Each germ layer gives rise to particular tissues and organs in the adult.

CNS Development During the first 26 days of development: – Ectoderm thickens along dorsal midline to form the neural plate – The neural plate invaginates, forming a groove flanked by neural folds – The neural groove fuses dorsally and forms the neural tube

Neural Tube The nervous system develops when the notochord induces its overlying ectoderm to become neuroectoderm and to develop into the neural plate. The neural plate folds along its central axis to form a neural groove lined on each side by a neural fold. The two neural folds fuse together and pinch off to become the neural tube. Fusion of the neural folds begins in the middle of the embryo and moves cranially and caudally. The cranial open end of the tube is the anterior (rostral) neuropore, and the caudal open end of the tube is the posterior (caudal) neuropore. The anterior neuropore closes on or before day 26 and the caudal neuropore closes before the end of the fourth week.

Figure 12.2

Neural Crest Some cells from the neural folds give rise to pleuripotent neural crest cells that migrate widely in the embryo and give rise to many nervous structures: · Spinal ganglia (dorsal root ganglia) · Ganglia of the autonomic nervous system · Ganglia of some cranial nerves · Sheaths of peripheral nerves · Meninges of brain and spinal cord · Pigment cells · Suprarenal medulla · Skeletal and muscular components in the head

Primary Brain Vesicles The anterior end of the neural tube expands and constricts to form the three primary brain vesicles – Prosencephalon – the forebrain – Mesencephalon – the midbrain – Rhombencephalon – hindbrain

Adult Neural Canal Regions Figure 12.3a, b

Secondary Brain Vesicles In week 5 of embryonic development, secondary brain vesicles form: – Telencephalon and diencephalon arise from the forebrain – Mesencephalon remains undivided – Metencephalon and myelencephalon arise from the hindbrain

Adult Neural Canal Regions Figure 12.3c

Adult Neural Canal Regions Figure 12.3c, e

Development of Brain The neural tube forms three primary brain vesicles. The primary brain vesicles give rise to five secondary brain vesicles, which give rise to various adult structures. Adult structuresSecondary vesicles Primary vesicles Cerebral hemispheres, consisting of the cortex and medullary center, basal ganglia, lamina terminalis, hippocampus, the corpus striatum, and the olfactory system TelencephalonForebrain vesicle (prosencephalon) Thalamus, epithalamus, hypothalamus, subthalamus, neurohypophysis, pineal gland, retina, optic nerve, mamillary bodies Diencephalon Midbrain colliculiMesencephalonMidbrain vesicle (mesencephalon) Pons and cerebellumMetencephalonHindbrain vesicle (rhombencephalon) MedullaMyelencephalon

Development of Spinal Cord The neural tube consists of three cellular layers from inner to outer: the ventricular zone (ependymal layer), the intermediate zone (mantle layer), and the marginal zone (marginal layer). The ventricular zone gives rise to neuroblasts (future nerve cells) and glioblasts (future supporting cells) which migrate into the intermediate zone form two collections of cells (the alar plate and the basal plate) separated by a groove called the sulcus limitans. Cells in the alar plate become afferent (sensory) neurons and form the dorsal (posterior) horn of the spinal cord. Cells in the basal plate become efferent (motor) neurons and form the ventral (anterior) horn of the spinal cord..

The spinal cord extends the entire length of the vertebral canal at week 8 of development. At birth, the conus medullaris extends to the L3 vertebra. In the adult, the conus medullaris extends to the L1 vertebra. Spinal lumbar punctures must be performed caudally to the conus medullaris to avoid damaging the spinal cord.

Development of Meninges The dura mater arises from paraxial mesoderm that surrounds the neural tube. The pia mater and arachnoid mater arise from neural crest cells. Cerebellum Rhombic lips of alar plate — approach each other — cerebellar plate — at w 12- midline vermis and lateral 2 cerebellar hemispheres,are seen. Transverse fissure separate the nodule from vermis and lateral flocculus Hypophysis (Pituitary Gland) The anterior pituitary gland (adenohypophysis) arises from an evagination of the oropharyngeal membrane known as Rathke ’ s pouch. The posterior pituitary gland (neurohypophysis) arises from an evagination of neuroectoderm from the diencephalon.

DEVELOPMENT OF THE BRAIN & SPINAL CORD Enormous growth, distortion and cellular differentiation occur in the neural tube. This is maximal in the rostral part, which develops into the brain, while the caudal part will forms the spinal cord.

The central cavity within the neural tube becomes: The ventricles of the brain & The central canal of the spinal cord. 17

Prof. Saeed makarem18 The neural crests cells form the sensory ganglia of the spinal & cranial nerves, and also the autonomic ganglia.

