Nervous System Development II: Spinal Cord

Question 1

From which parts of the Embryo does the Spinal Cord develop from?

Answer 1

The Spinal Cord develops from two parts.

1) The Neural Tube (caudal to the 4th pair of somites.)
2) Caudal Eminence.

Question 2

How does the Central Canal develop?

Answer 2

Due to the thickening of the Lateral walls of the Neural tube we get a narrowing of the Neural groove to form the Central Canal.

Question 3

Explain how the layers develop and what they will lead to the formation of in the Spinal Cord.

Answer 3

• Initially the wall of the Neural Tube is composed of Pseudostratified Columnar Neuroepithelium.
• This Neuroepithelium eventually begins to form Neuroblasts.
• These Neuroblasts will lead to the formation of the “Mantle Layer”.
• The Mantle Layer will form the Gray Matter of the Spinal Cord.
• The Outermost layer of the wall will form the “Marginal Layer”. This contains the Nerve Fibres emerging from the Gray Matter.
• So the Marginal Layer will form the White Matter of the Spinal Cord.

Question 4

Explain the Formation of the Basal and Alar Plates.

Answer 4

-Due to the continuous addition of Neuroblasts to Mantle Layer we see a Ventral and Dorsal thickening.

• The Ventral Thickening = Basal Plate
• The Dorsal Thickening = Alar Plate.

-We soon see a Longitudinal groove form between the Basal and Alar plate. This groove is called “Sulcus Limitans”.

Question 5

Explain the Histological Differentiation of Neuroblasts to Neurons.

Answer 5

• Initiallty Neuroblasts have a Central Process in the Lumen.
• When the Neuroblasts migrate to Mantle Layer, they lose this central process and become “Apolar”.
• Then they develop two processes and become Bipolar Neuroblast.
• Then one of these Processes becomes a Primitive Axon the other becomes a Primitive Dendrite. At this stage we call it a Multipolar Neuroblast.
• With time this becomes an Adult Neuron.

Question 6

What happens with the Axons of Neurons found in The Basal Plate and in the Alar Plate.

Answer 6

• The Axons of Neurons in the Basal Plate will penetrate through the Marginal Layer and become visible in the ventral aspect of the Spinal cord.
• Thus they form the “Ventral Motor Root of the Spinal Nerve”.
• Whereas the Axons of Neurons in the Alar Plate will penetrate up or down in the Marginal Layer and form”Association Neurons”.

Question 7

Explain the Histological Differentiation of Glial Cells.

Answer 7

• Glial Cells are the main supporting cells in the CNS.
• They are formed from the differentiation of Neuroepithelial cells (but after Neuroblast formation has stopped).
• Glial Cells in the Mantle Layer will form - “Astrocytes”.
• Glial Cells in the Marginal Layer will form “Oligodendroglial cells”.
• Oligodendroglial cells will form Myelin sheath for Spinal Nerves.

Question 8

Where do Microglial Cells develop from?

Answer 8

Microglial cells develop from Vascular Mesenchyme.

Question 9

What happens after Neuroepithelial Cells stop differentiating into Neuroblasts and Glial Cells?

Answer 9

The Neuroepithlial cells will become Ependymal cells.

These line the Central Canal.

Question 10

Explain the Histological Differentiation of Neural Crest cells into Dorsal Sensory Roots?

Answer 10

• Neural Crest cells are of Ectodermal origin.
• They migrate laterally from the Neural Tube and form “Sensory Ganglia”.
• The Neuroblasts in the Sensory Ganglia will develop two processes.
• The Central Process will penetrate the Dorsal portion of the Neural Tube and form the “Dorsal Sensory Root”.
• The Peripheral Process of the Neuroblasts (of the Sensory Ganglia) will join up with the Ventral Motor Roots to form “Trunk of Spinal Nerve”

Question 11

Explain the differentiation in the process of Myelination of Nerves in Peripheral Nerves and in Nerves in the Spinal Cord.

Answer 11

Peripheral Nerves:

-Schwaan cells derived from Neural Crest Cells migrate peripherally and wrap around axons to form “Neurolamma”.

Nerves in the Spinal Cord:

-The Myelin Sheath comes from Oligodendroglial cells (which themselves have Glial Origin).

Question 12

Explain the Positional Changes of the Spinal Cord.

Answer 12

• Initially the Spinal Cord lies perfectly in the Vertebral canal with the Spinal Nerves exiting through the opposite Intervertbral foramina on its level.
• However due to Dura Matter and Vertebral Canal growing at a faster rate than the Spinal Cord this layout changes.
• For a 24 Week old Fetus: Caudal end of Spinal cord is at S1 level.
• For a Neonate: Caudal end is at L2/L3 Level.
• For an Adult: Caudal end is at inferior border of L1.

Question 13

Explain the origins of the Meninges?

Answer 13

The origin of the Meninges is from 2 sources:

1) Neural Crest cells
2) Mesenchyme surrounding the Neural Tube.

Question 14

Explain the formation of the layers of the Meniges.

Answer 14

The outer layer of the Primordial Meninges (formed from Neural Crest cells and surrounding Mesenchyme) will form the “Dura Matter”.

The Inner layer is the LeptoMeninges. What we see happen is Fluid Filled Spaces forming in this Leptomeninges which end up joining together to eventually form the “Sub-Arachnoid Space”.

Proof that the Pia and Arachnoid matter comes from the same sheet is shown by the Trabecules which is found in the Sub Arachnoid Space.