DEVELOPMENT OF SPINAL CORD AND VERTEBRAL COLUMN Prof. Mujahid Khan

Origin of Nervous System 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

Origin of Nervous System 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

Development of Spinal Cord 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

Development of Spinal Cord 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

Development of Spinal Cord 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

Development of Spinal Cord 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

Development of Spinal Cord 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

Development of Spinal Cord 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

Development of Spinal Cord 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

Myelination of Nerve Fibers 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

Myelination of Nerve Fibers 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

Somites 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)

Somites 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

Vertebral Column 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

Vertebral Column 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

Intervertebral Disc 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

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

Nucleus Pulposus 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

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

Cartilaginous Stage During the sixth week chondrification centers appear in each mesenchymal vertebra

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

Bony Stage 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

Bony Stage 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

Bony Stage 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

Bony Stage 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

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

Spina Bifida 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

Spina Bifida Occulta 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

Spina Bifida Cystica 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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