2. College
of
Medicine
–
الطب
كلية
The nervous system develops from the
neural plate
It is a thickened, slipper shaped area of
embryonic ectoderm
The neural tube differentiates into the CNS
The neural crest gives rise to cells that form
most of the PNS and ANS, consisting of
cranial, spinal and autonomic ganglia
4. College
of
Medicine
–
الطب
كلية
Neurulation begins during the early part of
the fourth week
The cranial (upper) two thirds of the neural
plate and tube represent the future brain
The caudal (lower) one third of neural plate
and tube represents the future spinal cord
7. College
of
Medicine
–
الطب
كلية
These neuroepithelial cells give rise to all
neurons and macroglial cells in the spinal
cord
Soon a marginal zone composed of the
outer parts of the neuroepithelial cells
becomes recognizable
This zone becomes the white matter of the
spinal cord
9. College
of
Medicine
–
الطب
كلية
The primordial supporting cells of the central
nervous system, glioblasts or spongioblast
differentiate from neuroepithelial cells
This occurs after neuroblast formation has ceased
Some glioblasts become astroblasts and later
astrocytes
Other cells become oligodendroblasts and later
oligodendrocytes
11. College
of
Medicine
–
الطب
كلية
Proliferation and differentiation of
neuroepithelial cells in the developing spinal
cord produce thick walls but thin roof and
floor plates
Differential thickening of the lateral walls of
the spinal cord soon produces a shallow
longitudinal groove on each side called sulcus
limitans
13. College
of
Medicine
–
الطب
كلية
Sulcus limitans separates the dorsal part the alar
plate from the ventral part, the basal plate
The alar and basal plates produce longitudinal
bulges extending through most of the length of
developing spinal cord
The alar and basal plates are later associated with
afferent and efferent functions respectively
14. College
of
Medicine
–
الطب
كلية
Cell bodies in the alar plates form the
dorsal gray columns that extend the length
of the spinal cord
In transverse sections of the cord, these
columns are the dorsal gray horns
Neurons in these columns constitute
afferent nuclei
Groups of these nuclei form the dorsal gray
column
16. College
of
Medicine
–
الطب
كلية
As the basal plates enlarge, they bulge
ventrally on each side of the median plane
As this occurs, the ventral median septum
forms and a deep longitudinal groove, the
ventral median fissure develops on the
ventral surface of the spinal cord
17. College
of
Medicine
–
الطب
كلية
Myelin sheath in the spinal cord begin to
form during the late fetal period
Is continue to form during the first
postnatal year
The myelin sheaths surrounding nerve
fibers within the spinal cord are formed by
oligodendrocytes
19. College
of
Medicine
–
الطب
كلية
The plasma membrane of oligodendrocytes
wrap around the axon
The myelin sheaths around the axons of
peripheral nerve fibers are formed by the
plasma membranes of neurolemma
(Schwann) cells
Neurolemma or Schwann cells are
analogous to oligodendrocytes
These cells are derived from neural crest
cells
20. College
of
Medicine
–
الطب
كلية
Paraxial mesoderm differentiates and begins
to divide into cuboidal bodies called somites
by the end of 3rd week
These blocks of mesoderm are located on
each side of developing neural tube
About 38 pairs of somites form during the
somite period of human development (20-30
days)
22. College
of
Medicine
–
الطب
كلية
Each somite differentiates into two parts:
The ventromedial part is sclerotome
Its cells form the vertebrae and ribs
The dorsolateral part is the dermomyotome
Cells from myotome form myoblasts
Cells from dermatome form the dermis
24. College
of
Medicine
–
الطب
كلية
During the precartilaginous or mesenchymal
stage, mesenchymal cells are found in three
main areas:
Around the notochord
Surrounding the neural tube
In the body wall
26. College
of
Medicine
–
الطب
كلية
In a frontal section of a 4 week embryo, the
sclerotomes appear as paired condensations
of mesenchymal cells around the notochord
Each sclerotome consists of loose cells
cranially and packed cells caudally
27. College
of
Medicine
–
الطب
كلية
Some packed cells move cranially, where
they form the intervertebral disc
The remaining densely packed cells fuse
with the loose cells of the immediately
caudal sclerotome to form the
mesenchymal centrum
Centrum is a primordium of the body of a
vertebra
28. College
of
Medicine
–
الطب
كلية
Thus each centrum develops from two
adjacent sclerotomes and becomes an
intersegmental structure
The nerves lie in close relationship to the IV
discs
30. College
of
Medicine
–
الطب
كلية
The notochord degenerates and disappears where
it is surrounded by the developing vertebral bodies
Between the vertebrae, the notochord expands to
form the gelatinous center of the intervertebral
disc called nucleus pulposus
The nucleus later surrounded by circularly arranged
fibers that form the anulus fibrosus
31. College
of
Medicine
–
الطب
كلية
The nucleus pulposus and anulus fibrosus
together constitute the IV disc
The mesenchymal cells (connective tissues),
surrounding the neural tube, form the
vertebral arch
36. College
of
Medicine
–
الطب
كلية
Three primary centers are present by the end
of the embryonic period:
One in the centrum
One in each half of the vertebral arch
Ossification becomes evident in the vertebral
arches during the eighth week
37. College
of
Medicine
–
الطب
كلية
At birth each vertebra consists of three
bony parts connected by cartilage
The bony halves of the vertebral arch
usually fuse during the first 3 to 5 years
The arches first unite in the lumber region
This union progresses cranially
40. College
of
Medicine
–
الطب
كلية
Five secondary ossification centers appear
in the vertebrae after puberty:
One for the tip of the spinous process
One for the tip of each transverse process
One on the superior and one on the inferior
rim of the vertebral body
43. College
of
Medicine
–
الطب
كلية
Failure of fusion of the halves of the vertebral
arches result in major defect called spina bifida
It occurs more frequently in girls than boys
Most cases are open and covered by a thin
membrane
The closed spina bifida or spina bifida occulta is
covered by a thick membrane of skin
45. College
of
Medicine
–
الطب
كلية
It occurs in L5 or S1 vertebrae in about 10%
of otherwise normal people
In its most minor form, the only evidence of
its presence may be a small dimple with a
tuft of hair arising from it
It usually produces no clinical symptom
46. College
of
Medicine
–
الطب
كلية
It is a severe type of spina bifida
It involves protrusion of the spinal cord
and/or meninges through the defect in the
vertebral arches
It is called cystic because of a cyst like sac
associated with these anomalies
It occurs about once in every 1000 births
