1. Have Competence: becoming neuroblasts
Chapter 9. Ectoderm
Neurulation: instructing the ectoderm to form neural tube, the rudiment of central nervous system
Four stages that pluripotent ectodermal cells have to pass to become Neural precursor cells/Neuroblasts and then Neurons
Have Competence: becoming neuroblasts
Specification: stay or leave from neuroblast characteristics (intermediate; reversible)
Commitment (determination): entering neural differentiation pathway (neuronal subtype has been selected)
Differentiation: exit cell cycle and express neuronal genes
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2. Figure 9.1 Major derivatives of the ectoderm germ layer
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3. Figure 9.2 Gastrulation and neurulation in a chick embryo (Part 1)
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4. Figure 9.3 Three views of neurulation in an amphibian embryo, showing early, middle, and late neurulae in each case
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5. Formation of the Neural Tube
Primary neurulation: generating neural tube at the neural plate (anterior)
Forms brain and spinal cord inside, epidermis at the outside, and neural crest cells in between
Neural plate: formation -> shaping -> bending -> closure
Secondary neurulation: forming the neural tube from the mesenchymal cells (posterior)

6. Primary Neurulation
1. Formation of the neural plate
Signals from dorsal mesoderm elongate anterior-posterior axis into columnar cells (~50% ectodermal cells)
Convergent extension movement (intercalation into less layers)
2. Bending of the neural plate
Medial hinge point (MHP): cells at the midline of the neural plate; anchor to the notochord and form a hinge
Dorsal hinge point (DHP): connection between neural plate cells and other ectoderm; wedging for the neural tube closure, which is dependent on intrinsic cytoskeletal rearrangement and external power supported by surface ectoderm

7. Primary Neurulation 3. Closure of the neural tube
Junctional region between both side of neural plate becomes the source of the neural crest cells
Chick: zipping initiates at the primitive midbrain region
Human: three sites of neural tube closure; defects 1/1000 births
spina bifida: a failure in the closure at the posterior neuropore
anencephaly: failures in the closure at the anterior neuropore
craniorachischisis: failure in the whole neural tube closure
Mediated by adhesion molecules: transition from E-cadherin to N-cadherin and N-CAM
Critical genes: Pax3, Shh, openbrain
Dietary factors: cholesterol (DHA), folic acid (vitamin B12)
Other factors: socioeconomic situation, seasonal variation, teratogens from the food (fumonisin, a fungal material)

8. Figure 9.6 Expression of N- and E-cadherin adhesion proteins during neurulation in Xenopus
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9. Differentiation of the Neural Tube : Anterior-Posterior Axis
Form brain and spinal cord structures in anterior-posterior axis from 3 primary cavity and then 5 secondary vesicles
Rhombomere: the primitive hindbrain regional unit in periodic swelling structures; divide different types of cranial nerves
r2 ganglia: form 5th (trigeminal) cranial nerve
r4 ganglia: form 7th (facial) and 8th (vestibulo-acoustic) cranial nerve
r6 ganglia: form 9th (glossopharyngeal) cranial nerve
Ganglia: clusters of neuronal cell bodies
Cerebrospinal fluid in ventricles: inflate brain size by osmotic gradient generation; Na+/K+ ATPase plays role by setting up osmotic gradient
Occlusion: separate brain and spinal cord

10. Figure 9.9 Early human brain development (Part 2)
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11. The vertebrate nervous system
Most of the vertebrate CNS comes from the neural plate.
Specification of vertebrate neuronal precursors also involves lateral inhibition.
Delta activates Notch which inhibits synthesis of neurogenin (related to the achaete-scute proteins).
The cell expressing neurogenin then expresses neuroD (a transcription factor required for neuronal differentiation). 2

12. Differentiation of the Neural Tube : Dorsal-Ventral Axis
Differential structures within brain
Hypothalamus at the ventral, while cortex at the dorsal
Differential neuronal distribution in spinal cord:
Dorsal: sensory neurons
Ventral: motor neurons
Intermediate zone: interneurons

