1. Neurulation
Neurulation is the formation of the vertebrate nervous system in embryos.
The notochord induces the formation of the CNS by signaling the ectoderm above it to form the thick and flat neural plate.
The neural plate then folds in on itself to form the neural tube, which will then later differentiate into the spinal cord and brain.

2. Neurulation (cont’d)
Different portions of the neural tube then form by 2 different processes in different species:
1. Primary Neurulation – the neural plate creases inward until the edges come into contact and then fuse.
2. Secondary Neurulation – the tube forms by hollowing out of the interior of a solid precursor

8. 24-hr Chick
22-23 days of Human

9. 23-26 days of Human

11. Formation of the Neural Tube
Secondary Neurulation
Occurs beyond the caudal neuropore
lumbar and tail region
Exclusive mechanism for fish
Starts with formation of medullary cord
Cavitation of cord to form hollow tube

12. Secondary Neurulation

13. Differentiation of Neural Tube
Major morphological changes: differentiation of brain vesicles and spinal cord
Differentiation of neural tube cells
Development of peripheral nervous system

15. Differentiation of Brain Vesicles
Anterior neural tube bulges: 3 primary vesicles:

16. Then further differentiation into 5 secondary vesicles:

18. Differentiation of the Neural Tube
Neural tube must maintain dorsal-ventral polarity
Sensory neurons- dorsal
Motor neurons- ventral
Accomplished by “inductive cascades”
Dorsal: BMPs from epidermisRoof plate cells in neural tubeTGF-B cascadeCell differentiation
Ventral: Sonic hedgehog from notochord and retinoic acid from somitesFloor plate cells of neural tubeshh gradientCell differentiation

20. Differentiation of the Neural Tube
Histological changes
Neural tube initially a single layer of cells: germinal epithelium
Cells are called neural stem cells
Neurons
Glial Cells: Myelin sheath

27. Glial Guidance

29. Development of Peripheral Nervous System
Divisions:
Autonomic NS
Sympathetic vs. Parasympathetic
Somatic NS
Anatomy
Sensory Neurons
Enter dorsal part of spinal cord
Soma located outside of cordDorsal ganglia
Form Dorsal Root of Spinal Nerve
Motor Neurons
Soma in ventral gray matter
Somatic NS: Neurons run directly from SC to muscle
Autonomic: 2 Neurons
Sympathetic: Ganglia near SC
Parasympathetic: Ganglia near or in or near organ

31. Development of Peripheral Nervous System
Anatomy
Spinal Nerves:Spinal Cord
Sensory fibers of somatic nervous system: Dorsal root
Preganglionic neurons of sympathetic system: Ventral root
Motor fibers of somatic nervous system: Ventral root
Cranial Nerves: Brain stem
Preganglionic neurons of parasympathetic system: Ventral root

34. Origin of PNS Cells From neural tube: From neural crest:
All motor neurons of somatic nervous system
Preganglionic neurons of autonomic system
From neural crest:
Sensory nerves and associated ganglia
Postganglionic neurons of autonomic system

37. Neural Crest Cells Induced by organizing cells of notochord
Main functional groups:
Cranial neural crest:
Bones and connective tissue of face
Tooth primordia
Thymus, parathyroid, thyroid glands
Sensory cranial neurons
Parasympathetic ganglia and nerves
Parts of the heart (cardiac neural crest)

38. Neural Crest Cells
A group of cells, which breaks away from the closing neural tube and populate the periphery (as opposed to the CNS, which will develop from the tube).
Neural crest will supply all neurons of the PNS and a variety of other peripheral structures, ranging from melanocytes to craniofacial bones to cells of the adrenals.
This population of cells separates from the neural plate shortly after the fusion of the neural folds, and streams of dividing cells begin their journey through the embryo.
The expression of which genes are turned off during this migratory stage?
And, this occurs for individual cells as well as for groups of cells

39. Neural Crest Cells
For neural crest cells, migratory pathway is particularly important in cellular determination, as location (or path) controls the availability of inducing factors for particular cell fates.
Making Cells
The use of chimeras has been invaluable in the study of individual cell fates.
What is a chimera?
Cells with a different genome; e.g., chick/quail mix – heterochromatin marker not found in chick.
[3H] thymidine labeling has helped in the delineation of migratory pathways and development potential of neural crest cells.

