2. Dr. Nimir Dr. Safaa

3. Objectives Describe the formation of neural tube and neural crest. Describe the development of brain and spinal cord. Describe the positional changes of spinal cord. Describe the development of the spinal nerves and their spinal ganglia.

4. At the beginning of the third week the neural tube appear as neural plate. Neural plate lateral edges soon elevate to form the neural folds which then join to form neural tube with cranial and caudal neuropores. Cells of lateral margin of plate do not incorporate in the neural tube and form neural crest.

5. A–C Transverse sections through successively older embryos showing formation of the neural groove, neural tube, and neural crest. Cells of the neural crest migrate from the edges of the neural folds and develop into spinal and cranial sensory ganglia. D. Scanning electron micrograph of a chick embryo showing the neural tube and neural crest cells migrating from the dorsal region of the tube (compare with B and C).

6. Cephalic end of neural tube shows three dilations, the primary brain vesicles: (1) Prosencephalon, or forebrain. (2) Mesencephalon, or midbrain. (3) Rombencephalon, or hindbrain. Neural tube forms two flexures: Cervical flexure at the junction of the hindbrain and the spinal cord. Cephalic flexure in the midbrain region.

7. At 5 weeks prosencephalon consists of two parts: Telencephalon. Diencephalon. Rhombencephalon also consists of two parts: Metencephalon, which later forms the pons and cerebellum. Myelencephalon which later forms medulla & continues as spinal cord.

8. The wall of neural tube consists of neuroepithelial cells. Neuroepithelial cells give rise to primitive nerve cells, or neuroblasts which form mantle layer (a zone around neuroepithelial layer). Mantle layer later forms the gray matter of spinal cord. Outermost layer of spinal cord (marginal layer) contains nerve fibers emerging from neuroblasts in mantle layer and forms white matter of the spinal cord.

9. Basal, Alar, Roof, and Floor Plates: Addition of more neuroblasts to mantle layer, each side of neural tube shows a ventral and a dorsal thickening. Ventral thickenings basal plates which contain ventral horn cells, form motor areas of spinal cord. Dorsal thickenings alar plates form sensory areas. Sulcus limitans marks boundary between the two plates. The dorsal and ventral portions of neural tube are roof and floor plates, respectively do not contain neuroblasts and serve as pathways for nerve fibers.

10. Motor nerve fibers begin to appear in the fourth week, arising from nerve cells in the basal plates (ventral horns) of the spinal cord. Axons of neurons in dorsal sensory horn (alar plate) penetrate into marginal layer of cord to form association neurons. Neural crest cells migrate laterally and give rise to sensory ganglia (dorsal root ganglia).

11. The majority of primitive supporting cells, the gliablasts, are formed by neuroepithelial cells. Gliablasts migrate from neuroepithelial layer to mantle and marginal layers. Gliablasts differentiate into protoplasmic astrocytes, fibrillar astrocytes and oligodendroglial cell. Microglial cells are derived from vascular mesenchyme. When neuroepithelial cells cease to produce neuroblasts and glia blasts, they differentiate into ependymal cells lining the central canal of the spinal cord.

12. Positional Changes of the Cord: In third month of development, spinal cord extends entire length of embryo, and spinal nerves pass through intervertebral foramina at their level of origin. With increasing age vertebral column and dura lengthen more rapidly than neural tube. At birth this end is at the level of the third lumbar vertebra. In adult, spinal cord terminates at the level of L2–L3, whereas dural sac and subarachnoid space extend to S2.