13. Figure 9.13 Dorsal-ventral specification of the neural tube
Graded cues along dorso-ventral spinal cord
Notocord at the ventral side of neural tube: secrete Shh to induce floor plate cells, which in turn express Shh
Surface ectoderm at the dorsal side of neural tube: secrete BMPs to let roof plate cells, in turn, express TGFb family proteins (BMP4, BMP7, BMP5, Dorsalin, Activin)
Differential subsets of transcription factors dictate the fate of neural precursor cells
Ex) Nkx2.2: V0; Nkx6.1+Pax6: motor neuron; Pax6: V2 interneuron; Dbx2: V1 interneuron; Dbx1: D2 interneuron; Pax7: D1 interneuron
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14. Reciprocal suppression for boundary formation

15. Figure 9.13 Dorsal-ventral specification of the neural tube (Part 4)
Shh/dorsalin/Hb9 (motor neuron)
Nkx6.1/Pax6/Pax7
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16. Figure 9.14 Cascade of inductions initiated by the notochord in the ventral neural tube (Part 1)
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17. Differentiation of neurons
The nervous system
1) is the most complex organ system in the animal embryo.
2) provides communication via a network of varied neurons.
3) is connected by a process of action potential propagation and neurotransmitter release.
Supporting tissues (glia) include Schwann cells (surround peripheral neuron axons), oligodendrites and astrocytes (surround central neurons).
In human brain: 1011 neurons and 1012 glia
From common ancestor, ventricular (ependymal) cells in ventricular zone

18. Figure 9.15 Diagram of a motor neuron
Neurons
Dendrites
One neuron accommodates about 100,000 synapses with about 10,000 other neurons
Axons
Axons outgrow from cell body in variety of length led by growth cone
Axons navigate to their targets in recognizing numerous attractive or repulsive cues
Myelination of axons help electrical conductance: Oligodendrocytes (CNS) or Schwann cell (PNS)
Myelination defective mice: trembler (PNS: PMP20), jimpy (CNS: myelin proteolipid protein)
Secrete neurotransmitters
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19. Santiago Ramón y Cajal (1852 –1934)
Drawing of the neural circuitry of the rodent hippocampus
Drawing of a section through the optic tectum of a sparrow
Structure of the Mammalian Retina
Drawing of the cells of the chick cerebellum

20. Neural stem cells in the germinal epithelium
Neurons in brain: connected in network in structures of cortices (layers) and nuclei (clusters)
Germinal neuroepithelium contains rapidly dividing neural progenitor/stem cells, whose nuclei locate at the certain level according to their cell cycle status
Mitosis occurs at the luminal side after moving downward from the outside edge (cortical)
Stem cells remain at germinal neuroepithelium through the adhesion, while differentiating neurons leave after cell division
Symmetric horizontal cell division generates two stem cells at the luminal surface
Asymmetric cell division:
One having more Par3 and high notch activity remains as stem cells.
Other having less Par3 and less Notch, high Delta become a neuroblast(progenitor) and primed for neuronal differentiation
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21. Figure 9.19 Differentiation of the walls of the neural tube (Part 1)
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White layer
Gray matter

22. Figure 9.19 Differentiation of the walls of the neural tube (Part 2)
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23. Figure 9.19 Differentiation of the walls of the neural tube (Part 3)
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24. Neocortex formation
Migration of neuroblasts along radial glia from mantle zone to marginal zone results in the formation of neocortex.
Neocortex subdivided into six layers in differential functional properties – ex. thalamic inputs to layer IV, while outputs from layer VI.)
Neocortx subdivided into about 40 functional domains – visual cortex; auditory cortex
Inside-out layer formation (early-born neurons form the layer closest to the ventricle. Subsequent neuron travel greater distance to form the more superficial layers of the cortex)
Layer-specificity is instructed during cell division
Foxg1 (BF1) suppress the layer 1 neural fate

25. Determination of cortical laminar identity in the mouse cerebrum

26. Commitment occurs during S phase
Figure Determination of cortical laminar identity in the ferret cerebrum (Part 2)
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Commitment occurs during S phase

27. Figure Cortical neurons are generated from three types of neural precursor cells: radial glia cells, short neural precursors, and intermediate progenitor cells
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28. Adult neural stem cells
BrdU labeling showed the existence of proliferating cells in adult brain
Granular cell l ayer of hippocampus and subventricular zone for olfactory bulb (~0.3% of ventricle wall population)
Respond to Shh for a long time (about a year in mice)

