CNS DEVELOPMENT

Stages in Neural Tube Development Neural plate. Neural plate. Neural folds. Neural folds. Neural tube. Neural tube.

Time-Line Formation of nervous system occurs during the embryonic stage: Formation of nervous system occurs during the embryonic stage: End of second week to end of eighth week.

Early Human Development

Neural Plate

Neural Fold

Neural Tube

Time-Line Superior (anterior or cranial) neuropore closes by day 27. Superior (anterior or cranial) neuropore closes by day 27. Inferior (posterior or caudal) neuropore closes by day 30. Inferior (posterior or caudal) neuropore closes by day 30.

Subdivision of Cranial End of Neural Tube Tripartite brain. Tripartite brain. Pentapartite brain. Pentapartite brain.

Tripartite Brain Prosencephalon. Prosencephalon. Mesencephalon. Mesencephalon. Rhombencephalon. Rhombencephalon.

Pentapartite Brain Prosencephalon: Prosencephalon: Telencephalon (most anterior). Diencephalon. Mesencephalon. Mesencephalon. Rhombencephalon: Rhombencephalon:Metencephalon.Myelencephalon.

Telencephalon Primordia Lumina: Lumina: Lateral ventricles (I, II). Floor: Floor: Basal ganglia (nuclei). Olfactory lobes and nerves. Roof: Roof: Cerebral hemispheres.

Diencephalon Primordia Lumen: Lumen: Third ventricle. Roof: Roof:Epithalamus. Walls: Walls:Thalamus. Floor: Floor: Hypothalamus and infundibulum.

Mesencephalon Primordia Lumen: Lumen: Cerebral aqueduct (of Sylvius). Roof =Tectum: Roof =Tectum: Superior and inferior colliculi. Floor: Floor:Tegmentum.

Metencephalon Primordia Lumen: Lumen: Part of fourth ventricle. Roof: Roof:Cerebellum. Floor: Floor:Pons.

Myelencephalon Primordia Lumen: Lumen: Rest of fourth ventricle. Main part: Main part: Medulla oblongata. Roof: Roof: Posterior choroid plexus.

Histogenesis of Neural Tube Initial tube wall = Initial tube wall = Pseudostratified epithelium: Single layer of cells, but cells are of different heights. All cells are in contact with a basement membrane. Outermost membrane = Outermost membrane = External limiting membrane.

Histogenesis of Neural Tube Some neuroepithelial cells remain attached to the basement membrane and will form a single layer of ependymal cells that will line the entire ventricular system and the neural canal. Some neuroepithelial cells remain attached to the basement membrane and will form a single layer of ependymal cells that will line the entire ventricular system and the neural canal.

Histogenesis of Neural Tube Tube differentiates into two concentric rings by day 26: Tube differentiates into two concentric rings by day 26: Mantle layer and marginal layer.

Histogenesis of Neural Tube Other cells lose contact with the basement membrane and will migrate past the ependymal cells to form a new outer layer of densely packed cells collectively called the: Other cells lose contact with the basement membrane and will migrate past the ependymal cells to form a new outer layer of densely packed cells collectively called the: Mantle layer: Cells that make up the mantle layer are: Cells that make up the mantle layer are:NEUROBLASTS. Note that mantle layer is still covered by the external limiting membrane. Note that mantle layer is still covered by the external limiting membrane.

Spinal cord 10mm pig embryo cross-section © 2006 Marshall Andersen

Histogenesis of Neural Tube Neuroblasts in the mantle layer will begin to grow processes (axons) that will form a new outer layer: Neuroblasts in the mantle layer will begin to grow processes (axons) that will form a new outer layer: Marginal layer. The marginal layer is also located beneath the external limiting membrane. The marginal layer will form the white matter of the spinal cord and the brain. The marginal layer will form the white matter of the spinal cord and the brain. The mantle layer forms the gray matter of the brain and spinal cord (except for the cortices). The mantle layer forms the gray matter of the brain and spinal cord (except for the cortices).

DEVELOPMENTAL ANOMALIES

Anencephaly Failure of cranial end of neural tube to close. Failure of cranial end of neural tube to close.

Arnold-Chiari deformity Inferior cerebellum and medulla are elongated and protrude into vertebral canal. Inferior cerebellum and medulla are elongated and protrude into vertebral canal. Medulla and pons are small and deformed. Medulla and pons are small and deformed. Hydrocephalus. Hydrocephalus. Malformation of lower cranial nerves: Malformation of lower cranial nerves:Deafness. Tongue, facial muscle, lateral eye movement weakness.

Spina Bifida Occulta Results from a failure of the inferior neuropore to close. Results from a failure of the inferior neuropore to close. Vertebral arch(-es) fails to develop in caudal area. Vertebral arch(-es) fails to develop in caudal area. Spinal cord function is usually normal. Spinal cord function is usually normal.

Spina Bifida Cystica Characterized by a sac-like cyst at the caudal end of spine. Characterized by a sac-like cyst at the caudal end of spine. Spinal cord and/or meninges may be found in the cyst. Spinal cord and/or meninges may be found in the cyst. Spinal cord function may be impaired. Spinal cord function may be impaired. May be lower extremity dysfunction. May be lower extremity dysfunction. Bladder and bowel function may be impaired. Bladder and bowel function may be impaired.

Meningocele Form of spina bifida cystica. Form of spina bifida cystica. Only meninges found in sac. Only meninges found in sac. Spinal cord function may be impaired. Spinal cord function may be impaired. Signs and symptoms vary depending on location and severity of malformation. Signs and symptoms vary depending on location and severity of malformation.

Meningomyelocele Form of spina bifida cystica. Form of spina bifida cystica. Both meninges and spinal cord are found in sac. Both meninges and spinal cord are found in sac. Always results in abnormal growth of spinal cord. Always results in abnormal growth of spinal cord. Lower extremity paralysis. Lower extremity paralysis. Bowel and bladder dysfunction. Bowel and bladder dysfunction. Loss of sensation to lower limbs. Loss of sensation to lower limbs.

Myeloschisis Failure of caudal neural folds to close. Failure of caudal neural folds to close. Most severe of the defects. Most severe of the defects.

Holoprosencephaly Failure of prosencephalon to divide into two cerebral hemispheres. Failure of prosencephalon to divide into two cerebral hemispheres. Often associated with facial deformities: Often associated with facial deformities: Single orbit with two eyes or one eye or no eye. Proboscis-type nose located above eye. Cleft lip and palate.