Neuronal Development Dr. Donald Allen

Learning Objectives 1.Describe the normal development of the nervous system. 2.Describe the pathology of the following developmental disorders 1.Neural tube defects 2.Prenatal exposure to drugs 3.Cerebral palsy 4.Abnormal location of cells

3. Describe the changes that occur in the nervous system during infancy 4. Define and give an example of a critical period

Three Stages of Human Development in utero 1.Pre-embryonic 2.Embryonic 3.Fetal

Pre-embryonic Fertilization Cell Division Two groups of cells –Inner cell mass –Outer cell mass Nature|Genetics 2001

Inner cell mass Two layers form first – Third layer forms in middle –

Embryonic stage Organogenesis –Ectoderm –Mesoderm –Endoderm

Fetal Stage Further development Myelination

Development of the Nervous System Occurs during embryonic stage Two phases –Formation of neural tube –Formation of brain

Day 18 (after fertilization) Neural plate – thickening of ectoderm Extends from head to tail region A groove forms in center of neural plate, with folds on either side

Neural Plate

Day 22 The folds grow up and toward each other –Form neural groove When the folds touch, they form the neural tube Neural tube formation starts in the future cervical region –The groove closes rostrally and caudally

Neural Crest Some cells adjacent to the neural tube separate from the tube and the ectoderm Brown cells on previous slide

Neuropores Open ends of the neural tube Rostral neuropore closes by day 24 Caudal neuropore closes by day 26 If the neuropores do not close correctly, there will be neurological problems

Neuropores

More Day 26 The neural tube divides into two layers –Mantle layer –Marginal layer

Neural tube Forms almost all of nervous system The inside space in the tube – 4 weeks post-fertilization –Sulcus limitans –Alar plate (Association plate) –Basal plate (Motor plate)

Organization of brainstem Level of medulla and pons –Neural tube opens at top Same organization of cranial nerve regions as in spinal cord (somatic/visceral, sensory/motor)

Sacral Spinal Cord Secondary neurulation

Neural Crest Cells Cells that separate during formation of neural tube Cells migrate Form many different types of cells –Neurons –Glia –Non-neuronal cells

Neuronal and glial derivatives of neural crest cells Sensory neurons of somatic nervous system –Where are the cell bodies? Sensory neurons of some cranial nerves Schwann cells of PNS Postganglionic neurons of autonomic nervous system