Neurulation And Development Of The PNS Flashcards
What is ectoderm?
Neuroectoderm + surface ectoderm
What significant event happens at the beginning of week 3?
Notochord induces neuroectoderm to form the neural plate
What is neurulation?
Process by which the notochord and neuroectoderm are made physically and functionally distinct (formation of the neural tube)
What is primary neurulation?
Formation of general embryonic ectoderm into a neural plate -> lateral folding of the neural plate creates neural folds and a neural groove -> NT is formed by fusion of the neural folds and outer ectoderm
Where does neurulation start?
At the 5th somite
Gastrulation ends with what?
Formation of the tail bud (caudal eminence)
Primarily neurulation is complete with what?
Closure of caudal neuropore at somite 31 but the NT extends caudally into sacral and coccygeal regions
What is secondary neurulation?
Formation of the NT from the tail bud
What develops from the tail bud?
Caudal most NT, neural crest cells and somites
Describe secondary neurulation
Tail bud cells condense into a solid mass known as the medullary cord —> medullary cord cavitates forming a lumen —> merges with central canal of NT
What do NCC arise from?
The roof of the NT and are migratory
What happens when the lateral tail bud cells segments?
They form caudal somites
What does the caudal notochord grow into?
The sacral, coccygeal and tail regions
What are neural crest cells?
Neuroectodermal cells positioned along dorsal neural folds bilaterally along the entire cranial caudal NT
What process do NCC undergo?
Epithelial mesenchymal transition (EMT)
What happens to NCC when they undergo EMT?
Lose contacts and become migratory
Migrate ventrally from their dorsal positions along the length of the embryo
Give rise to numerous derivatives
List some NCC derivatives
Melanocytes, odontoblasts, Schwann cells
Spinal and autonomic ganglia
Enteric NS
Suprarenal (adrenal) medulla
What are neurocristopathies?
Defects in NCC
Ex. Goldenhar syndrome, micrognathia, TCS, Hirschsprung disease and piebaldism
What is Goldenhar syndrome (hemifacial macrosomia)?
A type of neurocristopathy characterized by incomplete development of the eyes, ears and nose on one side of the body
What is micrognathia?
A type of neurocristopathy in which a child has a very small jaw
What is TCS or mandibulofacial dystosis?
A type of neurocristopathy characterized by incomplete development of bones and tissue of the face
What is Hirschsprung disease?
A type of neurocristopathy that causes mega colon leading to difficulty passing a stool
What is piebaldism?
A type of neurocristopathy in which a person develops unpigmented or white patches of skin or hair
Due to a lack in melanocytes/NCC
What do the meninges develop from?
NCC and mesenchyme between 20-35 days
NCC and mesenchyme migrate and surround the NT to form what?
The primordial meninges (meninx primitiva)
What does the primitive meninx differentiate into?
The ectomeninx and endomeninx at approximately 34-48 days of development
What occurs between 45-60 days of development in regards to the ectomeninx of the meninges?
Becomes more compact and forms spaces for future venous sinuses (allows blood to drain from the CNS)
What occurs between 45-60 days of development in regards to the endomeninx of the meninges?
Becomes more reticulated and subarachnoid space/cisterns appear
The spinal ectomeninx dissociates from the vertebral bodies and gives rise to what?
The spinal epidural space which is essential for epidurals
True or false: Neural tube closure does not occur simulatanously with the length of the NT
True - cranial regions are more advanced than caudal regions (still undergoing gastrulation)
What is the result of the NT closure not occurring simultaneously with the length of the NT?
Results in the formation of the cranial and caudal neuropores
Where is NT closure initiated?
At several locations along the AP axis
What are some examples of cranial neural tube defects?
Anencephaly, encephalocele, meroencephaly and cranioraschischisis
Due to the anterior neuropore failing to close
What is anencephaly?
A cranial NTD characterized by absence of a major portion of the brain, skull, or scalp
What is encephalocele?
A cranial NTD that can be with or without brain tissue
Characterized by a sac-like protrusion or projection of the brain and the membranes that cover it through an opening in the skull
What is meroencephaly?
A cranial NTD that is a rare form of anencephaly characterized by malformed cranial bones leaving brain tissue open to amniotic fluid and stopping brain development
What is cranioachischisis?
A cranial NTD in which the entire brain and spinal cord remain open
What are the biggest factors causing NTDs?
Genetic, nutritional (especially folic acid intake) and environmental
What are examples of spinal NTDs?
