The central nervous system develops from the neural plate, which forms the neural tube. The neural tube undergoes primary and secondary folding and vesicles form the brain regions. The neural tube closes at specific points forming the cranial and caudal neuropores. Within the neural tube, the neuroepithelial layer gives rise to neuroblasts and glioblasts which form the gray and white matter. Neural crest cells contribute to peripheral ganglia. As development proceeds, the spinal cord undergoes positional changes relative to the lengthening vertebral column.

1. DEVELOPMENT
OF
CENTRAL NERVOUS
SYSTEM
DR. SUNDIP CHARMODE
ASSOCIATE PROFESSOR
DEPARTMENT OF ANATOMY
AIIMS RAJKOT

3. INTRODUCTION
• The central nervous system (CNS) appears at the beginning of the
third week as a slipper-shaped plate of thickened ectoderm, the
neural plate (a large placode; in the middorsal region in front of the
primitive node.
• Its lateral edges soon elevate to form the neural folds.

7. INTRODUCTION
• Fusion begins in the cervical region and proceeds in cephalic and
caudal directions (Fig. 18.3A). Once fusion is initiated, the open ends
of the neural tube form the cranial and caudal neuropores that
communicate with the overlying amniotic cavity (Fig. 18.3B).
• Closure of the cranial neuropore proceeds cranially from the initial
closure site in the cervical region (Fig. 18.3A) and from a site in the
forebrain that forms later.
• This latter site proceeds cranially, to close the rostral-most region of
the neural tube, and caudally to meet advancing closure from the
cervical site (Fig. 18.3B).

9. CLOSURE OF NEUROPORES
• Final closure of the cranial neuropore
occurs at the 18- to 20-somite stage
(25th day); closure of the caudal
neuropore occurs approximately 3
days later.
• The cephalic end of the neural tube
shows three dilations, the primary
brain vesicles: (1) the
prosencephalon, or forebrain; (2) the
mesencephalon, or midbrain; and (3)
the rhombencephalon, or hindbrain
(Fig. 18.4).

10. FORMATION OF FLEXURES
• Simultaneously, it forms two
flexures: (1) the cervical flexure at
the junction of the hindbrain and
the spinal cord and (2) the
cephalic flexure in the midbrain
region (Fig. 18.4).

11. INTRODUCTION
• By 5 weeks of development, the primary brain vesicles have
differentiated into five secondary vesicles:
• The prosencephalon forms the telencephalon and diencephalon,
• the mesencephalon remains, and
• The rhombencephalon forms the metencephalon and
myelencephalon (Fig. 18.5).
• A deep furrow, the Rhombencephalic Isthmus separates the
mesencephalon from the metencephalon, and the Pontine Flexure
marks the boundary between the metencephalon and
myelencephalon (Fig. 18.5).

13. INTRODUCTION
• Each of the secondary vesicles will contribute a different part of the brain.
• The main derivatives of these vesicles are indicated in Figure 18.5 and
include:
• Telencephalon (cerebral hemispheres),
• Diencephalon (optic vesicle, thalamus, hypothalamus, pituitary),
• Mesencephalon (anterior [Visual] and posterior [auditory] colliculi),
• Metencephalon (cerebellum, pons), and
• Myelencephalon (medulla oblongata).

15. INTRODUCTION
• The lumen of the spinal cord, the central canal, is
continuous with that of the brain vesicles.
• The cavity of the rhombencephalon is the fourth
ventricle, that of the diencephalon is the third
ventricle, and those of the cerebral hemispheres
are the lateral ventricles (Fig. 18.5).
• The lumen of the mesencephalon connects the
third and fourth ventricles.
• This lumen becomes very narrow and is then
known as the aqueduct of Sylvius.

16. INTRODUCTION
• Each lateral ventricle
communicates with the third
ventricle through the
interventricular foramina of Monro
(Fig. 18.5).

17. SPINAL CORD
Neuroepithelial, Mantle, and Marginal Layers
• The wall of a recently closed neural tube consists of Neuroepithelial
Cells.
• These cells extend over the entire thickness of the wall and form a
thick pseudostratified epithelium (Fig. 18.6).
• Functional complexes at the lumen connect them.
• During the neural groove stage and immediately after closure of the
tube, they divide rapidly, producing more and more neuroepithelial
cells.
• Collectively, they constitute the neuroepithelial layer or
neuroepithelium.

19. SPINAL CORD
• Neuroepithelial, Mantle, and Marginal Layers
• Once the neural tube closes, neuroepithelial cells begin to give rise
to another cell type characterized by a large round nucleus with pale
nucleoplasm and a dark-staining nucleolus.
• These are the Primitive Nerve Cells, or Neuroblasts (Fig. 18.7).
• They form the Mantle Layer, a zone around the neuroepithelial layer
(Fig. 18.8).

22. SPINAL CORD
• The mantle layer later forms the gray matter of the spinal cord.
• The outermost layer of the spinal cord, the Marginal Layer, contains
nerve fibers emerging from neuroblasts in the mantle layer.
• As a result of myelination of nerve fibers, this layer takes on a white
appearance and therefore is called the white matter of the spinal
cord (Fig. 18.8).

24. SPINAL CORD
• Basal, Alar, Roof, and Floor Plates
• As a result of continuous addition of neuroblasts to the mantle layer, each
side of the neural tube shows a ventral and a dorsal thickening.
• 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 (Fig. 18.8A).

