The central nervous system (CNS) Spinal cord

Prof. Dr. Malak A. Al-yawer

Department Of Anatomy/ Embryology Section

Learning Objectives At the end of this lecture, the medical student will be able to

Describe how the neural tube is derived from ectoderm Define neural tube and its cranial & caudal neuroporeses Define neuroepithelial tissue and state its role in the formation of

mantle and marginal layers Compare between the embryonic development of white and gray

matter of spinal cord Compare between the embryonic development of motor and

sensory areas of spinal cord List the developmental stages of neuroblasts State the embryonic origin of different types of glial cells Define neural crest cells & mention their fate State the embryonic origin of spinal nerves State the embryonic development of myelinated nerve fibers Describe the positional changes of the spinal cord with age State some clinical correlates

The neural plate

Appears at the beginning of the 3rd week as

A slipper-shaped plate of thickened ectoderm in front of the primitive node

• Neural folds

• Neural tube

• Cranial and caudal neuropores

• Failure of the neural tube to close results in defects such as spina bifida & anencephaly defects that can be prevented by folic acid.

A. Fetus with anencephaly (absent brain) due to a lack of closure of the cranial neural folds. B. Fetus with anencephaly and craniorachischisis. The neural tube has failed to close in cranial and upper spinal cord regions

The wall of a recently closed neural tube / neuroepithelial cells extend over its entire thickness

Neuroepithelial cells begin to give rise to primitive nerve cells (neuroblasts). They form the Mantle layer, a zone around the neuroepithelial layer - later forms the gray matter of the spinal cord.

Nerve fibers emerging from neuroblasts in the mantle layer form the marginal layer -outermost layer of the spinal cord - white matter of the spinal cord

Basal and Alar Plates

• As a result of continuous addition of neuroblasts to the mantle layer, each side of the neural tube shows

1. Ventral thickenings( Basal plates ) / motor areas of the spinal cord

2. Dorsal thickenings(Alar plates)/ sensory areas

• Sulcus limitans a longitudinal groove marks the boundary between the two.

Roof and floor plates

• are the dorsal and ventral midline portions of the neural tube respectively,

• do not contain neuroblasts

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

Intermediate horn

• a group of neurons accumulates between ventral and dorsal horns

• contains neurons of the sympathetic portion of the autonomic nervous system

• present only at thoracic (T1–T12) and upper lumbar levels (L2 or L3) of the spinal cord

Various stages of development of a neuroblast.

The majority of primitive supporting cells (gliablasts) are formed by neuroepithelial cells after production of neuroblasts ceases.

Ependymal cells

• When neuroepithelial cells cease to produce neuroblasts and gliablasts, they differentiate into ependymal cells lining the central canal of the spinal cord.

Neural Crest Cells

ectodermal in origin give rise to dorsal root

ganglia of the spinal nerves

differentiate also into 1. sympathetic neuroblasts, 2. Schwann cells, 3. Pigment cells, 4. Odontoblasts, 5. Meninges 6. mesenchyme of the

pharyngeal arches

Spinal nerves

Motor nerve fibers

begin to appear in the 4th week

arising from nerve cells in the basal plates

Distal processes ( dorsal root ganglia) join the ventral nerve roots to form a Spinal nerve

Myelination

begins in approximately the 4th month of intrauterine life Some 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.

Positional Changes of the Cord

3rd month of development • spinal cord - entire length of the embryo • spinal nerves - intervertebral foramina at their level of origin With increasing age the vertebral column and dura lengthen more rapidly than

the neural tube terminal end of the spinal cord gradually shifts to a higher

level. At birth, this end is at the level of the 3rd lumbar vertebra

Positional Changes of the Cord

In the adult, the spinal cord terminates at the level of L2 to L3, whereas the dural sac and subarachnoid space extend to S2.

Filum terminale Cauda equina

Lumbar puncture/ needle is

inserted at the lower lumbar level, (L4-L5), avoiding the lower end of the cord.

Summary The CNS originates in the ectoderm and appears as the neural plate at

the middle of the 3rd week. After the edges of the plate fold, the neural folds approach each other

in the midline to fuse into the neural tube The cranial end closes at approximately day 25, and the caudal end

closes at day 28. The CNS then forms a tubular structure with a broad cephalic portion,

the brain, and a long caudal portion, the spinal cord. Failure of the neural tube to close results in defects such as spina bifida

and anencephaly defects that can be prevented by folic acid. The spinal cord, which forms the caudal end of the CNS, is

characterized by the basal plate containing the motor neurons, the alar plate for the sensory neurons, and a floor plate and a roof plate as connecting plates between the two sides

The majority of primitive supporting cells (gliablasts) are formed by neuroepithelial cells after production of neuroblasts ceases

Neural crest cells are ectodermal in origin and give rise to a number of structures

Position of spinal cord changes with age