29. Figure 9.25 Evidence of adult neural stem cells
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New born neuron / Synaptophysin
BrdU / Neurons / Glia

30. Human cerebral cortical map

31. Figure 9.27 Retention of fetal neuronal growth rate in humans
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32. Figure Dorsal view of the human brain showing the progression of myelination (“white matter”) over the cortical surface during adolescence
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33. Development of the vertebrate eye
General
Developed from optic vesicle and lens placode
Optic vesicle: differentiate into retina, pigment epithelium, and optic stalk (optic nerve)
Key early factors for optic vesicle formation: Six3, Pax6, and Rx1
Evolutionary conserved functions of the early determinants
External factors control the expression of the key factors: e.g. Shh suppresses expression of Pax6
Environmental adaptation of development: Eyeless cave fish Astyanax mexicanus have impaired expression of Pax6 because of the elevated action of Shh in the optic field
Axial patterning
Dorsal-ventral & Proximal-distal axes
Dorsal BMP and ventral Shh: result in the grated expression of key factors. e. g. optic stalk: Pax2 and Vax1 by Shh; PE: Mitf by BMP; retina: Rx
Anterior-posterior axis
Otx2 at the anterior head region -> Six3 and Pax6
Retinal differentiation
Sensory neurons: rod and cone photoreceptors
Interneurons: bipolar cells, horizontal cells, amacrine cells
Loss of competence of retinoblasts along developmental stages
Lens and cornea
Lens is differentiated from lens placode, but cornea is originated from neural crest cells
Express crystallins: by Sox2, Pax6, and L-Maf

34. Figure 9.29 Development of the vertebrate (Part 1)
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35. Figure 9.29 Development of the vertebrate (Part 2)
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36. Figure 9.30 Dynamic formation of the eye field in the anterior neural plate (Part 1)
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37. Figure 9.30 Dynamic formation of the eye field in the anterior neural plate (Part 2)
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38. Figure 9.31 Sonic hedgehog separates the eye field into bilateral fields
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39. Figure 9.32 Surface-dwelling (A) and cave-dwelling (B) Mexican tetras (Astyanax mexicanus)
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40. Figure 9.33 Expression of Rx genes in vertebrate retina development (Part 1)
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41. Figure 9.33 Expression of Rx genes in vertebrate retina development (Part 2)
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42. Figure 9.34 Retinal neurons sort out into functional layers during development
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43. Figure 9.36 Differentiation of the lens and anterior portion of the mouse eye
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44. Figure 9.36 Differentiation of the lens and anterior portion of the mouse eye (Part 1)
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45. Figure 9.36 Differentiation of the lens and anterior portion of the mouse eye (Part 2)
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46. Figure 9.36 Differentiation of the lens and anterior portion of the mouse eye (Part 3)
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47. Epidermis and the origin of cutaneous structures
Skin
Protective role
Consistently renewable
Dermis (inner) and epidermis (outer)
Epidermis
Two layers: periderm and basal layer (or stratum germinativum)
Epidermal stem cells: in the basal layer; asymmetrically divide to generate new epidermal cells, keratinocytes
TGFa: autocrine factor produced in the basal cells to stimulate division
Keratinocyte growth factor (KGF) and FGF7: migration and differentiation
Cutaneous appendages
Hairs, scales, feathers
Requires a series of reciprocal inductive events between dermal mesenchyme and the ectodermal epithelium to form placode, a precursor of hair follicles
Wnt signaling is critical for the follicle development
Hair follicular stem cells: at the bulge of hair follicle; enable the cycling of hair

48. Figure 9.37 Layers of the human epidermis
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49. Figure 9.38 Early development of the hair follicle and hair shaft
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50. Figure 9.39 Patterning of hair follicle placodes by Wnt10 and Dickkopf
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51. Figure 9.39 Patterning of hair follicle placodes by Wnt10 and Dickkopf (Part 3)
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52. Figure 9.42 Model of follicle stem cell migration and differentiation
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