Development of the Spinal Cord

Question 1

What parts of the nervous system originate from the
a) Neural tube?
b) Neural crest?
c) Ectodermal placodes?

Answer 1

a) Central nervous system, somatic motor neurons, preganglionic autonomic neurons
b) Somatic sensory nerves, somatic and autonomic ganglia
c) Cranial sensory neurons, olfactory epithelia, epithelia of the inner ear

Question 2

State the neural crest derivatives.

Answer 2

• peripheral nervous system
• endocrine system
• integument system
• cardiovascular system
• craniofacial region

Question 3

State the site of development of the spinal cord.

Answer 3

Neural tube caudal to the fourth pair of somites

Question 4

State the source of development of the spinal cord.

Answer 4

Neuroectoderm

Question 5

State the sequence of development of the spinal cord.

Answer 5

• the lateral walls of the neural tube thicken, gradually reducing the size of the neural canal until only a minute central canal of the spinal cord is present at 9 to 10 weeks
• initially the wall of the neural tube is composed of a thick, pseudostratified columnar neuroepithelium
• these neuroepithelial cells constitute the ventricular zone (ependymal layer), which give rise to all neurons and macroglia in the spinal cord
• a marginal zone develops which will become the future white matter as axons grow into it
• dividing cells in the ventricular zone differentiate into glioblasts (spongioblasts) [mainly after neuroblast formation has ceased]
• the glioblasts migrate from the ventricular zone to the mantle layer, which is the gray matter
  (NB: Some glioblasts become astroblasts and later astrocytes, whereas others become oligodendroblasts and eventually oligodendrocytes)
• once neuroepithelial cells cease producing neuroblasts and glioblasts, they differentiate into ependymal cells, forming the central canal
• floor plate and roof plate will form
• enlargement of the basal plates ventrally leads to the ventral median septum and fissure developing
• the neurons in the mantle layer will extend their axons into the marginal layer
• myelination occurs during the fetal period into the first year of life
• the mantle layer differentiates into alar plate (dorsal) and basal plate (ventral)
• the alar plate will represent the cell bodies of sensory neurons, which are the dorsal grey horns
• the basal plate will represent the cell bodies of motor neurons, which are the ventral grey horns, and lateral grey horns
• they will be separated by sulcus limitans

Question 6

State the 3 zones of the neural tube.

Answer 6

• ventricular zone/ependymal layer/matrix cell layer
• mantle layer/intermediate zone
• marginal zone
• Click here for a diagram.

Question 7

Microglia are derived from ________________.

Answer 7

mesenchyme

Question 8

Histiogenesis
Neuroblasts initially develop a ____(a)____ dendrite. When they migrate into the mantle layer, it disappears and hence they become ____(b)____ [Hint: Classification of neurons according to number of processes]. New cytoplasmic processes soon form on opposite sides of the cell body, leading to a ____(c)____ neuroblast. Then primitive axon shows cytoplasmic ____(d)____, the primitive dendrites. Multipolar neuroblasts further develop into ____(e)____.

Answer 8

(a) transient
(b) apolar
(c) bipolar
(d) arborisations
(e) neurons

Question 9

Describe the development of the spinal ganglia.

Answer 9

• the unipolar neurons in the spinal ganglia (dorsal root ganglia) are derived from neural crest cells
• the peripheral processes of spinal ganglion cells pass in the spinal nerves to sensory endings in somatic or visceral structures
• the central processes enter the spinal cord and constitute the dorsal roots of spinal nerves

Question 10

Describe the development of the spinal meninges.

Answer 10

• the mesenchyme surrounding the neural tube condenses to form a membrane called the primordial meninx
• the external layer of this membrane thickens to form the dura mater
• the internal layer, the pia-arachnoid, composed of pia mater and arachnoid mater (leptomeninges), is derived from neural crest cells
• cerebrospinal fluid begins to form during the 5th week

Question 11

What happens in the 3rd, 5th and 9th months during maturation of the spinal cord?

Answer 11

3rd month: long intersegmental fibres form
5th month: myelination begins, continues past puberty
9th month: corticospinal fibres

Question 12

Discuss myelination of nerve fibers.

Answer 12

• myelin sheaths surrounding nerve fibers within the spinal cord begin to form during the late fetal period and continue to form during the first postnatal year
• in general, fiber tracts become myelinated at approximately the time they become functional
• motor roots are myelinated before sensory roots
• the myelin sheaths within the spinal cord are formed by oligodendrocytes
• the plasma membranes of these cells wrap around the axon, forming a number of layers
• the myelin sheaths around the axons of peripheral nerve fibers are formed by the plasma membranes of neurolemma cells (Schwann cells), which are analogous to oligodendrocytes