Ectodermal Derivatives II

Histogenesis of the Nervous System:
The Spinal Cord and Spinal Nerves

This page details the development of both the spinal cord and the spinal nerves.

The Spinal Cord

The spinal cord (as with the brain) develops from the neural tube. The early neural tube is composed of a single layer of pseudostratified columnar epithelium (use your browser's "Back" button to return). As the neural tube develops, this single layer differentiates to from three distinct layers which make up the mature spinal cord. Proliferation and differentiation of the neuroepithelial cells (the initial pseudostratified columnar cells) causes a second layer of cells to form around the outside of the initial columnar cells. This second layer of cells consists of neurons and neuroglial cells and is called the mantle or intermediate layer. It is this layer, rich in cell bodies, that makes up the grey matter of the spinal cord. The neurons in the mantle layer then send out axonal processes, which comprise the third layer, called the marginal layer. Since the axonal processes are myelinated by neuroglial cells called oligodendrocytes in the adult, the axonal processes appear white in gross anatomical section (myelin is largely composed of fat). As a result, this layer containing many axonal processes is called the white matter of the spinal cord. Note that the initial layer of cells has remained epithelial in nature, lining the neurocoele, and is now called the ependymal layer. The neurocoele will eventually regress to the small central canal of the adult spinal cord. To get a feel for this, please see an animation of spinal cord development. After all is said and done, the three layers which form the spinal cord are (from inside to out): the ependymal layer, the mantle (intermediate) layer and the marginal layer.

The Spinal Cord
Embryonic layers Adult layers

Ependymal Ependymal (epithelium)

Mantle Grey matter (mainly cell bodies)

Marginal White matter (mainly axons)

The Spinal Cord
Embryonic layers	Adult layers
Ependymal	Ependymal (epithelium)
Mantle	Grey matter (mainly cell bodies)
Marginal	White matter (mainly axons)

Spinal Cord of the Frog
Diagrammatic frog spinal cord Poor Frogger's spine

These pictures show the spinal cord from a young adult frog. The central canal, sitting in the centre of the cord lined by ependymal cells, is the adult remnant of the neurocoele. The grey matter is the H-shaped area surrounding the central canal. It, in turn, is surrounded by the white matter. Recall that the names, "grey" and "white" refer to the tissue's appearance when fresh. In this slide, the tissue has been stained, resulting in the white and grey matter actually being shades of red and orange! The grey matter is composed of nerve cell bodies and their dendrites, the supporting neuroglial cells and unmyelinated axons. The white matter is composed primarily of bundles of myelinated axons. Since the bundles (or tracts) of axons of the white matter run in vertical lines (in the human, anyway - animals are horizontal!), they are often referred to as white matter columns. The ventral white matter columns generally carry motor impulses down from the brain, whereas the dorsal columns generally carry sensory information to the brain. The lateral columns do both. Note that the grey matter is divided into dorsal and ventral "horns" (the ends of the H). The dorsal horns contains sensory neurons which carry sensory information from the body, whereas the ventral horns contain large motor neurons which relay motor signals to the body.
The commissures (from the Latin, commissura, meaning, "a joining together") are where the two halves of the spinal cord connect to one another. The grey matter has a dorsal and ventral commissure, whereas the white matter has only a ventral commissure. It is through these that a nerve impulse can cross to the other side of the body.

Spinal Cord of the Frog

These pictures show the spinal cord from a young adult frog. The central canal, sitting in the centre of the cord lined by ependymal cells, is the adult remnant of the neurocoele. The grey matter is the H-shaped area surrounding the central canal. It, in turn, is surrounded by the white matter. Recall that the names, "grey" and "white" refer to the tissue's appearance when fresh. In this slide, the tissue has been stained, resulting in the white and grey matter actually being shades of red and orange! The grey matter is composed of nerve cell bodies and their dendrites, the supporting neuroglial cells and unmyelinated axons. The white matter is composed primarily of bundles of myelinated axons. Since the bundles (or tracts) of axons of the white matter run in vertical lines (in the human, anyway - animals are horizontal!), they are often referred to as white matter columns. The ventral white matter columns generally carry motor impulses down from the brain, whereas the dorsal columns generally carry sensory information to the brain. The lateral columns do both. Note that the grey matter is divided into dorsal and ventral "horns" (the ends of the H). The dorsal horns contains sensory neurons which carry sensory information from the body, whereas the ventral horns contain large motor neurons which relay motor signals to the body. The commissures (from the Latin, commissura, meaning, "a joining together") are where the two halves of the spinal cord connect to one another. The grey matter has a dorsal and ventral commissure, whereas the white matter has only a ventral commissure. It is through these that a nerve impulse can cross to the other side of the body.

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Spinal Cord of the Cat
Diagrammatic cat hairball...er...spinal cord Cross cut cat cord

These images show the spinal cord of the cat. Although some differences are visible, note how similar this is to the spinal cord of the frog. The "H" of grey matter with its dorsal and ventral horns is still present around the central canal, surrounded by tracts of white matter. As well, the dorsal and ventral septae and commissures are seen here, just as they were in the frog spinal cord. This particular preparation looks somewhat different in colour than the frog spinal cord due to the staining used.

Spinal Cord of the Cat

These images show the spinal cord of the cat. Although some differences are visible, note how similar this is to the spinal cord of the frog. The "H" of grey matter with its dorsal and ventral horns is still present around the central canal, surrounded by tracts of white matter. As well, the dorsal and ventral septae and commissures are seen here, just as they were in the frog spinal cord. This particular preparation looks somewhat different in colour than the frog spinal cord due to the staining used.

