Neurulation & Development of PNS Flashcards
what induces neurulation?
notochord (sitting in the mesoderm, below ectoderm)
starts at week 3
what is primary neurulation?
process of forming neural plate, then neural folds, then a neural tube
occurs in the part of the ectoderm directly on top of the notochord (the rest becomes surface ectoderm)
what is the neural groove?
anchors the neural plate (sits right along the center of it)
remains throughout all of neurulation
what days does primary neurulation occur during?
days 17-20
what direction does primary neurulation occur in?
starts in the cervical region
moves up over then head and down towards the lumbar region at the same time (like a zipper going in both directions)
what are the different layers that form during primary neurulation? (3)
- both sides of the neural plate meet - pinch off and form the neural tube
- neural crest cells (lateral to the groove) - both sides meet in the middle - pinch off and form neural crest cells (between surface ectoderm and mesoderm)
- surface ectoderm on both sides meet (most superiorly) and form a whole surface ectoderm layer
what signals the end of primary neurulation?
ends when the caudal neuropore closes (around somite 31)
what is secondary neurulation?
formation of neural tube in the sacral/cocygeal segments from the tail bud - eventually merges with the caudal neuropore
where does the tail bud come from?
forms during gastrulation
what are the steps that cause the tail bud to form into the neural tube?
- tail bud condenses to form the medullary cord (a solid mass)
- medullary cord caviate (forms a lumen)
- this lumen merges with the central canal of the neural tube
- lateral segments of the tail bud become somites
- neural crest cells form and migrate
how does the location of neural crest cells change during neurulation?
start lateral on the neural plate- end up dorsal to the neural tube
what transition do neural crest cells under go?
epithelial to mesenchymal transition (become migratory)
how do neural crest cells migrate?
migrate ventrally
can migrate before the neural tube closes or after then neural tube closes
what are important neural crest cell derivatives?
neural crest cells named by region: cranial, cardiac, trunk, sacral
- cranial - chin formation
- cardiac - septa between aorta and pulmonary trunk
- melanocytes, odontoblast, Schwann cells, ganglia, enteric nervous system, adrenal medulla
what are neurocristopathies? (3 examples)
group of diseases (effect depends on which neural crest cells are affected )
eg; head/neck -goldenhar syndrome, micrognathia, TCS
trunk - Hirschsprung (impacted megacolon)
melanocytes - piebaldism
what days does the first round of meninges formation occur during?
days 25-30
what do meninges develop from?
neural crest cells and mesenchyme
what is the mininx primordial?
primordial meninges - original layer of neural crest cells/mesenchyme that migrate around the spinal cord
what happens to the primitive mininx during days 34-48
splits into the ectomeninx and endomeninx
what does the ectomeninx form?
dura matter
what does the endomeninx form?
arachnoid/pia matter
what role do the vertebra play in meninges development?
vertebra form at the same time as the neural tube
neural crest cells migrate towards the vertebra, then mix with mesenchyme
eventually form circular structure around the closed neural tube
what changes does the ectomeninx undergo during days 45-60
the ectomeninx becomes compact and forms a dural Venus sinus
in the spinal region– it disassociates from the vertebral bodies in order to form the epidural space
what is the dural Venus sinus
drains cranial blood (formed by the ectomeninx)
what happens to the endomeninx during days 45-60
the endomeninx becomes reticulated (forms a meshwork)
the subarachnoid space forms (arachnoid matter still keeps its connection to Pia matter)
which part of the neural tube closes first?
closure occurs at various points along the neural tube
cranial closure is more advanced than caudal closure
what forms when the neural tube closes?
cranial and caudal neural pores form
what is necessary during pregnancy in order to prevent neural tube defects?
folic acid
what are some examples of cranial neural tube deficiencies?
anencephaly/microencephaly (brain tissue open to amniotic fluid)
encephalocele
craniorachichisis (extends into the thoracic region)
what is spina bifida occulta?
