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Neurulation and Ectoderm
Chapters 12 & 13
Ectodermal Derivatives
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Three main derivatives
Neural tube
Skin
Neural crest
Neurulation is process that separates neuralectoderm, epidermal ectoderm, and
neural crest
I. Neurulation
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Formation of Neural Tube
Two major processes
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Primary Neurulation: folding of neural plate
Secondary Neurulation: cavitation from solid cord
Primary Neurulation anterior
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Fish & frogs: all but tail
Birds: anterior to hindlimbs (1-27th somite)
Mammals: anterior to sacrum
Secondary Neurulation posterior
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Fish & frogs: tail
Birds: posterior to hindlimbs (28th somite on back)
Mammals: sacrum and posterior
Figure 12.7 Secondary Neurulation in the Caudal Region of a 25-Somite Chick Embryo
I . Neurulation
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Primary Neurulation
Shaping & Bending of Neural Plate
Closing of Neural Tube
Separation from overlying Epidermis
Dorso-ventral Patterning
Brain Regions
Neuronal Development
Eye Formation
Neuronal Stem Cells
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I A. Primary Neurulation
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Midline cells elongate  neural plate
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Induction from notochord
Lateral cells flatten  epidermis
Edge of plate folds up (neural folds)
Folds move medially & fuse
I B. Shaping & Bending of Neural Plate
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Median Hinge Point (MHP)
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Cells anchor to notochord
Shorten, become wedge shaped
Bends plate inward
Dorsolateral Hinge Point (DLHP)
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Cells elongate via microtubules
Become wedge shaped (microfilaments at apex)
Bends edges inward
Colchicine inhibits elongation
Cytochalasin B inhibits wedge formation
Lateral epidermal ectoderm pushed medially
Folds forced together
I C. Closing of Neural Tube
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Anterior neuropore at front
Posterior neuropore at back
Failure to close Anterior Neuropore  anencephaly
Failure to close Posterior neuropore  spinal bifida
I D. Separation from overlying Epidermis
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Neural tube cells change CAM’s
Prior to neurulation
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E-cadherin (Ca++ dependent)
During neurulation
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N-cadherin (Ca++ dependent)
N-CAM (Ca++ independent)
I E. Dorso-ventral Patterning
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Dependent upon dual, opposing gradients
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Ventral floor (& notochord) secretes sonic hedgehog
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Diffuses dorsally, supresses gene expression for dorsal characteristics
Causes differentiation of motor neurons
Dorsal epidermis cells secrete BMP-4 and BMP-7
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Counteract sonic hedgehog
Stimulates TGF-β paracrine family
Adding second notochord induces second set of motor neurons
I F. Brain Regions
Starts with three primary regions/vesicles
• Prosencephalon (forebrain)
• Mesencephalon (midbrain)
• Rhombencephalon (hindbrain)
The 3 regions give rise to 5 secondary vesicles
• Prosencephalon (forebrain)
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Telencephalon
Diencephalon
Mesencephalon (midbrain)
Rhombencephalon (hindbrain)
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Metencephalon
Myelencephalon
I G. Neuronal Development
Dendrites
• Fine outgrowth, receptors
During 1st year after birth, enough dendrites form to make 100,000 connections for each
cortical neuron
• Average cortical neuron connects to 10,000 other neural cells
Axons
• Long extension of cell body, carry impulse away from cell body
• Forms as outgrowth of cell
• Elongates along length due to microtubules
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Colchicine causes length regression
Microspikes in growth cone area moved by microfilaments
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Cytochalasin B blocks microspikes
I H. Eye Formation
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Outgrowth of diencephalon
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Sonic hedgehog separates eye field into two bilateral fields
Mutation of sonic hedgehog can cause cyclopia
I I. Neuronal Stem Cells
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Evidence from mammals and birds that some cells in brain continue to divide after
embryonic development
BrdU (thymine analog) incorporates in cell undergoing replication (S period)
Brains of people, mice, birds treated with BrdU shows uptake and incorporation
II. Epidermis
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Basal layer gives rise to all cells of epidermis
Cells keratinize as move to surface
Melanin added in lowest two layers
8 weeks to move human cells from basal area to stratum corneum
Last 2 weeks in s.c.
Note arrow!
Two main growth factors stimulate epidermis
• TGF-α (transforming growth factor)
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From basal cells
Stimulate own division (autocrine)
KGF (keratinocyte growth factor)
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From underlying dermis (mesoderm)
Same as FGF-7
Autocrine or paracrine?
III. Neural Crest Cells
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Form from edge of neural plate
Form from edge of neural plate
Migrate laterally and ventrally
Cells migrate individually
Diverse cell fates
• Pigment cells
• Nerve cells and ganglia (spinal and autonomic ganglia)
• Cartilage and bone, especially of head, neck, face
• Adrenal medulla
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Connective tissue
Cranial Neural Crest
• Fates different from trunk neural crest
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Only cranial neural crest form cartilage and bone
Contribute to pharyngeal arch tissue
Last updated 15 April 2004
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