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Central Nervous System

Central Nervous System

LESSON 22 DEVELOPMENT OF THE NERVOUS SYSTEM Objectives

By the end of this lesson, you should be able to: 1. Describe the development of the 2. Describe the development of the 3. Describe the development of the 4. Describe the development of the hypophysis cerebri (pituitary gland)

CENTRAL NERVOUS SYSTEM • The appears in the middle of the 3rd week of the development as a thickened area of the embryonic ectoderm, the neural plate. • Its lateral edges become elevated to form the neural folds, which approach each other and fuse in the middle, thus forming the neural tube. • At the cranial and caudal end of the embryo the neural tube is temporarily open and communicates with the amniotic cavity by the way of the cranial and caudal neuropores. • The neural tube differentiates into the central nervous system, consisting of the brain and spinal cord, and the neural crest, which gives rise to the most of the peripheral nervous system. • The neural canal becomes the of the brain and the of the spinal cord. FORMATION OF THE NEURAL TUBE

1. Notochord 2. Intermediate zone of neural crest 3. Neural groove 4. Neural crest 5. Neural fold 6. Dorsal root ganglion 7. Neural tube 8. Surface ectoderm

EMBRO – DAY 19 1. Neural plate 2. Primitive node 3. Primitive streak 4. Cut edge of amnion

EMBRYO – DAY 20 1. Primitive streak 2. Cut edge of amnion 3. Neural fold 4. Neural groove 5. Somite 6. Primitive node

EMBRYO – DAY 23 1. Pericardial bulge 2. Anterior neuropore 3. Somite 4. Posterior neuropore DEVELOPMENT OF THE SPINAL CORD • The neural tube caudal to the fourth pair of the somites develops into the spinal cord. • The wall of the neural tube consists of the neuroepithelial cells which give rise to the neurons and macroglia cells (ependymal cells, oligodendroglia, astrocytes). • The neurepithelial cells differentiate into neuroblasts, which form the mantle layer (gray matter of the spinal cord). • Nerve fibers emerging from these neuroblasts constitute the marginal layer (the white matter of the spinal cord). • The microglia cells differentiate from the mesenchymal cells surrounding the central nervous system.

SPINAL CORD CONT… • Thickening of the lateral walls of the spinal cord causes the formation of the ventral and dorsal columns separated by a longitudinal groove, the . • The ventral thickenings (basal plates) or ventral horns form the motor areas, while the dorsal thickenings (alar plates) or dorsal horns form the sensory areas of the spinal cord. • A small intermediate horn contains neurons of the . • The roof and floor plate serve as pathways for nerve fibers crossing from one side to the other. Development of the spinal cord 1. Roof plate 2. Flor plate 3. Sulcus limitans 4. Alar plate 5. Basal plate 6. Intermediate horn 7. Mantle layer 8. Neuroepithelial layer 9. Marginal layer

Development of the spinal cord 1. Intermediate horn 2. Dorsal sensory horn 3. Ventral motor horn 4. Dorsal sensory root 5. Ventral motor root 6. Dorsal root ganglion 7. Trunk of spinal nerve 8. Central canal HISTIOGENESIS

• The primitive neuroblasts gradually acquire a primitive axon and dentrites and differentiate into unipolar, bipolar or multipolar neuroblasts. • The axons of the neurons in the ventral horns break through the marginal zone and become ventral motor root of the spinal nerve. • The axons in the dorsal horns ascend or descend in the marginal layer to form sensory neurons.

MYELINATION • In the spinal cord the myelin sheath is formed by the oligodendroglia cells, and by the Schwann cells outside the spinal cord. • Myelination begins at about 4th month of fetal life. • Schwann cells wrap themselves around the axons thus forming the neurilemma sheath. • The tracts in the nervous system become myelinated at about the time they start to function. MYELINATION 1. Motor horn cell 2. Oligodendroglia cell 3. Axon 4. Node of Ranvier 5. Schwann cell nucleus 6. Neurilemma sheath 7. Myelin sheath

POSITIONAL CHANGES OF THE CORD

• During the development, the vertebral column and the dura lengthen more rapidly than the neural tube and the terminal end of the spinal cord gradually shifts to a higher level. • The marks the tract of regression of the spinal cord, while the nerve fibers below are known as cauda equina.

