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Central Nervous System

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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 neural tube 2. Describe the development of the spinal cord 3. Describe the development of the brain 4. Describe the development of the hypophysis cerebri (pituitary gland) CENTRAL NERVOUS SYSTEM • The central nervous system 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 ventricular system of the brain and the central canal 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 sulcus limitans. • 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 autonomic nervous system. • 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 filum terminale 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 forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon). • Two flexures appear simultaneously: the cervical flexure 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 diencephalon. • The hindbrain partly divides into the metencephalon (future pons and cerebellum) and myelencephalon, separated by the pontine flexure. • The cavity of the rhombencephalon is known as the fourth ventricle, that of the diencephalon as the third ventricle, and those of the cerebral hemispheres as the lateral ventricles. 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 medulla oblongata. • 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 pia mater), which invaginates and differentiates into the plexus choroideus (site of the production of the cerebrospinal fluid). • The cavity of the rostral part of the myelencephalon becomes the caudal part of the fourth ventricle. Development of the medulla oblongata 1. Choroid plexus 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 cerebral aqueduct. • 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, thalamus 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
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