The document discusses the development of the spinal cord and vertebrae in humans. It explains that the spinal cord develops from the neural tube and that neurulation begins in the fourth week of development. It also describes how the somites differentiate and contribute to the formation of the vertebrae, ribs, and muscles. Finally, it outlines the process of ossification of the vertebrae from embryonic stages through adulthood.

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Prof. Mujahid Khan

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 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

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 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

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 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

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 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

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 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

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 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

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 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

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 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

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 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

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 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

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 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

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 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)

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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

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 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

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 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

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 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

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 Thus each centrum develops from two
adjacent sclerotomes and becomes an
intersegmental structure
 The nerves lie in close relationship to the IV
discs

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 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

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 The nucleus pulposus and anulus fibrosus
together constitute the IV disc
 The mesenchymal cells (connective tissues),
surrounding the neural tube, form the
vertebral arch

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 During the sixth week chondrification centers
appear in each mesenchymal vertebra

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 Ossification of typical vertebrae begins during
the embryonic period
 It usually ends by the twenty-fifth year

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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

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 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

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 These articulations permit the vertebral
arches to grow as the spinal cord enlarges
 These joints disappear when the vertebral
arch fuses with the centrum during the third
to sixth years

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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

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 All secondary centers unite with the rest of
the vertebra around 25 years of age

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 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

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 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

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 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

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