For the real amazement, if you wish to be amazed, is this process. You start out as a single cell derived from the coupling of a sperm and an egg; this divides in two, then four, then eight, and so on, and at a certain stage there emerges a single cell which has as all its progeny the human brain. The mere existence of such a cell should be one of the great astonishments of the earth. People ought to be walking around all day, all through their waking hours calling to each other in endless wonderment, talking of nothing except that cell. Lewis Thomas (1979) Nervous Development This chapter will briefly introduce the key developmental principles associated with the formation of the nervous system and the relationships it forms with other body systems. You should be familiar with the basic structure of the nervous system central nervous system consisting of brain and spinal cord and peripheral nervous system comprised of cranial nerves and spinal nerves. You should understand the basic design of the special senses the eye and the internal ear. Understand the process of neurulation and the early development of the key components of the nervous system. Recognize how the neural tube and neural crest contribute to the formation of the central and peripheral nervous system. Understand the basic development of the eye from the neural tube, optic placode, and surrounding mesenchyme. Understand how the internal ear emerges from the otic placode and its association with the neural tube. 157
Review of Nervous System Anatomy Be comfortable with the following basic anatomy Basic structure of the nervous system Central nervous system Brain Spinal cord Peripheral nervous system Cranial nerves Spinal nerves Autonomic nervous pathways Basic eye anatomy Fibrous tunic Sclera Cornea Vascular tunic Choroid coat Ciliary body Iris Sensory tunic Retina Photosensitive layer Pigment layer Basic internal ear anatomy Boney labyrinth Membranous labyrinth Cochlear division Vestibular division E m b r y o l o g y L e c t u r e M a n u a l b y M a r k N i e l s e n 158
N e r v o u s D e v e l o p m e n t Origin of Nervous System Review of neurulation Neural plate Neural groove Neural fold Body ectoderm Neural tube Rostral neuropore Caudal neuropore Neural crest Neural canal 159
Development of the Neural Tube Ventricular zone Neuroblasts Future ependymal glioblasts Marginal zone Future white matter glioblasts Astrocytes Oligodendrocytes Intermediate zone Neuroblasts from ventricular zone Differentiate into neurons Mesenchyme Microglial cells Dura mater Crest cells Pia-arachnoid mater E m b r y o l o g y L e c t u r e M a n u a l b y M a r k N i e l s e n 160
N e r v o u s D e v e l o p m e n t Development of Spinal Cord Lateral wall intermediate zone Alar plate Dorsal gray horn Basal plate Ventral gray horn Lateral gray horn Sulcus limitans Roof plate Floor plate 161
Peripheral Nervous System Development Establishing the peripheral nervous pathways Neural Crest Cells Pseudounipolar sensory neurons Cell bodies Relation to neural tube Relation to somite Relation to gut tube Section depicting sensory neuron growth, left S2-S4, right T1-L2 Autonomic postganglionic motor neurons Sympathetic trunk migrations Sympathetic collateral migrations Sympathetic adrenal migrations Parasympathetic terminal migrations Enteric migrations Section depicting neural crest migration, left S2-S4, right T1-L2 Association with smooth muscle and gland cells Sympathetic postganglionic Parasympathetic postganglionic E m b r y o l o g y L e c t u r e M a n u a l b y M a r k N i e l s e n 162 Section depicting postganglionic neuron growth, left S2-S4, right T1-L2
N e r v o u s D e v e l o p m e n t Neural Tube Cells Skeletal motor neurons Cell bodies Relation to somite Autonomic preganglionic motor neurons Cell bodies Section depicting skeletal motor neuron growth, left S2-S4, right T1-L2 Sympathetic preganglionic Parasympathetic preganglionic Association with migrating neural crest cells Section depicting preganglionic neuron growth, left S2-S4, right T1-L2 163
Resulting Structure of Peripheral Nervous System Embryonic migrations and neuronal connections Section depicting complete neuronal development, left S2-S4, right T1-L2 E m b r y o l o g y L e c t u r e M a n u a l b y M a r k N i e l s e n 164
N e r v o u s D e v e l o p m e n t Section depicting complete neuronal development for levels C1-C8, T3-S1, and S5-Co1 165
Development of the Brain Brain vesicles Three primary vesicles Prosencephalon Mesencephalon Rhombencephalon Five secondary vesicles Telencephalon Diencephalon Mesencephalon Metencephalon Myelencephalon Brain flexures Ventral flexures Midbrain flexure Cervical flexure Dorsal flexure Pontine flexure E m b r y o l o g y L e c t u r e M a n u a l b y M a r k N i e l s e n 166
N e r v o u s D e v e l o p m e n t Ventricles Telencephalon Lateral ventricles Diencephalon Third ventricle Mesencephalon Cerebral aqueduct Metencephalon and myelencephalon Fourth ventricle Wall of neural tube in brain Expansion of roof plate Shifting of alar and basal plates Addition of new cell columns 167
Cranial Nerve Development Somitic motor cranial nerves Basal plate origin Oculormotor nerve Trochlear nerve Abducens nerve Hypoglossal nerve Pharyngeal arch cranial nerves Crest, lateral horn, and new column origin First arch - trigeminal nerve Second arch - facial nerve Third arch - glossopharyngeal nerve Fourth and sixth arches - vagus nerve Lateral plate mesoderm nerve Accessory nerve Special sensory cranial nerves Olfactory nerve Optic nerve Vestibulocochlear nerve E m b r y o l o g y L e c t u r e M a n u a l b y M a r k N i e l s e n 168
N e r v o u s D e v e l o p m e n t Arch 1 Arch 2 Arch 3 Arch 4 Neural tube, notochord, prechoral mesoderm Neural crest Premuscular mesoderm Muscle migration and development Motor neuron development Peripheral ganglia development Arch 1 Arch 2 Arch 3 Arch 4 169
Development of Parts of the Brain Medulla Pons and cerebellum Midbrain Diencephalon Cerebrum E m b r y o l o g y L e c t u r e M a n u a l b y M a r k N i e l s e n 170
N e r v o u s D e v e l o p m e n t Development of the Eye Eye components from neural tube Optic groove and vesicle Optic stalk Optic cup Outer layer of optic cup Inner layer of optic cup Intraretinal space Eye components from surface ectoderm Lens placode Lens pit Lens vesicle Mesenchyme and neural crest 171
Development of basic eye components Optic cup derivatives Outer layer of optic cup - pigment layer of the retina Inner layer of optic cup - neural layer of the retina Ciliary body Epithelium from optic cup Smooth muscle from mesenchyme Iris Epithelium from optic cup Connective tissue from neural crest Smooth muscle from optic cup epithelium Lens vesicle derivatives Subcapsular lens epithelium Primary and secondary lens fibers E m b r y o l o g y L e c t u r e M a n u a l b y M a r k N i e l s e n 172
N e r v o u s D e v e l o p m e n t Mesenchymal derivatives Central artery of the retina and hyaloid canal Tunica vasculosa lentis Lens capsule Suspensory ligaments of lens Anterior and posterior chambers Choroid and sclera Neural crest derivatives Vitreous humor Choroid and sclera Surface ectoderm derivatives External corneal epithelium Conjuntiva 173
Development of the Ear Key components Otic placode Otic pit Otic vesicle Morphogenesis of the otic vesicle Dorsal utricular part Endolymphatic duct and sac Utricle Semicircular ducts Absorption focus Ampulla Ventral saccular part Saccule Cochlear duct Surrounding mesenchyme E m b r y o l o g y L e c t u r e M a n u a l b y M a r k N i e l s e n 174