Development of the Nervous System Flashcards
What will the ectoderm form?
Entire nervous system- both peripheral and central
epidermis of the skin
placodes in the head region
Two parts of the ectoderm
Midline columnar epithelium- will form neural plate
lateral cuboidal epithelium- epidermis and sensory placodes
What is the first structure to form in the establishment of the nervous system? + stages
Neural plate- just cranial to the primitive node
- Formation of the neural plate is induced by the organiser (the primitive node)
- ectodermal cells differentiate into a thick plate of pseudostratified, columnar neuroepithelial cells- neuroectoderm
- Forms at the cranial end then grows in the cranial to caudal direction
How does the organiser induce neural induction?
Chordin secreted, which forms a secondary axes
secretes 10 transcription factors.
Also antagonises the Bmp signalling pathway, changing the formation surface ectoderm to midline neuroectoderm
Organiser’s importance evidence
Transport primitive node to another organism, induces another neural plate- thus two are formed in the organism
Neurulation definition
Formation of the neural tube
Shape of neural plate before neurulation (day 22)
Wide cranial portion that will form brain
narrow caudal portion that will give rise to spinal cord
Why does the caudal portion grow rapidly?
Somite pairs are continually, rapidly added so the spinal cord lengthens faster than the cranial neural plate
4 stages of neurulation
- formation of neural plate
- shaping of the neural plate
- bending of the neural plate
- closure of the neural groove
Stages of bending of neural plate
- Neural folds formed at lateral edges, consisting of neuroepithelium and adjacent surface ectoderm
- during folding neural folds elevate dorsally by rotating about a central pivot point overlying the notochord cells called the median hinge point
- Neural groove- groove delimited by the bending neural plate- is formed
- neural folds brought into apposition along the dorsal midline
What occurs differently in the cranial region + why?
Neural plate at the future brain level is much broader
- dorsolateral hinge points form, that allow the neural folds to come together in the dorsal midline
What is the next stage?
Closure of the neural groove
Stages of closure of the neural groove
Day 22
- Neural crest cells are formed at the interface between the epithelial layers
- arise from the neural folds by undergoing EMT
- The neural folds adhere to one another. Begins in the cervical region then moves cranially and caudally
- The cells rearrange in the folds to form two separate epithelial layers- roof plate of the neural tube and the overlying surface ectoderm
- neural tube formed
- neural crest cells then migrate into the mesoderm
What are the ends of the neural tube called?
Cranial neuropore and caudal neuropore
Example of a neural defect where the neural tube does not close properly
Anencephaly- absence of a major portion of the brain, skull and scalp due to the cranial neuropore failing to close
What is required for the shaping of the neural plate + their origin?
Forces are generated by changes in cell behaviour, particularly cell shape, position and number.
Intrinsic neurulation forces- generated within the neural plate itself
Extrinsic neurulation forces- arise outside the neural plate
What does the shaping of the neural plate include + explained briefly?
Convergent extension- transverse narrowing and longitudinal lengthening.
Cell elongation- Cells thicken apicobasally as they get taller, which requires the presence of paraxial microtubules. Contributes to narrowing, as cell diameters decrease to compensate for the elongation
Cell rearrangement- cells move from lateral to medial in the neural plate, thus increasing its cranial-caudal length
cell division- many cells orientated to place daughter cells longitudinally
Structure of hinge points
localised regions of neuroepithelial cells that change their chai to wedge like and become firmly attached to an adjacent structure through the deposition of ECM
Thus median hinge point firmly attached to notochord and dorsolateral hinge points to the adjacent surface ectoderm of the neural folds
Explain neural determination
Signals from notochord and mesoderm- secrete noggin, chordin and follistatin.
All factors bind BMP-4, an inhibitor of neuralation
Explain patterning of neural tube
Wnt pathway involved in convergent extension
Shh, secreted by notochord, initiates the formation of the median hinge point
Notochord produces chorine, a Bmp antagonist which results in the induction neural crest cells
What are neural crest cells?
Unique population of cells that arise from the dorsal part of the neural tube
Migration of neural crest cell stages
- undergo EMT
- migrate from various craniocaudal levels of neural folds into the underlying mesoderm
- Migrates via two pathways
- different neural crest cells react different to morphogens, thus become repulsed or attracted by different distances
Two pathways explained
Ventral- through anterior part of somites- become sensory/dorsal root autonomic ganglia, sympathetic and enteric neurones, Schwann cells and cells of adrenal medulla
Dorsal- through dermis, enter ectoderm through holes in basal lamina- form melanocytes
How are neurones formed?
- Neural tube is the embryonic precursor to the central nervous system
- Shh is secreted by the notochord and forms a concentration gradient from the ventral cells
- high concentrations induce floor plate
- high conc- ventral neurons induced, lower conc, more intermediate, lowest conc, more dorsal neurones
Secondary neuralation stages
- Neural tube extends past the caudal neuropore, due to the level of closing neuropore being merged with the tail bud
- tail bud undergoes morphogenesis to form the most caudal extent of the neural tube
- Neural crest cells and somites develop in this caudal region
- medullary cord- condensation of central tail bud formed
- fuses with more cranial neural tube, after cavititation.
- neural crest cells then arise and form caudal spinal ganglia
- lateral tail bud cells undergo segmentation to form the caudal somites
- grows into sacral and coccygeal regions
Explain spina bifida
Incomplete formation of neural arch- neural groove not completely formed. Can cause a sac of fluid from a gap in the spine to protrude.
What morphogens control the fates of the neural tube?
Shh secretion induces different ventral fates
BMP secretion induces different dorsal fates
Segmental segregation explained
Spinal nerve growth from the neural tube splits the sclerotome into rostral and caudal segments- known as resegmentation of the sclerotome.
Caudal half and rostral half between one nerve fuse to produce the vertebrae
Segmentation of the nerves definition
Develops the separate central nervous system and peripheral nervous system
Guides the developing axons
Segmentation of nerves process
- Axons of motor and sensory neurones develop from the neural tube region and through each of the successive somite’s anterior
- The division of the scelotome into anterior and posterior parts enable the nerves to leave at different stages
Where does the dorsal root ganglion originate from?
Neural crest cells
What is neural patterning?
Neural tube becomes patterned in a dorsal- ventral axis
Explain stages of neural patterning
- Floor plate cells form at the ventral midline, as well as the more dorsally located motor neurones and interneurons- as a result of Shh secretion from the notochord
- the different concentrations of Shh are responsible for the identity of different neuronal progenitor cells
- at the dorsal end of the neural tube, BMPs are responsible for neuronal patterning- initially secreted by overlying ectoderm
Where do the cranial ganglia originate from?
Neural ectoderm of brain and spinal cord
How is the spinal cord structured and formed in relation to ventral root?
- floor plate- thin region that overlies the notochord in neural tube secretes Shh and ventrally patterns spinal cord
- basal plate- thick wall on either side of floor plate- Ventral horn develops here and extends axons out to innervate developing muscle
4 main domains of neural crest cells + what they contain
Cranial- craniofacial mesenchyme- cranial ganglia
Trunk- melanocytes, dorsal root ganglia (travel through anterior portion of each sclerotome)
Vagal and sacral- ganglia of enteric nervous system and parasympathetic ganglia
cardiac- melanocytes, cartilage, connective tissue, regions of the heart- Schwann cell
Two ways in which neural crest cells migrate
Repulsive- move away from chemical produced
Attractive- move towards
Which way do neural crest cells migrate?
Rostral half of somites- sensory and sympathetic neurones
Dorsolateral- between epidermis and dermamyotome
