Development of the Nervous System PDF

Development of the Nervous System  Development of the Nervous System From surface ectoderm by induction of the notochord & adjacent mesoderm Develops as neural plate which gives rise to the neural folds that fuse and form the neural tube.  Development of the neural crest From the edges...

Development of the Nervous System  Development of the Nervous System From surface ectoderm by induction of the notochord & adjacent mesoderm Develops as neural plate which gives rise to the neural folds that fuse and form the neural tube.  Development of the neural crest From the edges of the neural folds that detach and fuse Migrate to different regions of the body  Neural crest Give rise to:  Peripheral nervous system and their ganglia  Chromaffin cells of the adrenal medulla  Schwann cells  Odontoblasts  Memninges of the forbrain  Parafollicular cells of the thyroid gland  Melanocytes  Connective tissue and bones of the face and skull  Smooth muscles of blood vessels of the face and forebrain  Cells of the trancoconal cushions of the heart  Development of the Develop from the cranial part of neural tube Brain During the 4th week shows 3 primary brain vesicles as: 1. Prosencephalon (forebrain) 2. Mesencepalon (midbrain) 3. Rhombencephalon (hindbrain).  Development of the Brain Shows 2 ventral brain flexures as: 1. Mesencephalic (midbrain) flexure – In the region of mesencephalon 2. Cervical flexure – Between rhombencephalon and the spinal cord.  Development of the Brain During the 5th week 5 secondary brain vesicles are formed as Pontine flexure appears dorsally dividing rombencephalon into: 1. Myelencephalon – caudally 2. Metencephalon – cranially  Development of the Brain Prosencephalon gives rise to: 1. Telencephalon 2. Diencephalons Mesencephalon remains as a single vesicle Development of the Brain  Development of the spinal cord Develops from the neural tube caudal to the 4th somits.  Development of the spinal cord Initially lined with neuroepithelial cells a thick pseudostratified epithelium that differentiates into: 1. Ventricular (ependymal or neuroepithelial) zone – Cells divide here and migrate to the outer two zones 1. Mantle zone 2. Marginal zone  Mantle zone of the spinal cord Neuroblasts give rise to the gray matter of the spinal cord Divided into: – Basal and alar plate separated from the basal plate by sulcus limitans – Roof and floor plate  Mantle zone of the spinal cord 1. Basal plate - ventrally & Develops into: a. General somatic efferent (GSE) column that innervate the somatic skeletal musculatures. b. General visceral efferent (GVE) column that are autonomic motor to the visceral organs.  Mantle zone of the spinal cord 2. Alar plate – dorsally & develops into: a. General somatic afferent (GSA) neurons - Are extroceptors (pain, temperature and touch) b. General visceral afferent (GVA) neurons - Are introceptors from the visceral organs.  Mantle zone of the spinal cord Roof and floor plates - Form nerve fibre crossings with no neuroblasts present.  Marginal zone of the spinal cord Is the most outer zone Develops into the white matter of the spinal cord.  Histological differentiation of the spinal cord 1. Neuroblasts appear 1st & migrate to mantle zone 2. Glioblasts (spongioblasts) appear 2nd from same neuroepithelial & migrate into both mantle and marginal zones as astrocytes & oligodendrocytes. Histological differentiation of the spinal cord 3. Microglia are formed from the monocytes arriving through blood as vessels appear 4. Ependymal cells are finally formed from the remaining neuroepithelial cells  Development of the meninges Memenchyme surrounding the neural tube condense and gives rise to the primitive meninx with two layers: 1. Outer thicker layer that develop into the dura mater. 2. Inner thinner layer that form an outer arachnoind mater and most inner pia mater  Positional changes of the spinal cord Because of the faster rate of elongation of the vertebral canal and dura & arachnoid mater Conus medullaris – Extends the entire length of the embryo up to the 3rd month, but terminate at: S1 vertebral level at 6th month Between L2 & 3 vertebral level at birth Between L1 & 2 vertebral level in adult  Positional changes of the spinal cord In the adult, lower end of: – Dura mater remain attached to the coccygeal vertebrae – Arachnoid mater ends at the level of S2(3) vertebrae. Pia mater forms filum terminale that attaches to the pereosteum of the 1st coccygeal vertebra. Lower spinal nerves descend as cauda equina Derivatives of the Myelencephalon Develops into the medulla. Because of the development of the pontine flexure: The upper part is opened dorsally like metencephalon and the 4th ventricle is formed The lower part remained closed like spinal cord Derivatives of the lower closed part of Myelencephalon Neuroblasts from alar plate migrate dorsally into the marginal zone to form gracile and cuneate nuclei. Corticospinal fibres are crowded in the ventral aspect of the marginal zone to give rise to the pyramids. Derivatives of the upper opened part of Myelencephalon The roof plate is formed only by the ependymal cells covered with pia mater to give the tela choroida. Alar plate is located dorsolateral to the basal plate.  Derivatives of the upper opened part of Myelencephalon Alar plate in addition to the GSA & GVA, also gives rise to: 1. Special somatic afferent (SSA) column located dorsolateral to GSA as centre for vestibulochoclear innervations. 