Development of the nervous system

2. Development of the nervous system Dr. Nimir Dr. Safaa

3. Objectives • Describe 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 plexuses • Describe the development of the cranial nerves and their ganglia. • Describe the congenital anomalies of brain and spinal cord.

4. Meninges: • The meningeal layers originate from: • Paraxial mesenchymein 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.

5. Rhombencephalon:Hindbrain • Rhombencephalonconsists of the myelencephalon, most caudal of the brain vesicles,andmetencephalon, which extends from the pontine flexure to rhombencephalic isthmus.

6. 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 sulcuslimitans can be clearly distinguished

7. 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.

8. 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.

9. 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).

10. Roof plate of myelencephalon consists of a single layer of ependymal cells covered by pia materThe two combined are known as telachoroidea. • Proliferation of vascular mesenchymeforms sac-like invaginations that project into the underlying ventricular cavity and form the choroid plexus, which produces cerebrospinal fluid.

11. 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 ofmotor 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 submandibularand sublingual glands.

12. 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 neuronsof the trigeminal nerve. • (2) Special afferent. • (3) General visceral afferent.

13. Cerebellum • Dorsolateralparts 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 flocculusfrom hemispheres. • This flocculonodular lobe is phylogenetically the most primitive part of the cerebellum.

14. 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 Purkinjecells and give rise to granule cells.

15. Basket and stellate cells are produced by proliferating cells in the cerebellar white matter. • The cortex of the cerebellum, consisting of Purkinjecells, 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.

16. 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 cruscerebri. • 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.

17. Prosencephalon (forebrain): • It isconsists oftelencephalonwhich 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.

18. 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.

19. 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.

20. 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.

25. Spinabifida cystica

26. Spina bifida occulta

27. 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).

28. Meningoencephalocele

29. Microcephaly

30. 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.

31. An Arnold-Chiarimalformation 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).