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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 DevelopmentThis 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.

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Review of Nervous System AnatomyBe comfortable with the following basic anatomy

Basic structure of the nervous systemCentral nervous system

Brain

Spinal cord

Peripheral nervous systemCranial nerves

Spinal nerves

Autonomic nervous pathways

Basic eye anatomyFibrous tunic

Sclera

Cornea

Vascular tunicChoroid coat

Ciliary body

Iris

Sensory tunicRetina

Photosensitive layer

Pigment layer

Basic internal ear anatomyBoney labyrinth

Membranous labyrinthCochlear division

Vestibular division

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Origin of Nervous SystemReview of neurulationNeural plate

Neural groove

Neural fold

Body ectoderm

Neural tube

Rostral neuropore

Caudal neuropore

Neural crest

Neural canal

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Development of the Neural TubeVentricular zoneNeuroblasts

Future ependymal glioblasts

Marginal zoneFuture white matter glioblasts

Astrocytes

Oligodendrocytes

Intermediate zoneNeuroblasts from ventricular zone

Differentiate into neurons

MesenchymeMicroglial cells

Dura mater

Crest cellsPia-arachnoid mater

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Development of Spinal CordLateral wall intermediate zoneAlar plate

Dorsal gray horn

Basal plateVentral gray horn

Lateral gray horn

Sulcus limitans

Roof plate

Floor plate

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Peripheral Nervous System DevelopmentEstablishing the peripheral nervous pathways

Neural Crest CellsPseudounipolar sensory neurons

Cell bodies

Relation to neural tube

Relation to somite

Relation to gut tube

Autonomic postganglionic motor neuronsSympathetic trunk migrations

Sympathetic collateral migrations

Sympathetic adrenal migrations

Parasympathetic terminal migrations

Enteric migrations

Association with smooth muscle and gland cells

Sympathetic postganglionic

Parasympathetic postganglionic

Section depicting sensory neuron growth, left S2-S4, right T1-L2

Section depicting neural crest migration, left S2-S4, right T1-L2

Section depicting postganglionic neuron growth, left S2-S4, right T1-L2

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Neural Tube CellsSkeletal motor neurons

Cell bodies

Relation to somite

Autonomic preganglionic motor neuronsCell bodies

Sympathetic preganglionic

Parasympathetic preganglionic

Association with migrating neural crest cells

Section depicting preganglionic neuron growth, left S2-S4, right T1-L2

Section depicting skeletal motor neuron growth, left S2-S4, right T1-L2

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Resulting Structure of Peripheral Nervous SystemEmbryonic migrations and neuronal connections

Section depicting complete neuronal development, left S2-S4, right T1-L2

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Section depicting complete neuronal development for levels C1-C8, T3-S1, and S5-Co1

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Development of the BrainBrain vesiclesThree primary vesicles

Prosencephalon

Mesencephalon

Rhombencephalon

Five secondary vesiclesTelencephalon

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon

Brain flexuresVentral flexures

Midbrain flexure

Cervical flexure

Dorsal flexurePontine flexure

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VentriclesTelencephalon

Lateral ventricles

DiencephalonThird ventricle

MesencephalonCerebral aqueduct

Metencephalon and myelencephalonFourth ventricle

Wall of neural tube in brainExpansion of roof plate

Shifting of alar and basal plates

Addition of new cell columns

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Cranial Nerve DevelopmentSomitic motor cranial nervesBasal plate origin

Oculormotor nerve

Trochlear nerve

Abducens nerve

Hypoglossal nerve

Pharyngeal arch cranial nervesCrest, 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 nerveAccessory nerve

Special sensory cranial nervesOlfactory nerve

Optic nerve

Vestibulocochlear nerve

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Arch 1 Arch 2 Arch 3 Arch 4

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

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Development of Parts of the BrainMedulla

Pons and cerebellum

Midbrain

Diencephalon

Cerebrum

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Development of the EyeEye components from neural tubeOptic groove and vesicle

Optic stalk

Optic cupOuter layer of optic cup

Inner layer of optic cup

Intraretinal space

Eye components from surface ectodermLens placode

Lens pit

Lens vesicle

Mesenchyme and neural crest

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Development of basic eye componentsOptic cup derivatives

Outer layer of optic cup - pigment layer of the retina

Inner layer of optic cup - neural layer of the retina

Ciliary bodyEpithelium from optic cup

Smooth muscle from mesenchyme

IrisEpithelium from optic cup

Connective tissue from neural crest

Smooth muscle from optic cup epithelium

Lens vesicle derivativesSubcapsular lens epithelium

Primary and secondary lens fibers

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Mesenchymal derivativesCentral artery of the retina and hyaloid canal

Tunica vasculosa lentisLens capsule

Suspensory ligaments of lens

Anterior and posterior chambers

Choroid and sclera

Neural crest derivativesVitreous humor

Choroid and sclera

Surface ectoderm derivativesExternal corneal epithelium

Conjuntiva

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Development of the EarKey componentsOtic placode

Otic pit

Otic vesicle

Morphogenesis of the otic vesicleDorsal utricular part

Endolymphatic duct and sac

Utricle

Semicircular ductsAbsorption focus

Ampulla

Ventral saccular partSaccule

Cochlear duct

Surrounding mesenchyme