to CNS Development
• Appearance of the CNS:
o The CNS begins forming during the third week of development as the neural plate,
a slipper-shaped thickened ectoderm located in the mid-dorsal region, just cranial to
the primitive node.
o The neural plate forms in response to signals from the notochord and prechordal
mesoderm.
• Neural Groove and Neural Folds:
o Lateral edges of the neural plate elevate to form neural folds, while the mid-region
invaginates to form the neural groove.
o As development proceeds, neural folds converge at the midline, eventually fusing to
form the neural tube.
• Neuropores:
o After fusion begins, the cranial and caudal ends remain open temporarily, forming
neuropores that communicate with the amniotic cavity.
o Cranial neuropore: Closure begins cranially at the cervical site and progresses
rostrally and caudally, closing fully by the 25th day (18–20 somite stage).
o Caudal neuropore: Closes approximately three days later (~28th day).
• Primary Vesicles:
o Following neural tube formation, the cephalic end develops three primary vesicles
(forebrain, midbrain, and hindbrain), while the caudal end forms the spinal cord.
2. Development of Brain Vesicles
• Primary Brain Vesicles:
o The cephalic end of the neural tube expands into three primary brain vesicles:
1. Prosencephalon (Forebrain):
§ The most anterior vesicle.
§ Gives rise to:
§ Telencephalon: Forms the cerebral hemispheres.
§ Diencephalon: Forms the thalamus, hypothalamus, and optic
vesicles.
2. Mesencephalon (Midbrain):
§ Forms the midbrain structures.
, § Cavity becomes the cerebral aqueduct connecting the third and
fourth ventricles.
3. Rhombencephalon (Hindbrain):
§ Divides into:
§ Metencephalon: Forms the pons and cerebellum.
§ Myelencephalon: Develops into the medulla oblongata.
• Brain Flexures:
o As the neural tube grows and elongates, flexures develop due to differential growth
rates:
1. Cephalic Flexure: Occurs in the midbrain, marking the anterior curvature.
2. Cervical Flexure: Forms between the hindbrain and spinal cord.
3. Pontine Flexure: Appears within the hindbrain during later stages of
development, dividing the myelencephalon and metencephalon.
3. Neural Tube Layers
• Histological Layers:
o The neural tube develops into three distinct layers:
1. Neuroepithelial Layer:
§ Initially forms the entire thickness of the neural tube wall.
§ Contains rapidly dividing pseudostratified neuroepithelial cells.
§ Neuroepithelial cells produce:
§ Neuroblasts: Primitive nerve cells.
§ Glioblasts: Supportive cells of the CNS.
2. Mantle Layer:
§ Located around the neuroepithelial layer.
§ Forms gray matter in the spinal cord and brain.
§ Contains migrating neuroblasts that differentiate into neurons.
3. Marginal Layer:
§ Outermost layer containing axonal processes of neuroblasts.
§ Forms white matter due to axonal myelination.
• Neuroblast Differentiation:
o Neuroblasts initially appear as apolar cells (lacking processes).
o Develop into bipolar neuroblasts, with two cytoplasmic processes on opposite sides.
o Eventually mature into multipolar neuroblasts, which form axons and dendrites.
