Somitogenesis Flashcards
(27 cards)
what are somites? from what embryological structure are they formed?
transient embryological structures formed from the pinching of the presomitic mesoderm (cranial-to-caudal pinching)
form as paired epithelial spheres on either side of the neural tube
from what embryological structure are somites formed from?
presomitic/ somitic paraxial mesoderm
in which direction does the presomitic mesoderm (PSM) grow? how does this relate to how somites form?
PSM grows in the caudal direction
somites form sequentially, with the most cranial somite being somite1 & the most caudal somite is the most recently formed
what tissues do somites give rise to?
axial skeleton - ribs, vertebrae
axial muscles - vertebral, thoracic & abdominal muscles
appendicular muscles - flexors and extensors
what tissue subdivision do somites NOT give rise to out of:
A) axial muscles
B) axial skeleton
C) appendicular muscles
D) appendicular skeleton
D: appendicular skeleton - (upper & lower limbs) it’s derived from the limb buds
describe what is involved in the formation of the presomitic mesoderm
mesodermal cells in the cranial primitive streak and elongating tail bud are proliferating rapidly
mesodermal cells in the primitive streak undergo specification to adopt a preaxial mesodermal/PSM fate
specified preaxial mesodermal cells migrate away from the central embryo & join the elongating PSM in the tail bud region - preaxial mesodermal cells are considered PSM cells
Wnt, FGF, RA and BMP signalling pathways are involved in regulating the formation, maintenance, and differentiation of the PSM
what three main compartments do somites differentiate into? what is the 4th compartment?
sclerotome
dermatome
myotome
(4th) - syndetome
what is the presomitic mesoderm (PSM), and where does it form?
PSM is a tissue that forms on either side of the neural tube in the embryo
arises from mesodermal cells that are specified into preaxial mesodermal cells & migrate away from the central embryo to the elongating tail bud
what is the role of the sclerotome in somite formation?
sclerotome differentiates & gives rise to hard tissues like vertebrae & ribs
what does the dermatome in the somite give rise to?
dorsal dermis of the body
what do the myotomes from the somites give rise to?
myotome splits into:
- epimere = forms epaxial/ back muscles
- hypomere = forms hypaxial muscles (limb muscles & body wall)
what does the syndetome from somites contribute to?
tendons & cartilage
links axial muscles to vertebrae
what model explains the regulation of somite formation?
‘clock and wavefront’ model
- clock refers to cyclic gene expression that controls timing
- wavefront controls the periodicity of somite formation
how does somite formation differ between species?
timing and the number of somites can vary between different species - e.g. cornsnakes form approx. 300 somites; humans form 44
what does the ‘clock’ in the clock and wavefront model do?
controls timing of somite formation via cyclic gene expression (e.g. Notch, Wnt, FGF)
what does the ‘wavefront’ in the clock and wavefront model do?
controls periodicity of somite formation through two opposing signals which form gradients across PSM - Wnt/FGFs (posterior/caudal) and RA (anterior/cranial)
- high Wnt/FGF; low RA = promotes undifferentiated and proliferative state of PSM cells
- low Wnt/FGF; high RA = cells exit clock, differentiate and form somites
how does somite formation progress?
sequentially from cranial to caudal (Somite 1 - first to form, most cranial)
why is RA considered a counter-signal in somitogenesis?
antagonises Wnt/FGF to create the anterior wavefront that enables differentiation - spatial opposition across PSM ensures correct timing of somite formation
how does the sclerotome compartment of somites give rise to vertebrae & ribs?
epithelial somite cells undergo multiple rounds of EMT - become sclerotomal mesenchymal cells which proliferate rapidly and migrate to surround the neural tube and notochord
sclerotomal mesenchymal cells differentiate into cartilage, then undergo endochondral ossification to form bone - contribute differently to vertebrae structure:
- sclerotomal cells surrounding the neural tube = form neural arches = vertebral arches (spinous & transverse processes)
- sclerotomal cells surrounding the notochord = form centrum = vertebral body
how does the embryonic segmental organization of somites relate to the adult dermatome map?
segmental pattern of somites preserved as dermatomes migrate - formed stripe segmentally-innervated areas of skin (one dermatome innervated by a specific spinal nerve, arising form a specific spinal cord segment)
pattern of segmental organisation distorted by morphological folding of embryo itself- but pattern/ dermatome map still remains with segmental spinal nerve innervation
what ensures that each dermatome is innervated by a specific spinal nerve? what distorts the dermatome in the limbs compared to trunk?
each dermatome retains innervation from the spinal nerve of its original somite segment
limb rotation during development distorted limb dermatomes
trunk develops without rotational distortion
How can understanding dermatomes help in clinical diagnosis?
each dermatome corresponds to a specific spinal nerve - sensory loss/ referred pain in a dermatome can help identify the location of nerve injury or compression
sclerotome formation & differentiation
- signalling
- pathway of formation
- what it differentiates into
signalling;
- ventral NT & notochord secrete Shh > induces Pax1 in sclerotome region of somite (ventromedial region)
pathway of formation:
- epithelial somite cells undergo multiple rounds of EMT > from sclerotomal region of somite, these cells become sclerotomal mesenchymal cells which proliferate rapidly & migrate to surround the notochord and NT entirely
sclerotomal mesenchymal cells differentiate into cartilage, then undergo endochondral ossification to form bone - contribute differently to vertebrae structure:
- sclerotomal cells surrounding the neural tube = form neural arches = vertebral arches (spinous & transverse processes)
- sclerotomal cells surrounding the notochord = form centrum = vertebral body
differentiate into:
- occipital bone
- vertebrae (cranial half of one somites fuses with caudal half of adjacent somite = fusion forms vertebrae)
- ribs
myotome (epimere) formation
- signalling
- pathway of formation
- what it differentiates into
signalling:
- dorsal neural tube secretes Wnts > induces Myf5 in epaxial myotome (dorsomedial region of dermomyotome)
pathway for formation:
- dorsomedial cells of dermomyotome/somite committed to epimere fate by Myf5 > cells remain close to NT, don’t migrate as much
- cells differentiate to form epaxial (back) muscles for posture and spinal extension
- segmental innervation from spinal nerves of epaxial muscles maintained
differentiation:
- epaxial (back) muscles for posture & spinal cord extension
- innervated by dorsal rami of spinal nerves
