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Development of the nervous system

Dr. NimirDr. SafaaObjectivesDescribe the development of meninges.Describe the development of brain vesicles from the neural tube.Describe the development of the different parts of brain.Describe the development of brain ventricles and choroid plexusesDescribe the development of the cranial nerves and their ganglia.Describe the congenital anomalies of brain and spinal cord.

Meninges:The meningeal layers originate from:Paraxial mesenchyme in the trunk and caudal regions of the head. Neural crest in regions rostral to the mesencephalon. The meninges may be divided in development into the pachymeninx (dura mater) and leptomeninges (arachnoid mater, subarachnoid space with arachnoid cells and fibres, and pia mater). All meningeal layers are derived from loose mesenchyme which surrounds the developing neural tube, termed primary meninx.

Rhombencephalon:HindbrainRhombencephalon consists of the myelencephalon, most caudal of the brain vesicles,and metencephalon, which extends from the pontine flexure to rhombencephalic isthmus.

Myelencephalon:It is brain vesicle that gives rise to medulla oblongata. It differs from the spinal cord in that its lateral walls are everted. Alar and basal plates separated by sulcus limitans can be clearly distinguished

The basal plate, similar to that of spinal cord contains motor nuclei. These nuclei are divided into three groups: (1) Somatic efferent.(2) Special visceral efferent.(3) General visceral.Somatic efferent group continues rostrally into mesencephalon as somatic efferent motor column.In myelencephalon, it includes neurons of hypoglossal (XII) nerve. In the metencephalon and the mesencephalon, the column contains neurons of abducens (VI) ,trochlear (IV), and oculomotor(III) nerves.

Special visceral efferent group extends into the metencephalon, forming special visceral efferent motor column. Its motor neurons supply striated muscles of pharyngeal arches. In myelencephalon, the column is represented by neurons of accessory (XI),vagus (X), and glossopharyngeal (IX) nerves.The general visceral efferent group contains motor neurons that supply involuntary musculature of the respiratory tract, intestinal tract, and heart.

The alar plate contains three groups of sensory relay nuclei:(1)Somatic afferent (general sensory) receives sensations of pain, temperature, and touch from pharynx by way of glossopharyngeal nerve (IX). (2) Special afferent receives from taste buds of tongue, palate, oropharynx, epiglottis and from vestibulocochlear nerve (VIII) for hearing and balance. (3) General visceral afferent, group receives interoceptive information from respiratory gastrointestinal tract and heart by vagus nerve (X).

Roof plate of myelencephalon consists of a single layer of ependymal cells covered by pia mater The two combined are known as tela choroidea. Proliferation of vascular mesenchyme forms sac-like invaginations that project into the underlying ventricular cavity and form the choroid plexus, which produces cerebrospinal fluid.

Metencephalon:Similar to myelencephalon it is characterized by basal and alar plates.Two new components form:(1) Cerebellum, a coordination center for posture and movement .(2) Pons, the pathway for nerve fibers between spinal cord , cerebral and cerebellar cortices.Basal plate of metencephalon contains three groups of motor neurons:(1) Somatic efferent , which gives rise to the nucleus of the abducens nerve.(2) Special visceral efferent containing nuclei of the trigeminal and facial nerves.(3) General visceral efferent, with axons that supply the submandibular and sublingual glands.

Marginal layer of basal plates of metencephalon makes a bridge for nerve fibers connecting cerebral cortex and cerebellar cortex with the spinal cord.Hence, this portion of the metencephalon is known as pons (bridge). In addition to nerve fibers, pons contains pontine nuclei, which originate in the alar plates of metencephalon and myelencephalon.Alar plates of the metencephalon contain three groups of sensory nuclei:(1) Somatic afferent which contains neurons of the trigeminal nerve.(2) Special afferent.(3) General visceral afferent.

CerebellumDorsolateral parts of alar plates bend and form rhombic lips which join and form the cerebellar plate. In a 12-week embryo, this plate shows a small midline vermis, and two lateral hemispheres. A transverse fissure soon separates the nodule from vermis and lateral flocculus from hemispheres.This flocculonodular lobe is phylogenetically the most primitive part of the cerebellum.

Initially cerebellar plate consists of neuroepithelial, mantle, and marginal layers. Cells formed by the neuroepithelium migrate to surface of the cerebellum to form external granular layer.In the sixth month of development external granular layer gives rise to various cell types. These cells migrate toward the differentiating Purkinje cells and give rise to granule cells.

Basket and stellate cells are produced by proliferating cells in the cerebellar white matter. The cortex of the cerebellum, consisting of Purkinje cells, Golgi II neurons, and neurons produced by the external granular layer, reaches its definitive size after birth .The deep cerebellar nuclei, such as dentate nucleus, reach their final position before birth.