DEVELOPMENT OF THE BRAIN • The neural tube cranial to the fourth pair of the somites develops into the brain. • During the 4th week, three primary brain vesicles are formed: the (prosencephalon), (mesencephalon) and (rhombencephalon). • Two flexures appear simultaneously: the cervical at the junction of the hindbrain and the spinal cord, and cephalic flexure in the midbrain region. • During the 5th week, the forebrain divides into two vesicles, the telencephalon (future cerebral hemispheres) and . • The hindbrain partly divides into the (future and ) and myelencephalon, separated by the pontine flexure. • The cavity of the rhombencephalon is known as the , that of the diencephalon as the , and those of the cerebral hemispheres as the . Neural tube - 4th week 1. Prosencephalon 2. Mesencephalon 3. Rhombencephalon

Development of the brain - 6th week 1. Telencephalon 2. Lateral ventricle 3. Interventricular foramen of Monro 4. 3rd ventricule 5. Optic cup 6. 4th ventricle 7. Future aqueduct of Sylvius 8. Central canal

Brain vesicles - 6th week 1. Diencephalon 2. Optic cup 3. Telencephalon 4. Primitive cerebral hemisphere 5. Metencephalon 6. Mesencephalon 7. Rhombencephalic isthmus 8. Myelencephalon 9. Pontine flexure

Brain vesicles (midline section) - 6th week 1. Diencephalon 2. Mesencephalon 3. Myelencephalon 4. Telencephalon 5. Metencephalon 6. Rhombencephalic isthmus 7. Roof of rhombencephalon 8. Central canal

Early development of the brain 1. Rhombencephalon (hindbrain) 2. Diencephalon 3. Heart 4. Prosencephalon (forebrain) 5. Optic outgrowth 6. Cervical flexure 7. Midbrain flexure 8. Myelencephalon 9. Mesencephalon (midbrain) 10. Metencephalon 11. Telencephalon 12. Spinal cord 13. 4th ventricle 14. Pontine flexure

MYELENCEPHALON • Myelencephalon gives rise to the . • Its lateral walls rotate around the longitudinal axis, so that the alar and basal plates separated by the sulcus limitans can be clearly distinguished. • The basal plate contains motor nuclei, while sensory nuclei are in alar plate. • The roof plate of the myelencephalon forms the tela choroidea (ependymal roof covered by ), which invaginates and differentiates into the plexus choroideus (site of the production of the ). • The cavity of the rostral part of the myelencephalon becomes the caudal part of the fourth ventricle.

Development of the medulla oblongata 1. 2. 4th ventricle 3. Tela choroidea 4. Sulcus limitans 5. Alar plate 6. Basal plate 7. Somatic afferent group 8. Special visceral afferent group 9. General visceral afferent group 10. Somatic efferent group 11. Special visceral efferent group 12. General visceral efferent group 13. Olive nucleus

METENCEPHALON • The walls of metencephalon form the pons and the cerebellum, while its cavity forms the cranial part of the fourth ventricle. • The cerebellum develops from thickenings of the alar plates, which enlarge and fuse. • It overgrows and overlaps the pons and medulla. • At the 8th developmental week, the cerebellum consists of the neuroepithelial, mantle and marginal layer. • During the further development, it gradually differentiates into gray and white matter and the several nuclei. • The pons is the region of the brainstream and the site of the passage of the nerve fibers connecting the cerebral and cerebellar cortices. Development of the pons and cerebellum 1. Choroid plexus 2. Medulla 3. Tela choroidea 4. Pons 5. 4th ventricle 6. Developing anterior lobe of cerebellum 7. Midbrain

Development of the pons and cerebellum 1. Developing cerebellum 2. Tela choroidea 3. 4th ventricle 4. Somatic afferent group 5. Special visceral afferent group 6. General visceral afferent group 7. Somatic efferent group 8. Special visceral efferent group 9. General visceral efferent group 10. Pontine nuclei

