1. DEVELOPMENT OF SPINAL CORD AND VERTEBRAL COLUMN
Prof. Mujahid Khan

2. 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

4. 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

5. 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

7. 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

9. 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

11. 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

13. 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

14. 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

16. 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

17. 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

19. 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

20. 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 ( days)

22. 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

24. 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

26. 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

27. 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

28. 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

30. 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

31. 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

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

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

36. 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

37. 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

39. 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

40. 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

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

43. 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

45. 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

46. 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

48. THE END