Mesencephalon (Midbrain):In mesencephalon each basal plate contains two groups of motor nuclei: (1) Somatic efferent represented by oculomotor and trochlear nerves. (2) General visceral efferent represented by nucleus of Edinger-Westphal.Marginal layer of each basal plate enlarges and forms crus cerebri. Alar plates of mesencephalon appear as two longitudinal elevations separated by a shallow midline depression. With further development, a transverse groove divides elevations into superior and inferior colliculi.

Prosencephalon (forebrain):It consists of telencephalon which forms cerebral hemispheres and diencephalon, which forms optic cup and stalk, pituitary, thalamus, hypothalamus, and epiphysis.Diencephalon:It develops from median portion of prosencephalon and consist of a roof plate and two alar plates but lack floor and basal plates.

Roof plate of diencephalon give rise to choroid plexus of the third ventricle.The most caudal part of roof plate develops into pineal body, or epiphysis.

Alar Plate, Thalamus, and Hypothalamus:Alar plates form lateral walls of diencephalon. Hypothalamic sulcus divides the plate into a dorsal region forming the thalamus and a ventral region forming the hypothalamus.

Cranial Nerves:By the fourth week of development, nuclei for all 12 cranial nerves are present. All except the olfactory (I) and optic (II) nerves arise from brainstem, and of these only the oculomotor (III) arises outside the region of the hindbrain.

Cranial NerveBrain RegionTypeInnervationOlfactory (I)TelencephalonSVANasal epithelium (smell)Optic (II)DiencephalonSSARetina (vision)Oculomotor (III)MesencephalonGSESup., inf., med. Rectus, inf. oblique, levator palpebrae sup. m.GVE (ciliary ganglion)sphincter pupillae, ciliary m.Trochlear (IV)Metencephalon (exits mesencephalon)GSESup. oblique m.Trigeminal (V)MetencephalonGSA (trigeminal ganglion)Skin, mouth, facial m., teeth, ant. two-thirds of tongueGVA (trigeminal ganglion)proprioception: skin, muscles, jointsSVE (branchiomotor)M. of mastication, mylohyoid, ant. belly of digastric, tensor velipalatini, post. belly of digastric m.Abducens (VI)MetencephalonGSELateral rectus m.Facial (VII)MetencephalonSVA (geniculate ganglion)Taste ant. two-thirds of tongueGSA (geniculate ganglion)Skin ext. auditory meatusGVA (geniculate ganglion)Ant. two-thirds of tongueSVE (branchiomotor)M. of facial expression, stapedius, stylohyoid, post. belly of digastricGVESubmandibular, sublingual, and lacrimal glandsVestibulocochlear (VIII)MetencephalonSSA (vestibular and spiral ganglia)Semicircular canals, utricle, saccule (balance) spiral organ of Corti (hearing)Glossopharyngeal (IX)MyelencephalonSVA (inferior ganglion)Post. one-third of tongue (taste)GVA (superior ganglion)Parotid gland, carotid body and sinus, middle earGSA (inferior ganglion)External earSVE (branchiomotor)StylopharyngeusGVE (otic ganglion)Parotid glandVagus (X)MyelencephalonSVA (inferior ganglion)Palate and epiglottis (taste)GVA (superior ganglion)Base of tongue, pharynx, larynx, trachea, heart, esophagus, stomach, intestinesGSA (superior ganglion)External auditory meatusSVE (branchiomotor)Constrictor m. pharynx, intrinsic m. larynx, sup. two-thirds esophagusGVE (ganglia at or near viscera)Trachea, bronchi, digestive tract, heartSpinal Accessory (XI)MyelencephalonSVE (branchiomotor)Sternocleidomastoid, trapezius m.Hypoglossal (XII)MyelencephalonGSEM. of tongue (except palatoglossus

Spina bifida cystica

Spina bifida occulta

A fetus at 20 weeks' gestation with severe neural tube defects, including acrania, cerebral regression (meroencephaly [anencephaly]), iniencephaly (enlargement of foramen magnum), and a sacral dimple (arrow).

Meningoencephalocele

Microcephaly

A, An infant with hydrocephalus and bilateral cleft palate. B and C, The brain of a 10-year-old child who had developed hydrocephalus in utero as a result of aqueductal stenosis.

An Arnold-Chiari malformation in a 23-week gestational age fetus. In situ exposure of the hindbrain reveals cerebellar tissue (arrow) well below the foramen magnum. B, Magnetic resonance image of a child with Arnold-Chiari malformation. Note the cerebellar tonsils lie inferior to the foramen magnum (red arrow).