All a type of spina bifida including spina bifida occulta, meningomyelocele, meningocele and myeloschisis
Due to the posterior neuropore failing to close
What is spina bifida occulta?
A type of spinal NTD in which the neural arch does not fuse
Characterized by a tuft of hair on the back and sx include back pain
What is meningomyelocele?
A type of spinal NTD that involves protrusion of the meninges and spinal cord but remain covered
What is meningocele?
A type of spinal NTD that involves protrusion of the meninges without spinal cord involvement (Tx = surgery)
What is myeloschisis?
A type of spinal NTD (most severe) in which the meninges and spinal cord protrude
Nervous tissue is completely bare/uncovered
What is the ventricular zone of the NT?
Stem cells positioned closest to the lumen of the tube
Cells divide and migrate
Will become ependyma
What is the mantle/intermediate zone of the NT?
Migrating cells that undergo differentiation into neurons and/or glia
Extend axons away from the lumen toward the outer surface while cell bodies stay put
Becomes the gray matter
What is the marginal zone of the NT?
Lacks neuronal cell bodies but contains axon bundles that will eventually be myelinated
Forms the white matter
Which cell types arise from neuroectoderm during differentiation of the NT zones?
A polar neuroblast -> neuron
Glioblast -> macroglia or oligodendrocytes
Ependyma
What significant event occurs at the end of the 4th week of development?
Mantle layer of the NT reorganizes into an alar (dorsal) plate and a basal (ventral) plate which are divided into these regions by the sulcus limitans
What structure dorsally connects the right and left alar plates?
Roof plate
What structure connects the left and right basal plates?
Connected ventrally by a floor plate
True or false: alar and basal plates extend the entire length of the SC
True - then undergo differentiation due to signaling from roof and floor plates
What is the nervous tissue composed of once the NT closes?
Germinal neuroepithelium that spans the full width of the NT from lumen/canal to outer surface and will give rise to a range of cell types in the brain and SC
How does increased SHH concentrations direct neuron identity?
Induces motor neuron development
How do decreased SHH concentrations direct neuron identity?
Multiple interneurons are induced
Shh activates what downstream targets and for what type of neuron formation?
Shh activates Nkx2.2 and Nkx6.1 for ventral neuron formation
What is the function of BMP/TGF-beta signaling?
Counteracts Shh gradient
What does BMP4 activate?
Pax3 and pax7 which are responsible for induction of dorsal associated neurons
Which type of neurons form first: motor or sensory?
Motor neurons form before sensory neurons
What are interomedial columns?
The most dorsal cells of the basal plate that undergo segregation to form the interomedial columns —> eventually form the lateral horns
What is T1-L3?
Visceral MNs of sympathetic division (ANS)
What is S2-S4?
Visceral MNs of parasympathetic division (ANS)
CNS derivatives form from what structure?
The neural tube
What does the PNS develop from?
The neural tube including NCC, neuroectoderm and ectodermal placodes
The PNS includes what type of neurons?
Sensory and motor (somatic and visceral) neurons of the head, trunk and extremities
What do NCC and ectodermal placodes form in the PNS?
Sensory ganglia of the head
What do neural crest cells alone form in the PNS?
Form sensory neurons on DRG, Schwann cells and ENS (trunk)
Describe formation of the ventral root and the lateral horn
Axons exiting ventral horn pass through cranial sclerotome —> form ventral root —> ventral root then joined by axons from the lateral horn
Describe the formation of the DRG and dorsal horn
Neural crest cells form the DRG sensory neurons —> axons then extend from the DRG —> dorsal horn is formed
What is the two chain pathway of the ANS?
1st preganglionic neuron and fibers
2nd ganglionic neuron and postganglionic fibers
When does development of the ANS occur?
During the 5th week
What are preganglionic fibers of the ANS derived from?
Neuroectoderm
What are ganglionic neurons of the ANS derived from?
Neural crest cells
Describe the sympathetic division ganglia of the ANS
Trunk and collateral ganglia
Describe ganglia of the parasympathetic division of the ANS
CN ganglia in the head and neck
Terminal ganglia in the trunk
When does the anterior neuropore close?
Day 24
When does the posterior neuropore close?
Day 26/27
What does the ectomeninx form in the adult?
Dura mater
What does the endomeninx form in the adult?
Arachnoid and pia mater
Which nervous system cell is the only one derived from mesenchyme?
Microglial cells