As development continues, a longitudinal groove, called the sulcus limitans, appears on the inner surface of the lateral walls of the embryonic spinal cord and caudal part of the brain. The dorsal and ventral cell groupings thus forming the alar plate and the basal plate, respectively. 19

sensory functions, motor. Nerve cells that develop within the alar plate have predominantly sensory functions, while those in the basal plate are predominantly motor. 20

Further development also brings about the differentiation of grey and white matter. The grey matter is located centrally around the central canal, While the white matter forming an outer coat. 21

This basic developmental pattern can still easily be recognized in the adult spinal cord. 22

DEVELOPMENT OF THE BRAIN During development, the rostral portion of the neural tube undergoes massive differentiation and growth to form the brain. three primary brain vesicles three primary brain vesicles can be identified: 1. Prosencephalon or (forebrain), 2. Mesencephalon or (midbrain), 3. Rhombencephalon or (hindbrain).

The longitudinal axis of the developing CNS (neuraxis) does not remain straight but is bent by : 1- Cephalic, or Midbrain flexure occurring at the junction of the forebrain and midbrain 2-Cervical flexure between the brain and the spinal cord. 24

five further differentiation distinguishes five secondary brain vesicles produced by: Division of the prosencephalon into 2 lateral vescicle the telencephalon and a median part the diencephalon. Also, division of the rhombencephalon into the metencephalon and myelencephalon.

The telencephalon undergoes the greatest further development and gives rise to the two cerebral hemispheres. 26

The cerebral hemispheres consist of: Outer layer of grey matter (the cerebral cortex) Inner mass of white matter, within which various groups of grey matter nuclei are buried 27

The metencephalon develops into the Pons anteriorly & cerebellum posteriorly. Prof. Saeed makarem28

The myelencephalon forms the medulla oblongata (medulla). Prof. Saeed makarem29

As the brain develops, its central cavity also undergoes considerable changes in size and shape forming a system of ventricles, which contain cerebrospinal fluid (CSF).

malformations Spina Bifida · Spina bifida occulta is a defect of the vertebral column only, and is a common problem affecting as many as 10% of live births. · Spina bifida with meningocele (spina bifida cystica) is a defect of the vertebral column with protrusion of the meninges through the defect. · Spina bifida with myelomeningocele is a defect of the vertebral column protrusion of the meninges and herniation of the spinal cord through the defect. · Spina bifida with myeloschisis results from the failure of the caudal neuropore to close at the end of the fourth week of development. Newborn infants are paralyzed distal to the lesion. These defects usually occur in the cervical and/or lumbar regions and may cause neurologic deficits in the lower limbs and urinary bladder. Neural tube defects can be detected by the presence of alpha-fetoprotein (AFP) in the fetal circulation after the fourth week of development.

Anencephaly Anencephaly is the failure of the anterior neuropore to close, resulting in a failure of the brain to develop. Microcephaly Microcephaly (small head) results from microencephaly (small brain), or the failure of the brain to grow normally. This can be the result of exposure to large doses of radiation up to the sixteenth week of development, or from certain infectious agents (cytomegalovirus, herpes simplex virus, and toxoplasma gondii) Hydrocephalus Hydrocephalus is an accumulation of CSF in the ventricles of the brain, caused most commonly by stenosis of the cerebral aqueduct. In the absence of surgical treatment in extreme cases the head may swell to three times its normal size. Arnold-Chiari Malformation Arnold-Chiari malformation is herniation parts of the cerebellum (medulla oblongata and cerebellar vermis) through the foramen magnum of the skull. Fetal Alcohol Syndrome Ingestion of alcohol during pregnancy is the most common cause of infant mental retardation. It also causes microcephaly and congenital heart disease.

Neural Tube Related Birth Defects Anterior neural pore Posterior neural pore failure to close = anencephaly failure to close = spina bifida

ANENCEPHALY In anencephaly, the brain and skull are minute and the infant does not usually survive.

SPINA BIFIDA In spina bifida, the lower part of the spinal cord & nerve roots are underdeveloped and may lie uncovered by skin or the bony spine on the infant's back 35

MENINGIOMYELOCELE It is Spina Bifida with It is Spina Bifida with the meninges of the spinal cord appear on the back of the infant. the meninges of the spinal cord appear on the back of the infant. Such infants are left with withered, paralysed and anaesthetic lower limbs together with incontinence of the bowel and bladder. Such infants are left with withered, paralysed and anaesthetic lower limbs together with incontinence of the bowel and bladder.