MESENCEPHALON

• The mesencephalon undergoes lesser changes than any other part of the developing brain. • The neural canal narrows to form the . • Neuroblasts migrate from the alar plates and aggregate to form large groups of neurons. • The basal plates give rise to the neurons in the tegmentum. Development of the mesencephalon

1. Basal plate 2. Alar plate 3. Sulcus limitans

Development of the mesencephalon

1. Nucleus ruber 2. Stratified nuclear layer of colliculus 3. Crus cerebri 4. Substantia nigra 5. Visceral efferent 6. Somatic efferent DIENCEPHALON

• Three swellings develop in the lateral walls of the third ventricle, later becoming the epithalamus, and hypothalamus. • The roof of diencephalon gives rise to the plexus choroideus and epiphysis. • The downward extension of the diencephalon forms the infundibulum (future stalk and the pars nervosa of the hypophysis). • Lateral evagination of diencephalon participates in the formation of the eye.

Diencephalon and telencephalon

1. Lateral ventricle 2. Choroid plexus 3. Foramen of Monro 4. 3rd ventricle 5. Ependymal roof of the 3rd ventricle 6. Corpus striatum 7. Hypothalamus 8. Neopallium 9. Hippocampus Hypophysis

• The hypophysis or pituitary gland develops entirely from the ectoderm derived from two sources: – ectodermal outpocketing of the stomodeum, known as Rathke's pouch and a downward extension of the diencephalon, the infundibulum. • Rathke's pouch appears during the fourth week and loses its connection with the oral cavity by the end of the 8th week. • The cells of the Rathke's pouch increase rapidly in number and give rise to the adenohypophysis (the anterior lobe), pars tuberalis and pars intermedia. • The infundibulum gives rise to the stalk, and the neurohypophysis (the posterior lobe). Development of the hypophysis

1. Oral cavity 2. Infundibulum 3. Rathke's pouch 4. Notochord 5. Diencephalon Development of the hypophysis

1. Anterior lobe 2. Lumen of diencephalon 3. Pars nervosa 4. Sphenoid bone 5. Pars intermedia

TELENCEPHALON • The telencephalon consists of a median part (lamina terminalis) and two lateral vesicles (cerebral hemispheres). • The lateral ventricles communicate with the cavity of the third ventricle through the interventricular foramina of Monro. • During the 6th and 7th week, the basal part of the hemispheres expand, thus forming the corpus striatum (later divided into caudate and lentiform nucleus). • The hemispheres cover the diencephalon, midbrain and hindbrain. Surface of the hemispheres forms the cerebral cortex. • In the region of attachment to the roof of diencephalon, the wall of the hemispheres remains very thin. • The choroid plexus and the hippocampus form here. • Further differentiation of the brain takes place after the embryonic period. Diencephalon and telencephalon

1. Lateral ventricle 2. Choroid plexus 3. Foramen of Monro 4. 3rd ventricle 5. Ependymal roof of the 3rd ventricle 6. Corpus striatum 7. Hypothalamus 8. Neopallium 9. Hippocampus

PERIPHERAL NERVOUS SYSTEM • The peripheral nervous system mostly develops from the neural crest cells. • The neural crest cells are groups of ectodermal cells that temporarily form an intermediate zone between the neural tube and surface ectoderm. • These cells migrate and differentiate into the sensory ganglia, sympathetic neuroblasts, Schwann cells, pigment cells, odontoblasts, and cartilage cells of the branchial arches. • Neuroblasts of the sensory ganglia penetrate the dorsal part of the neural tube and mostly end in the dorsal horn, thus giving rise to the dorsal root neurons. • The peripherally growing processes of the neuroblasts in the sensory ganglia terminate in the sensory receptor organs.

Differentiation of the neural crest

1. Neural groove 2. Neural crest 3. Neural fold 4. Notochord 5. Intermediate zone 6. Neural tube 7. Surface ectoderm 8. Mesoderm Conclusion

Development of the neural tube Development of the spinal cord Development of the brain Myelencephalon – medulla oblongata Metencephalon – pons and cerebellum Mesenchephalon - midbrain Diencephalon – hypothalamus, thalamus, epithalamus, epiphysis, hypophysis cerebri Telencephalon – cerebral hemispheres

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