Topic 6: Neurulation Flashcards by Miyaki Jan Lim

Laying down the rudiments of the CNS (brain & spinal cord)

Defining the various brain divisions

Marks the start of organogenesis

Continuing the establishment of the embryonic axis

Neurulation

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Mesenchymal type

Migrates to different parts of the body and gives rise to different structures (ex. ganglia)

Generating the neural crest cells (NCCs)

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The specification of the dorsal ectoderm by the inducing effect of the chordamesoderm / axial mesoderm / notochordal process

Dorsal ectoderm →→ Neural plate

Neural induction

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Expressing the genes coding for N-cadherin
Neural cell adhesion protei

Presumptive neural plate cells

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Formation of Neural Plate rundown

Morphogenic changes:
Shaping bending, and folding

Result:
Neural plate bends and edges elevate

The neural plate cells undergo columnarization

Presumptive neural plate cells:
Low columnar cells → tall columnar cells
Tall columnar cells make it distinct from surrounding cells

Presumptive epidermal cells
Flat squamous cells

Eventually, the N-cadherin bearing neural cells separate from the E-cadherin-containing epidermal cells

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Formation of the Neural Tube by Closure of the Neuropore

run down

The neural folds then merge, cut off the neural groove, and form the neural tube

The closure of the neural tube disconnects the neural crest cells from the epidermis

Neural crest cells will move away from the crest ready to undergo mesenchymal migration via amoeboid movement

Migration of the neural crest cells is the final step in the neural tube closure
The neural crest cells have neither cadherin, and they disperse

Other mesoderm cells differentiate into the dorsal mesoderm → somites

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Expressing the genes coding for E-cadherin
Epithelial cell adhesion protein

Presumptive epidermal cells

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Dorsal-ventral specification of the Neural tube

Two primary signaling centers

Ectodermal cells of the epidermis produce BMP4 and BMP7 (TGF-β) → roof of the neural tube is exposed

Notochordal cells produce Sonic hedgehog protein → floor of the neural tube is exposed

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Formation of the Neural Groove and Neural Folds

run down

Neural plate deepens and forms the neural groove

Its margins are increased by convergent extension
Cell rows shift into one another and form the neural folds

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Two concentration gradients are established
during

Different concentration gradients activate the expression of different sets of genes
↓
Different exposure level of cells result to different identities
↓
Cells then differentiate to become interneurons and motor neurons

Notochord eventually degenerates → persists as nucleus pulposus of the vertebral discs

The neural tube is temporarily open at both ends

The cavity of the neutral tube (neurocoel) later forms the ventricles of the central nervous system

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Dorsal-ventral specification of the Neural tube

Two primary signaling centers

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Extent of use depends on the class of vertebrates

Humans

2° neurulation
Around level of 35th somite

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what sides do these neurons form
Motor neurons:
Sensory neurons:
Interneurons:

Motor neurons: ventral side
Sensory neurons: dorsal side
Interneurons: in-between the gradient

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Opening at the caudal end
Spinal cord will develop towards the posterior region
Closes around 28th day

Posterior neuropore

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Neural Tube Defects (NTDs)

Spina bifida types
More severe than occulta
Spinal cord bulges out dorsally
With neurological disorder

Spina bifida cystica

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Opening at the head end
Brain will develop in the anterior region
Closes around 24th to 26th day

Anterior neuropore

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Neural Tube Defects (NTDs)

Spina bifida types

Obscure / concealed
Mild form, no pain, no neurological disorder

Spina bifida occulta

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Neural Tube Defects (NTDs)

Failure of the posterior neuropore to close
Different degrees of severity

Spina bifida

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Neural Tube Defects (NTDs)

Failure of the anterior neuropore to close
Portions of the forebrain is missing
Usually fatal

Anencephaly

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Allows the passage of amniotic fluid

Posterior neuropore

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Primary vs Secondary Neurulation

Primary neurulation In human embryo:

Development of the neural tube as induced by the notochord and the mesoderm

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Secondary Neurulation

In avians

Secondary neurulation in the caudal region of a 25-somite chick embryo

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Primary vs Secondary Neurulation

Development of the neural tube as induced by the notochord and the mesoderm

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Primary neurulation

Extent of use depends on the class of vertebrates

Fishes

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Exclusively secondary neurulation

Primary vs Secondary Neurulation The caudal end of the neural tube then develops into the neural notochord then canalized Starts out as medullary cord then cavitates Medullary cord undergoes epithelialization