40. Neural Crest Cells
Interaction: Neural crest migration/movement is rigid and occurs in a ventral (1st cells give rise to ventral structures) - to- dorsal order in the head. Migratory pathways are linked to neuronal fate. What is the mostly likely result of transplant experiments when early cells will switch their fate? Extrinsic cues  ? Whether these come from the pathway itself or the final destination (target-derived cues) is not clear. As in the CNS, the earlier the cell is, the more pleuripotent it is (the more flexibility of fate).

41. Neural Crest Cells Main functional groups:
The stream of neural crest cells migrates via a ventral route to form:
Trunk neural crest:
Melanocytes (via the dorsal route)
Sensory neurons (DRG)
Sympathetic ganglia and nerves (ANS)
Medulla of adrenal glands (chromaffin cells)
Note that they migrate segmentally (sclerotome) – only in the rostral compartment.

43. Neural Crest Cells Migration: Epithelial to mesenchyme transition
Migrational pathways are established by juxtacrine signals:
Fibronection, laminin in ECM + integrins
Ephrin proteins: Restrict movement
Contact inhibition
Use of existing structures
Migration ceases when these signals are reversed

44. Migration of Neural Crest Cells
Unlike cells in the CNS, which migrate radially along glial fibers, neural crest cells “crawl along” independently (like fibroblasts). Motility is promoted by integrins – bind cell surface to ECM (how does this contrast with cadherins?) Prominent ECM components along neural crest cell pathway: fibronectin, laminin, collagen. The ECM provides attractive (permissive) cues for movements, as well as a substrate on which to bind. A set of repulsive cues in neighboring structures keeps cells in their precise migratory pathway

45. Migration of Neural Crest Cells
As the cells reach their destination, the expression of cadherins is once again activated (had been repressed during free movement)  cells aggregate into ganglia when they undergo terminal neuronal and glial differentiation. Side bar: retroviral labeling: A cell can be labelled permanently and heritably by injection with a retrovirus carrying a gene (e.g., β-galactosidase)  incorporated into cells’ DNA and then expressed. A substance, which will turn blue from the action of the enzyme, can then be introduced in a histochemical test.

47. Neural Crest Cells Differentiation:
Largely based on location along neural tube and their migration route:

48. Neural Crest Cells Differentiation: Migration routes along trunk:
Ventral pathway: cells move through anterior portion of somite toward ventral side of embryo
Cells become: sensory neurons, sympathetic ganglia, medulla of adrenal gland
Dorsolateral pathway: cells move between epidermis and somite
Cells become: melanocytes
Basic organization of the PNS is established by the migratory pathways of the neural crest cells

50. Neural Crest Cells Differentiation: How do they know what to become?
Most cells are pleuripotent- fate determined by position
Paracrine factors play a role
Example: Endothelin-3 and Wnt
Some exceptions: only NC cells from head make bone
Individual cells may differentiate early in migration

51. Differentiation of Neurons
Within nerve tube:
Dorsal Interneurons
Ventral Motor neurons

52. Differentiation of Neurons
Motor neurons:
Tissues they innervate depends on:
Anterior-posterior location along the nerve tube
When the cells were “born”

54. Axonal Pathways Spinal Cord 2. Sensory ganglia 3. Autonomic motor
neurons
1. Motor neurons from ventral root

55. Axonal Pathways Establishing pathways and connections:
Pathway selection
Target selection
Address selection

56. Axonal Pathways Pathway Selection
Pathway axon takes influenced by extracellular matrix and cells encountered: signals by both paracrine and juxtacrine factors:
Cell adhesion and contact guidance: Haptotaxis
Growth cone repulsion
Ephrin and semaphorin proteins
Labeled Pathways Hypothesis: Pioneer Neurons
Diffusible molecules

57. Axonal Pathways Target Selection:
Neurotrophins on target cells (muscle or another neuron)
Address selection: constructing the synapse