2. Special visceral afferent (SVA) column located between the GSA and GVA as centre for taste Derivatives of the upper opened part of Myelencephalon 3. The olivary nucleus by some neuroblasts that have migrated from the alar plate ventrally into the marginal zone. Derivatives of the upper opened part of Myelencephalon Basal plate, in addition to GSE & GVE, also gives rise to: – Special visceral efferent (SVE) located between GSE and GVE for innervation of muscles developed from the branchial arch muscular components  Derivatives of the Metencephalon Develops into the Pons and Cerebellum  Development of the Pons Ventral aspect of the marginal zone of metencephalon. – Expands by fibres connecting the cerebrum with the spinal cord and cerebellum as a bridge. – Is occupied by pontine nuclei that develop from nuroblasts migrated from the alar plate.  Development of the Pons The mantle zone develops in a similar manner as the upper part of medulla showing the same 7 nuclear cell columns.  Development of the cerebellum Formed by the dorsolateral parts of the opened alar plates of metencephalon that gives rise to cerebellar swellings (rhombic lips)  Development of the cerebellum Cerebellar swellings (rhombic lips) – Enlarge & fuse to form cerebellar plate that develops into vermis. – The cerebellar hemispheres develop as lateral expansions from the vermis.  Development of the 4th ventricle Develops from the primitive lumen of neural tube in the opend region of the rohobencephalon  Derivatives of the Mesencephalon Develops into the midbrain. Is relatively unchanged and is primitive in its developmental structure Derivatives of the Mesencephalon Neuroblasts from the basal plate migrate into the ventral aspect of the marginal zone and give rise to: – Red nucleus, reticular nuclei & substancia nigra  Derivatives of the Mesencephalon Neuroblasts from alar plate migrate into the marginal zone & give rise to tectum (with the superior & inferior colliculi) Derivatives of the Mesencephalon The canal of the neural tube of the region becomes the cerebral aqueduct (of Sylvius).  Derivatives of the Diencephalon Alar plate develops into: – Epithalamus, thalamus & hypothalamus.  Derivatives of the Diencephalon Roof plate gives the choroid plexus of the 3rd ventricle and more caudally the pineal body.  Derivatives of theTelencephalon Develops as bilateral evaginations from proscencephalon.  Development of the basal nuclei Basal portion of the vesicle proliferates and bulges into the lateral ventricle as corpus striatum that will be divided by internal capsule into: 1. Caudate nucleus dorsomedially 2. Lentiform nucleus ventrolaterally.  Development of the basal nuclei Basal portion of the vesicle: – Gradually covers the lateral aspects of diencephalon, mesencephalon and cephalic portion of metencephlalon. – Medial wall of basal portion fuses with the diencephalon to make thalamus and caudate nucleus come into close contact. Development of the cerebral hemispheres Grow in anterior, posterior and inferior directions forming frontal, parietal, occipital and temporal lobes of the hemisphere.  Development of the cerebral hemispheres Region overlying the corpus striatum is overgrown by surrounding regions and buried in the lateral sulcus forming the Insula. Development of the cerebral hemispheres The giri and sulci appear gradually starting from the terminal period of fetal life, thereby increase the surface area of the cortex.  Development of the commissural fibres All commissural fibres develop within the lamina terminalis Anterior commissure connecting olfactory areas appears 1st  Development of the commissural fibres Hippocampal commissure (fornix) connecting hippocampal formations appears second. Corpus callosum appears later and gets bigger as the cerebral the hemispheres expand.  Development of the lateral ventricles Lumen of telencephalon develops into the two lateral ventricles and region of evagination of the telencephalon develop into the interventricular foramen (of Monro).  Congenital Anomalies of the Spinal Cord Most are due to failure of fusion of the vertebral arch resulting in the Spina bifida that may involve the : – Neural tube – Meninges – Muscles – Skin.  Congenital Anomalies of the Spinal Cord Spina bifida occulta – By failure of fusion of the vertebral arch usually located at sacrolumbar region (L5 or S1). – Normal spinal cord and nerves – Seen in about 10% of normal persons.  Congenital Anomalies of the Spinal Cord Spina bifida cystica Also protrusion of spinal cord &/or meninges Most commonly in the lumbar region. Usually associated with different degrees of neurological disorders. Occur as the following two types:  Congenital Anomalies of the Spinal Cord 1. Spina bifida with meningocele – Protrusion of the meninges – May be associated with abnormalities of the spinal cord.  Congenital Anomalies of the Spinal Cord 2. Spina bifida with meningomyelocele – Protrusion of spinal cord and nerves  Congenital Anomalies of the Spinal Cord 3. Spina bifida with myeloschisis or myelocele (rachishisis) – Because of failure of closure of the neural groove. – Seen as a flattened mass of nervous tissue exposed to surface. Congenital Anomalies of the Brain Cranium bifidum – Ossification defect usually in the squamous part of the occipital bone that is confluent with foramen magnum – Various types Congenital Anomalies of the Brain 1. Cranium bifidum with meningocele – Protrusion meninges Congenital Anomalies of the Brain 2. Cranium bifidum with meningencephalocele – Also contains part of the brain. Congenital Anomalies of the Brain 3. Cranium bifidum with meningohydroencephalocele – Also contains ventricle. Congenital Anomalies of the Brain Anencephaly (acrania or meroanencephaly) The neural tube is not closed and there is no calvaria formed. Brain is found as mass of neural tissue that degenerates. Congenital Anomalies of the Brain Microcephalus Small brain and skull with simple gyri & sulci in the cortex of the cerebral hemispheres Congenital Anomalies of the Brain Hydrocephalus By increased CSF in the ventricles because of imbalance between production and absorption. Congenital Anomalies of the Brain Arnold-Chiari malformation Elongation of medulla and herniation of cerebellar tissue through foramen magnum Causes obstruction of CSF flow and then hydrocephaly.