spinal neural tube defect
can be asymptomatic
vertebral arch does not form (but it is still covered by skin)
sx: skin pigment, hair tuft over section, dimple in sacral region (dermal sinus)
what is meningocele?
spinal neural tube defect
no vertebral arch forms, some meninges are involved (spinal cord is not involved)
sx: protruding sac
what is meningomyelocele?
spinal neural tube defect
similar to meningocele but spinal cord is involved as well (protrusion of sac and spinal cord)
what is myeloschesis?
spinal neural tube defect
no lateral vertebral structures form
meninges and spinal cord are exposed
neural tube is not closed`
what is the sulcus limitans?
divides dorsal and ventral portions of the neural tube
extends from 4th somite to caudal region
what are the zones of the neural tube?
go laterally - from central canal out
ventricular zone
intermediate zone
marginal zone
what is found in the ventricular zone of the neural tube?
stem cells that undergo mitosis - grow and divide and then move to the intermediate zone as neuroblasts
what is found in the intermediate zone?
neuroblast cells that differentiate into:
- neurons (initial wave of growth of the spinal cord)
- glioblast (after enough neurons are made) - differentiate into macroglia (astrocytes/oligodendrocytes) or ependymal (needed for CSF)
what is found in the marginal zone?
where white matter is formed
how is the spinal cord organized?
how are these all organized : roof plate, floor plate, alar plate, basal plate, sulcus limitans, notochord
roof plate- centrally on the dorsal side
alar plate - 2, one on each side - forms dorsal region
sulcus limitans - separate alar and basal plates
basal plate - 2, one on each side - forms ventral side
floor plate - centrally on the ventral side
notochord - adjacent to floor plate
which plates of the spinal plate have growth?
roof/floor - no growth (connect both alar plates and both basal plates together )
alar/basal plates- growth (establish sensory/motor processing)
what does sonic hedge hog do in the neural tube?
causes neuron differentiation
increased SHH induces motor neuron differentiation
how is the SHH gradient established? what is the effect of it?
ventral side has increased levels (closer to notochord) , dorsal side has decreased levels
ventral neurons differentiate into motor neurons
dorsal neurons differentiate into interneurons
what type of neurons form first? motor or sensory?
motor neurons
closer to notochord
where is BMP/TGFB released from?
roof plate
where is SHH released from?
notochord
what counters the effects of SHH?
BMP/TGF B (released from the opposite side of spinal cord - gradient runs in the opposite direction of SHH)
what does BMP 4 do?
activate PAX 3/ PAX 7 (help process sensory from DRG)
where are the lateral horns located?
intermediolateral column
most dorsal portions of the basal plate (closest to the sulcus limitans)
where do SNS and PSNS motor neurons originate? (which spinal segments)
SNS: T1-L3
PSNS: S2-S4
what fibers are in the spinal nerve?
sensory and motor mixed
what fibers are in the spinal roots?
motor or sensory only
anterior - motor only
posterior - sensory only
what fibers are in the rami?
mixed motor and sensory (based on location they are traveling to)
what is the process of somatic motor neuron formation?
occurs first
starts In the ventral horn
passes through cranial sclerotome and forms the ventral root (anterior root) and goes out to the target musculature
what are pioneer fibers?
the first somatic motor neurons that begin extending - spontaneously start forming
what is the process of the autonomic nervous system formation?
fibers arise in the lateral horn - follow the same ventral (anterior) root that already exists (from somatic motor neuron formation)
2 chain pathway - leaves ventral root to synapse on synaptic chain–> then hops back on spinal nerve to go out to the body
what gives rise to preganglionic neurons of the ANS ?
neuroectoderm
what gives rise to post ganglionic neurons of the ANS ?
neural crest cells
what is the process of sensory neuron formation?
starts in the DRG (outside the spinal cord)
extends in 2 direction - towards the spinal cord dorsal horn and outwards using the ventral root
meets the dorsal root at the spinal nerve and goes out to the body