Secondary neurulation

Extent of use depends on the class of vertebrates Birds

Anterior region of n.t. 1° neurulation Caudal end to the 25-27th somite 2° neurulation

Secondary Neurulation In human embryo:

Ends in the 6th week; around level of 35th somite

Extent of use depends on the class of vertebrates Amphibians

Mostly 1° neurulation Tail end 2° neurulation

Primary neurulation: neural tube formation in the chick embryo run down

Involves hinge point cells to form the neural tube Cells anchored in hinge point cells undergoes changes in shape (wedging) Cells or the neural plate can be distinguished as elongated cells in the dorsal region of the ectoderm. Folding begins as the medial neural hinge point (MHP) cells anchor to the notochord and change their shape, while the presumptive epidermal cells move towards the center. The neural folds are elevated as presumptive epidermis continues to move toward the dorsal midline. Convergence or the neural folds occurs as the dorsolateral hinge point (ULHP cells become weave-snapped and epidermal cells push toward the center. The neural folds are brought into contact with one another, and the neural crest cells link the neural tube with the epidermis. The neural crest cells then disperse, leaving the neural tube separate from the epidermis.

Neural Tube Formation run down

Starts out as elongated primitive streak Neurulation, Cephalic end → well-advanced Neurulation, Caudal end → still undergoing late gastrulation ↓ Regionalization of the tube

Neural tube formation does not occur simultaneously throughout the ectoderm

true

Visible in 24-hr chick embryo (whole mount)

Neural tube formation

Differentiation of the Neural tube Occurs simultaneously in 3 ways:

Gross anatomical level Tissue level Cellular level

Differentiation of the Neural tube Occurs simultaneously in 3 ways: Neuroepithelial cells → numerous types of nerve cells (neurons) & supportive cells (neuroglial cells)

Cellular level

Periodic swellings on the rhombencephalon With the neural crest cells and will determine where cranial nerves will arise

Rhombomeres

Differentiation of the Neural tube Occurs simultaneously in 3 ways: NT and neurocoel → chambers of the brain & spinal cord

Gross anatomical level

Differentiation of the Neural tube Occurs simultaneously in 3 ways: Cells in the wall of NT → functional regions of the brain & spinal cord

Tissue level

Early human brain development The __ primary brain vesicles are subdivided as development continues

three

r2 to r6

R2 - cn5 R4 = cn7 & 8 R6 = cn9 R3 and R5 = will undergo apoptosis

hindbrain subdivisions

rhombomeres

Will form the ganglia

rhombomeres

Developmental events of the Human embryo Visible primitive streak with the primitive node

16D embryo

Developmental events of the Human embryo Pear-shaped Cephalic end broader than caudal end

18D embryo

Developmental events of the Human embryo Amnion is cut to expose dorsal view Neural plate visible

19D embryo (approx.)

Developmental events of the Human embryo Neural groove & fold formed Somites forming lateral to the neural structures Fusion of the neural folds begins in the cervical region (level of 5th somite)

20D embryo (approx.)

Developmental events of the Human embryo 7 distinct somites on the sides of the closed neural tube

22D embryo (approx.)

Developmental events of the Human embryo Pericardial bulge on each side of the midline in the cephalic part Anterior neuropore and posterior neuropore allow passage of amniotic fluid

23D embryo (approx.)

Developmental events of the Human embryo 25 somites 3 p.a. formed Sensory placodes formed (lens & otic) Closure of the posterior neuropore

28D embryo (approx.)

Developmental events of the Human embryo 18-20 somites 14-somite stage; bulging pericardial structure 1st & 2nd p.a. forming Closure of crania neuropore

25D embryo (approx.)

Bias: ventral cells with greater expression of molecular maker _______ than animal cells

(Epi1)

Neural Induction as a Multistep Process

Dorsal mesoderm → DLB (organizer) → sends planar induction signals at gastrula stage; after involution → formation of chordamesoderm Chordamesoderm → sends vertical signals to the overlying prospective neuroectoderm/dorsal ectoderm

In later cleavage, only dorsal blastomeres are able to induce animal cells to form _______

dorsal mesoderm

is primed to respond to signal stimuli

Neurectoderm

Axis Induction Natural inhibitor of dorsal axis formation

Bone morphogenetic protein (BMP4)

Inductive signal may act by ________ Interfere with inhibitory signal that normally blocks a default program

disinhibition

Topic 6: Neurulation Flashcards

(55 cards)