Neural Tube: Internal Organization

Neural tube: Internal Organization

Overview

Within the spinal cord gray matter, the alar plate forms the dorsal (sensory) horns and the basal plate forms the ventral (motor) horns.
Nerve roots that emanate from these gray matter horns maintain this pattern: the dorsal nerve root carries sensory input and the ventral nerve root carries motor output.

overview of the internal organization of the neural tube (coronal section)

From top to bottom, indicate:

The ectoderm and mesoderm.

From lateral to medial, indicate:

The somite derivatives:

Dermatome (which differentiates into axial dermis),
Myotome (which differentiates into paraspinal and abdominal musculature)
Sclerotome (which differentiates into spine and posterior, basal skull).

Then, outline the neural tube.
Leave one side blank, so we can learn the neural tube's three cytoarchitectural zones in the order in which they develop, embryologically.

Cytoarchitectural zones

First, let's establish the zones in their order of development:

Ventricular zone, which produces the neuroepithelial cells that will populate the next two zones:
The intermediate (aka mantle) zone and
The marginal zone.

We'll see that after those zones are populated, the ventricular zone will populate with ependymal cells.

Now, let's learn the cell-types that form these zones.

Indicate that the intermediate zone forms the gray matter layer when the first wave of cells from the ventricular zone, the neurons, migrate peripherally to populate the mantle zone.

Emphasize the "n" in mantle to help remember neuron.

Then, indicate that the marginal zone forms the white matter layer when the ventricular zone generates glioblasts, which differentiate into glial cells (eg, oligodendrocytes and astrocytes). The oligodendrocytes give the layer its whitish color (from the fatty myelin sheaths).

We can remember this layer is the next to form because these cells wrap around the axons, which sprout from the neurons of the mantle zone.
Finally, emphasize the "g" in the marginal to remember "glioblast".

Finally, show that the ventricular zone, which is the first to form, is ultimately is populated with the last cell type it produces: the ependymal cells. They line the ventricular system and central canal.

Myelin

Before we move on, let's remind ourselves that whereas in the central nervous system, oligodendrocytes produce myelin, in the peripheral nervous system, the Schwann cells do.

The oligodendrocytes are formed from neural tube derived glioblasts whereas the Schwann cells are neural crest cell derived.
Remember that the physiological significance of the myelin sheath is that it ensures fast speeds of transmission along axons via a process called saltatory conduction.
Clinical Correlation: multiple sclerosis

The Internal Organization of the Neural Tube

The sulcus limitans divides the neural tube into dorsal and ventral regions.
Within the dorsal (aka posterior) region, lies the alar plate, which comprises the sensory neuronal populations of the neural tube.
Within the ventral (aka anterior) region, lies the basal plate, which comprises the motor neuronal populations of the neural tube.

The peripheral extension of the dorsal/ventral – sensory/motor divisions

Dorsal nerver root ganglion

The dorsal nerve root ganglion is a sensory ganglion (a cluster of pseudounipolar neurons) [it grows centrally into the alar plate].

Ventral nerve root

The ventral nerve root is a motor root that emanates from the basal plate.

development of the neural tube and its folding

We see a coronal section through the germ disc.

The notochord induces the overlying ectoderm to differentiate into the neural plate.
Sonic hedgehog (Shh) is the primary mediator to signal this induction.
When the neural plate has begins its folding, the neural groove forms the base of the developing neural tube and the neural crests comprise the neural fold tips.
With further differentiation of the neural tube, it descends into the mesodermal layer and is separating from the ectoderm.
The neural crests abut along the dorsal surface of the neural tube and migrate into the mesoderm: we learn the neural crest cell migration and differentiation elsewhere.
The neural crests produce biological mediators: bone morphogenetic proteins (BMPs) and Wnt, which signal the dorsal neural tube to form the roof plate.
The roof plate then uses these same mediators to form the bilateral alar plates.
The notochord signals the base of the neural tube to form the floor plate (again, via sonic hedgehog), and the floor plate uses Shh to form the bilateral basal plates.

Alar and basal plates induction of the formation of the peripheral nervous system

Ventrally, show that motor neurons from the basal plate produce ventral nerve roots which extend out peripherally. The mixed spinal nerves innervate myotomal muscle masses.
Dorsally, lies the dorsal root ganglia, which form from neural crest cells. Eventually, these ganglia connect centrally with the alar plate and extend peripherally to help form the mixed spinal nerve.
Neural crest cells migrate peripherally to form the sympathetic chain, a key component of the peripheral autonomic nervous system, which connects to the thoracolumbar spinal neurons.

Spinal Cord & PNS

See: Spinal Cord Cross-Section

The ventral horn derives from the basal plate and produces motor innervation.
The dorsal horn derives from the alar plate and receives sensory innervation.
The intermediate zone (aka intermediate gray), which contains both autonomic and cerebellar pathway neurons; here, we focus on the intermediolateral cell column (the autonomic pathway neurons).

As a tricky nomenclature point consider that we also referred to the mantle zone (the gray matter) cytoarchitectural zone as the "intermediate zone" is also used in neural tube development.*

The ventral root carries motor fibers.
The dorsal root ganglion houses the primary sensory neuron, is pseudounipolar –it sends fibers centrally toward the dorsal horn of the spinal cord via the dorsal nerve root, and also peripherally to merge with the ventral root to form the mixed spinal nerve.
The mixed spinal nerve divides into the:

Anterior ramus (ventral ramus), which innervates the limbs and anterior torso.
Posterior ramus (dorsal ramus), which innervates the back.

Remember: rami are not the roots!

The sympathetic ganglion, a neural crest cell derivative, lies along the anterior ramus. It attaches to the anterior ramus via the:

Gray ramus communicans, proximally, so-named because it is unmyelinated.
White ramus communicans, distally, so-named because it is myelinated.

The fiber pathways

The ventral horn produces motor, output fibers that exit via the ventral root, merge with the mixed spinal nerve and innervate the periphery via the anterior and posterior rami.
The sensory, input fibers enter from the periphery (their cell body is housed in the dorsal root ganglion and they innervate the dorsal horn of the spinal cord. Again, the dorsal root ganglion houses pseudounipolar neurons.
We show a sympathetic motor fiber emerge from the intermediolateral cell column, pass down the ventral root, along the anterior ramus, enter the sympathetic chain via the white ramus, then down the chain a couple of representative segments to synapse in an inferior sympathetic ganglion.
We show the post-synaptic fiber sympathetic pass ascend and exit to innervate an organ in the periphery (for instance the heart, lungs, or gut).

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