Development of the spinal cord

• The spinal cord develops from the caudal 2/3 of the neural tube

Development of the Spinal Cord

The cells of the neural tube form three layers:

An inner ventricular zone of undifferentiated cells

A middle mantle zone of cell bodies of neurons (future grey matter)

An outer marginal zone of nerve fibers or axons of neurons (future white matter)

The wall of the neural tube consists of neuroepithelial cells.The neuroepithelial cells begin to give rise to neuroblasts. They form the mantle layer.

Mantle Layer of Spinal Cord

Neurons of mantle layer (future grey matter) differentiate into:1. A dorsal alar plate (future dorsal horn): containing sensory neurons2. A ventral basal plate (future ventral horn): containing motor neurons The 2 areas are separated by a longitudinal groove (sulcus limitans).

Proliferation and bulging of both alar & basal plates cause: Formation of longitudinal dorsal & ventral median septa Narrowing of the lumen to form a small central canal

Central canal

Dorsal median septum

Ventral median fissure

BASAL, ALAR, ROOF, AND FLOOR PLATES

• The ventral thickenings, the basal plates, which contain ventral motor horn cells, form the motor areas of the spinal cord;

• The dorsal thickenings, the alar plates, form the sensory areas.

• A longitudinal groove, the sulcus limitans, marks the boundary between the two.

The dorsal and ventral midline portions of the neural tube, known as the roof and floor plates, respectively, do not contain neuroblasts;

• They serve primarily as pathways for nerve fibers crossing from one side to the other.

• In addition to the ventral motor horn and the dorsal sensory horn, a group of neurons accumulates between the two areas and forms a small intermediate horn.

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This horn, containing neurons of the Sympathetic portion of the autonomic nervous system, is present only at thoracic (T1–T12) and upper lumbar levels (L2 or L3) of the spinal cord.

Marginal Layer of Spinal cord

Marginal layer (future white matter) increases in size due to addition of ascending, descending & intersegmental nerve fibers.

Marginal layer is divided into: dorsal, lateral and ventral funiculi Myelination of nerve fibers starts at 4th month & continues during

the 1st postnatal year. Motor fibers myelinate before sensory fibers.

Dorsalfuniculus

Lateral funiculus

Ventral funiculus

Meninges

These are 3 membranes covering the neural tube:

Outer thick dura mater: mesodermal in origin

Middle arachnoid mater: ectodermal in origin

Inner thin pia mater: ectodermal in origin

A cavity appears between the arachnoid & the pia mater (subarachnoid space) & becomes filled with cerebrospinal fluid (CSF).

Positional Changes of Spinal Cord Initially, the

spinal cord occupies the whole length of the vertebral canal.

As a result a faster growth of vertebral column, the caudal end of spinal cord (conus medullaris) shifts gradually to a higher level.

Spina Bifida

Cause: Failure of fusion of the halves of vertebral arches of one or more vertebrae

Incidence: 0.04-0.15% Sex: more frequent in

females Types:

1. Spina bifida occulta (20%)

2. Spina bifida cystica (80%)

1)Spina bifida occulta: is a defect in the vertebral arches that is covered by skin

2) Spina bifida cystica: is a severe neural tube defect in which neural tissue and/or meninges protrude through a defect in the vertebral arches and skin to form a cyst like sac.

1) Spina Bifida Occulta• It is the least severe variation.• It is a defect in the vertebral

arches that is covered by skin and usually does not involve underlying neural tissue.

• It occurs in the lumbosacral region (L4 to S1) and the only evidence of its presence may be a small dimple with a tuft of hair arising from it

• Cause: • lack of fusion of the vertebral

arches, affects about 10% of normal people.

2) Spina bifida cystica Most lie in the lumbosacral

region and result in neurological deficits, but they are usually not associated with mental retardation.

A-Spina bifida with meningocele: The meninges project

through the defect in the vertebral arches, forming a cystic swelling beneath the skin and containing cerebrospinal fluid.

The spinal cord and nerves usually are normal.

b) Spina bifida with meningomyelocele: The normal spinal cord, or cauda equina, lies within the meningeal

sac, which projects through the vertebral arch defect. The spinal cord or nerve roots are adherent to the inner wall of the sac.

c) Spina bifida with myeloschisis or rachischisis:• This is the most severe type of spina bifida. In these cases, the

spinal cord in the affected area is open because the neural folds failed to fuse. As a result, the spinal cord is represented by a flattened mass of nervous tissue& devoid of skin

• Myelocele: An oval raw area is found on the surface; this represents the neural groove whose lips are fused. The central canal discharges clear

cerebrospinal fluid onto the surface.

Syringomyelocele: This condition is rare. A meningomyelocele is present, and in addition, the central canal of the spinal cord at the level of the bony defect is grossly dilated

Spinal Nerves• Motor nerve fibers begin to

appear in the fourth week, arising from nerve cells in the basal plates (ventral horns) of the spinal cord.

• These fibers collect into bundles known as ventral nerve roots.

• Dorsal nerve roots form as collections of fibers originating from cells in dorsal root ganglia (spinal ganglia).

Central processes from these ganglia form bundles that grow into the spinal cord opposite the dorsal horns. Distal processes join the ventral nerve roots to form a spinal nerve.

Almost immediately, spinal nerves divide into dorsal and ventral primary rami. Dorsal primary rami innervate dorsal axial musculature, vertebral joints, and the skin of the back. Ventral primary rami innervate the limbs and ventral body wall and form the major nerve plexuses (brachial and lumbosacral).

Neural Tube Components

Myelination• Schwann cells myelinate the peripheral nerves.• These cells originate from neural crest, migrate peripherally, and wrap

themselves around axons, forming the neurilemma sheath. • Beginning at the fourth month of fetal life, many nerve fibers take on a whitish

appearance as a result of deposition of myelin, which is formed by repeated coiling of the Schwann cell membrane around the axon.

• The myelin sheath surrounding nerve fibers in the spinal cord has a completely different origin, the oligodendroglial cells.

• Some of the motor fibers descending from higher brain centers to the spinal cord do not become myelinated until the first year of postnatal life. Tracts in the nervous system become myelinated at about the time they start to function.

A. Motor horn cell with naked rootlet. B. In the spinal cord oligodendrogliacells surround the ventral rootlet; outside the spinal cord, Schwann cells beginto surround the rootlet. C. In the spinal cord the myelin sheath is formed by oligodendroglia cells; outside the spinal cord the sheath is formed by Schwann cells.

SummaryMyelination- commences in the fourth fetal month in the spinal cord

motor roots.1. Oligodendrocytes accomplish myelination of the CNS.2. Schwann cells accomplish myelination of the PNS.3. Myelination of the corticospinal tracts is not

completed until the end of the second postnatal year (i.e., when the corticospinal tracts become myelinated and functional).

4. Myelination of the association neocortex extends into the third decade.