26. SPINAL CORD
• 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.

27. SPINAL CORD
• 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 (Fig. 18.8B).
• This horn, containing neurons of the sympathetic portion of the
autonomic nervous system (ANS), is present only at thoracic (Tl—
T12) and upper lumbar levels (L2 or L3) of the spinal cord.

29. Histological Differentiation
Nerve Cells
• Neuroblasts, or primitive nerve cells, arise exclusively by division of
the neuroepithelial cells.
• Initially, they have a central process extending to the lumen (transient
dendrite), but when they migrate into the mantle layer, this process
disappears, and neuroblasts are temporarily round and apolar (Fig.
18.9A).
• Later, two new cytoplasmic processes appear on opposite sides of the
cell body, forming a bipolar neuroblast (Fig. 18.9B).

31. Histological Differentiation
Nerve Cells
• The process at one end of the cell elongates rapidly to form the
primitive axon, and the process at the other end shows a number of
cytoplasmic arborizations, the primitive dendrites (Fig. 18.9 C).
• The cell is then known as a multipolar neuroblast and with further
development becomes the adult nerve cell or neuron.
• Once neuroblasts form, they lose their ability to divide.
• Axons of neurons in the basal plate break through the marginal zone
and become visible on the ventral aspect of the cord.

32. Histological Differentiation
NERVE CELLS
• Known collectively as the ventral motor root of the spinal nerve, they
conduct motor impulses from the spinal cord to the muscles (Fig.
18.10).
• Axons of neurons in the dorsal sensory horn (alar plate) behave
differently from those in the ventral horn.
• They penetrate into the marginal layer of the cord, where they ascend
to either higher or lower levels to form association neurons.

34. GLIAL CELLS
• The majority of primitive supporting cells, the glia blasts, are formed by
neuroepithelial cells after production of neuroblasts ceases.
• Glia Blasts migrate from the neuroepithelial layer to the mantle and
marginal layers.
• In the mantle layer, they differentiate into protoplasmic astrocytes and
fibrillar astrocytes (Fig. 18.11).
• These cells are situated between blood vessels and neurons where they
provide support and serve metabolic functions.

36. GLIAL CELLS
• Another type of supporting cell possibly derived from glia blasts is the
Oligodendroglial Cell.
• This cell, which is found primarily in the marginal layer, forms
myelin sheaths around the ascending and descending axons in the
marginal layer.

37. GLIAL CELLS
• In the second half of development, a third type of supporting cell, the
Microglial Cell, appears in the CNS.
• This highly phagocytic cell type is derived from vascular mesenchyme
when blood vessels grow into the nervous system (Fig. 18.11).
• When neuroepithelial cells cease to produce neuroblasts and glia
blasts, they differentiate into ependymal cells lining the central canal
of the spinal cord.

38. NEURAL CREST CELLS
• During elevation of the neural plate, a group of cells appears along
each edge (the crest) of the neural folds (Fig. 18.2).
• These neural crest cells are ectodermal in origin and extend
throughout the length of the neural tube.
• Crest cells migrate laterally and give rise to sensory ganglia (dorsal
root ganglia) of the spinal nerves and other cell types (Fig. 18.2).
• During further development, neuroblasts of the sensory ganglia form
two processes (Fig. 18.10A).

39. NEURAL CREST CELLS
• The centrally growing processes penetrate the dorsal portion of the
neural tube. In the spinal cord, they either end in the dorsal horn or
ascend through the marginal layer to one of the higher brain centers.
• These processes are known collectively as the dorsal sensory root of
the spinal nerve (Fig. 18.1OB).

40. POSITIONAL CHANGES OF SPINAL CORD
• In the third month of development, the spinal cord extends the entire
length of the embryo, and spinal nerves pass through the
intervertebral foramina at their level of origin (Fig. 18.13A).
• With increasing age, however, the vertebral column and dura
lengthen more rapidly than the neural tube, and the terminal end of
the spinal cord gradually shifts to a higher level.
• At birth, this end is at the level of the third lumbar vertebra (Fig.
18.13C).

42. POSITIONAL CHANGES OF SPINAL CORD
• As a result of this disproportionate growth, dorsal and ventral roots of
spinal nerves run obliquely from their segment of origin in the spinal
cord to the corresponding level of the vertebral column where the
appropriate roots unite to form spinal nerves.
• The dura remains attached to the vertebral column at the coccygeal
level.
• In the adult, the spinal cord terminates at the level of L2—L3,
whereas the dural sac and subarachnoid space extend to S2.

43. POSITIONAL CHANGES OF SPINAL CORD
• At the end of the cord, a threadlike extension of pia mater passes
caudally, goes through the dura, which provides a covering layer at S2
and extends to the first coccygeal vertebra.
• This structure is called the filum terminale, and it marks the tract of
regression of the spinal cord as well as providing support for the cord
(the part covered by dura and extending from $2 to the coccyx is also
called the coccygeal ligament).

45. POSITIONAL CHANGES OF SPINAL CORD
• Dorsal and ventral roots of spinal nerves below the terminal end of
the cord at L2—L3 collectively constitute the cauda equina (horse’s
tail).
• When cerebrospinal fluid is tapped during a lumbar puncture, the
needle is inserted at the lower lumbar level (L4—L5), avoiding the
lower end of the cord.