Nervous system is one of the earliest systems to begin development and the last to be completed after birth. The entire nervous system develops from the ectoderm.
Ectoderm forms the neural plate during 3rd week of development. Neural groove forms in the midline of the neural plate, either side of which are the neural folds. Groove continues to deepen until about week 4. Neural folds begins to fuse and form neural tube. Neural tube consists of 2 openings :
- Cranial (anterior) neuropore
- Caudal (posterior neuropore): The cranial neuropore closes earlier than the caudal neuropore.
The enlarged cranial part of the neural tube forms brain and the caudal tubular part forms the spinal cord. A population of cells at the edge of the neural plate that lie dorsally when the neural tube fuses forms the neural crest and it lies dorsal to the neural tube, as a pair of streaks.
Derivatives of neural crest:
- Adrenal medulla
- Pia and arachnoid sheath
- Dorsal root ganglion
- Craniofacial skeleton
- Schwann cells
- Thyroid parafollicular cells
- Symphatetic ganglia
- Parasymphatetic ganglia
Fates of cranial part of neural tube:
1. Forebrain vesicle –> Prosencephalon
- telencephalon – cerebral hemisphere
- diencephalon – thalamus, hypothalamus, epithalamus
2. Midbrain vesicle –> Mesencephalon: Tectum, tegmentum, substantia nigra, crus cerebri
3. Hindbrain vesicle –> Rhombencephalon
- Metencephalon – pons and cerebellum
- Myelencephalon – medulla oblongata
Formation of basal and alar plate:
- The wall of the neural tube consists of a single layer of pesudostratified columnar epithelial cells, called matrix cells.
- Matrix cells rapidly divide and form neuroblasts and glioblasts.
Neuroblasts give rise to following layers:
- Mantle (intermediate zone)
- Marginal (Layer external to mantle)
Mantle further forms 2 areas separated by sulcus limitans:
- Basal lamina or plate (ventrally) – motor
- Alar lamina or plate (dorsally) – sensory
Glioblasts give rise to:
- Microglial cells
- Ependymal cells (arise from matrix cells that line neural tube)
Development of spinal cord:
- Neuroblasts of basal plate (motor) forms motor cells of anterior gray column and their axon forms anterior nerve root of spinal nerves.
- Neuroblasts of alar plate (sensory) forms sensory cells of posterior gray column and interneurons.
- Neural crest cells migrate posterolaterally on each side of spinal cord and give rise to dorsal root ganglia and dorsal root of spinal nerves.
- Progressive growth of basal plate on either side forms anterior median fissure
- Progressive growth of alar plate on either side forms posterior median septum and central canal (form lumen of neural tube)
- Develops from myelencephalon
- As a result of expasion of 4th ventricle, alar plates come to lie lateral to basal plate.
- Neurons of basal plate form motor nuclei of cranial nerves (IX,X,XI,XII) in medulla.
- Neurons of alar plate form sensory nuclei of cranial nerves in medulla (V,VIII,IX,X) in medulla.
- Remaining cells of alar plate migrate ventrolaterally to form olivary nuclei.
- Develops from ventral part of metencephalon.
- It has also cellular contributions from the alar part of the Myelencephalon.
- Basal plate forms : the motor nuclei of cranial nerves (V,VI,VII).
- Alar plate forms : Sensory nucleus (V,VII), Vestibulo-cochlear nuclei (VIII) and pontine nuclei.
- Axons of the pontine nuclei go to the developing cerebellum of the opposite side (Rhombic lip) & form transverse pontine fibers and middle cerebellar peduncle.
- Develops from dorsal part of metencephalon.
- On each side, alar plate bends medially to form the rhombic lip
- Right and left rhombic lip grows caudally and fuses with across midline forming cerebellum
- At 12th week: small midline portion differentiates into vermis and 2 lateral portion differentiates into cerebellar hemispheres
- Migration of neuroblasts from matrix cells in ventricular zone to cerebellum leads to formation of cerebellar cortex.
- Neuroblasts remaining close to ventricular surface forms cerebellar nuclei (d,e,g,f = dentate, emboliform, globose, fastigii)
- Develops from mesencephalon.
- Cavity of mesencephalon remains narrow to form cerebral aqueduct.
Basal plate forms:
a) Motor nuclei of the 3rd & 4th cranial nerves
b) Red nuclei
c) Substantia nigra
d) Reticular formation
- The marginal zone of basal plates on each side enlarges to form basis pedunculi (The descending motor tracts are situated in that region – Cortico pontine, corticobulbar & corticospinal tracts)
- The 2 alar plates & the roof plate forms the Tectum.
- The alar plate forms the sensory neurons of the superior & inferior colliculi.
- Starts developing at 5th week.
- Thickening of the walls lead to reduction of intraventricular foramen.
Cerebral hemispheres grow & expand:
- Anteriorly : Form frontal lobes (1st)
- Laterally & superiorly : Parietal lobes (2nd)
- Posteriorly & inferiorly : occipital & temporal lobes (Last)
- Falx cerebri and flax cerebellum is formed by the condensation of mesenchyme between hemispheres.
- Develops from telencephalon.
- Migration of cells from mantle layer to marginal layer–>surface expansion–>cortex folds on itself–>formation of sulci and gyri
- By development cortex are of 3 types: hippocampal, pyriform and neocortex.
At the 12th week, cortex become very cellular due to migrating neuroblasts; different areas of the cortex will show specific cell types due to the influence of the ascending & descending tracts
- Motor cortex : Pyramidal cells
- Sensory areas : Granular cells
- Develops from telencephalon.
- Remaining cells of mantle layer forms caudate nucleus and lentiform nucleus.
- Many axons from ascending & descending tracts that pass between the thalamus & caudate nucleus medially and lentiform nucleus laterally as a bundle forms internal capsule.
- Similarly, cortical projections lateral to lentiform nuclei forms external capsule.
Thalamus and hypothalamus:
- Develops from diencephalon.
- Diencephalon is divided by epithalamic sulcus and hypothalamic sulcus into 3 surfaces.
- Part above epithalamic sulcus forms epithalamus (habenular nuclei and pineal body)
- Middle part forms the thalamus.
- Part below hypothalamic sulcus forms hypothalamus.
- Lamina terminalis (The cephalic end of the neural tube) forms a bridge between the 2 cerebral hemispheres. Optic chiasma is formed by inferior part of lamina terminalis.
- Anterior commissures : 1st commissure to develop. It connects olfactory bulbs of the temporal lobes on either side with each other.
- Fornix : 2nd commissure to develop. Connects hippocampus in each hemisphere
- Corpus callosum: 3rd commissure to develop. It connects frontal and parietal lobes of both hemispheres.
- The remaining of the lamina terminalis (Lies between the corpus callosum & the fornix) form the septum pellucidum.
Development of Eye:
- Outpouching of forebrain vesicle forms optic vesicle (3rd week).
- The optic vesicles come into contact with the epithelum and induce the epidermis. The epithelium thickens to form the lens placode.
- The lens differentiates and invaginates until it pinches off from the epithelium. The lens acts as an inducer back to the optic vesicle to transform it into the optic cup and back to the epidermis to transform it into the cornea.
- The optic cup then delaminates into two layers: The neural retina and the retinal pigment epithelium.
- The periocular mesenchyme migrates in during the formation of the optic cup and is critical for the induction of the retinal pigment epithelium and the optic nerve.
Development of Nervous System: Embryology Lecutre Video