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The Spinal Nerves

The spinal nerves are a regular series of paired nerves in the peripheral nervous system (PNS) which arise from the spinal cord (a part of the central nervous system (CNS)) and exit the spinal column between each adjacent vertebrae. Each nerve is made up of sensory fibres carrying impulses to the spinal cord from the periphery and motor fibres carrying motor impulses from the spinal cord to the periphery. At the spinal cord, these different fibres split and enter/exit the spinal cord through different routes. The sensory fibres enter into the dorsal horn of the spinal cord grey matter via the dorsal root, where they synapse with CNS neurons. The motor fibres exit the ventral horn of the spinal cord grey matter via the ventral root. The voluntary (somatic) motor nerves have cell bodies within the ventral horn of grey matter and extend long axons to their target organs. The motor nerves of the sympathetic division of the autonomic nervous system may or may not have cell bodies in the ventral grey matter, but synapse in a ganglion (a bundle of nerve cell bodies in the PNS) just outside the spinal column. As a result, the sympathetic fibres leaving the spinal cord are called "pre-ganglionic" fibres. After having synapsed in the sympathetic ganglion, the fibres continuing on to the periphery are called "post-ganglionic" fibres.

The spinal nerves and their supporting cells, as with all of the PNS, are derived from the CNS, ectodermal placodes and, especially, neural crest cells.

Schematic spinal column

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The Ventral and Dorsal Roots and the Dorsal Root Ganglion
10 mm pig embryo spinal cord This shows the spinal cord of the 10 mm pig embryo. The three layers of the developing spinal cord can be appreciated here. Note the developing dorsal root ganglion. The dorsal root ganglion is where the sensory nerve cell bodies are located for sensory nerves entering the spinal cord from the periphery. Sensory neurons are a special classification of neuron, which have essentially only one long axon, with the neuronal body sitting along side the axon, between the two ends. The ventral root (motor) fibres do not have a similar ganglion. Don't confuse the dorsal root ganglion, which is within the spinal column, immediately beside the spinal cord and only has sensory fibres passing through it, with the sympathetic ganglion, which is located in the periphery, and has only sympathetic motor fibres passing through it (this ganglion is found after the ventral and dorsal spinal nerve roots have combined - piece of cake, huh?!).

Mammalian spinal cord and dorsal root ganglion This image is a cross section through the adult mammalian spinal cord. All the structures of the spinal cord are well visualized, as are the ventral and dorsal roots and the dorsal root ganglion. Be aware that the dorsal root ganglion is found on both sides of the spine (they are paired, just like the spinal nerves are paired), it is just this series of sections that shows the dorsal root ganglion only on the left side.

The Ventral and Dorsal Roots and the Dorsal Root Ganglion
This shows the spinal cord of the 10 mm pig embryo. The three layers of the developing spinal cord can be appreciated here. Note the developing dorsal root ganglion. The dorsal root ganglion is where the sensory nerve cell bodies are located for sensory nerves entering the spinal cord from the periphery. Sensory neurons are a special classification of neuron, which have essentially only one long axon, with the neuronal body sitting along side the axon, between the two ends. The ventral root (motor) fibres do not have a similar ganglion. Don't confuse the dorsal root ganglion, which is within the spinal column, immediately beside the spinal cord and only has sensory fibres passing through it, with the sympathetic ganglion, which is located in the periphery, and has only sympathetic motor fibres passing through it (this ganglion is found after the ventral and dorsal spinal nerve roots have combined - piece of cake, huh?!).

This image is a cross section through the adult mammalian spinal cord. All the structures of the spinal cord are well visualized, as are the ventral and dorsal roots and the dorsal root ganglion. Be aware that the dorsal root ganglion is found on both sides of the spine (they are paired, just like the spinal nerves are paired), it is just this series of sections that shows the dorsal root ganglion only on the left side.

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The Spinal Column

Spinal column - frog

Spinal column - close encounters of the 4th (ventricle) kind

These two images are of the spinal column of a young frog. The spinal column (or vertebral column) consists of the spinal cord, dorsal and ventral spinal nerve roots, surrounding membranes and the vertebrae themselves. The sections seen here are actually catching the caudal end of the hindbrain (the medulla oblongata here) within the spinal column. Note that the central canal is not formed here, but rather there is an ependymal cell lined fissure in the dorsal (top) portion of the medulla. This is the 4^th ventricle. Further caudal, the dorsal aspect of the spinal cord will fuse to form the central canal. The thin membranes surrounding the spinal cord are the meninges. They are comprised of connective tissue of mesenchymal origin and cover and protect the nervous components of the CNS. The meninges are composed of three layers. The outer layer is the tough dura mater ("tough mother" in Latin - I'm serious!), which is protective and has a mesodermal origin. The middle arachnoid layer is highly vascular, giving it a spider's web look when fresh (arachnid means, "spider"). The inner layer is pia mater ("delicate mother" in Latin), which is very thin and thought to be of neural crest origin. Between these layers, there is a continuous flow of cerebral spinal fluid, which bathes the spinal cord. The bone seen surrounding the spinal cord and meninges is the bone of the vertebrae. The basal portion of the vertebrae is called the vertebral centrum, while the upper sides are called the vertebral arches. There is also some cartilage still present in these sections. Finally, the muscles dorsal and lateral to the spinal column are the large skeletal (voluntary) muscles of the back.

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Animation of Spinal Cord Development

Proceed to Histogenesis of the Nervous System: The Brain

Back to the Histology II Index

Special thanks to Hans Christoffersen for the preparation of this page.

Send comments to:
Sandra K. Ackerley (ackerley@uoguelph.ca),
Department of Zoology, University of Guelph, Guelph, Ontario, Canada N1G 2W1.

Histogenesis of the Nervous System: The Spinal Cord and Spinal Nerves

The Spinal Cord

The Spinal Nerves

Histogenesis of the Nervous System:
The Spinal Cord and Spinal Nerves