Mesoderm and segmentation Flashcards
(12 cards)
what are the four types of mesodermal tissue
Axial mesoderm: forms the notochord
Paraxial mesoderm: forms the somites
intermediate mesoderm: forms the kidneys and gonads
lateral mesoderm: forms the heart, blood peritoneum
what tissues are derived from somites
axial skeleton, skeletal muscles and part of the dermis
Somites are also important in spinal chord formation
why were somites originally evolved
they originally formed muscles called myotomes that are used in side to side movement such as in fish
what are somites decided into
dermomyotome: holds the precursors for dorsal dermis and muscle
myotome: froms contractile muscles
scleromyotome: forms the vertebral column and ribs
describe the process of paraxial segmentation
paraxial mesoderm starts has mesenchyme either side of the notochord
paraxial mesoderm will then form balls of cells called somites
the formation of somites occurs rostrally, appearing in regular intervals
what is the signalling process behind paraxial segmentation
Notch-Delta signalling acts as a cyclical clock to activate the transcription factor Hairy at regular intervals:
-Lunactic fringe is both produced by and inhibits Notch signalling
-this causes the system to reset itself and continue cyclically
FGF8 is expressed in gradient from caudal to Raustral
-once FGF8 drops below a threshold segmentation can occur
what roles does Wnt signalling play in somite formation
Wnt is involved in transitioning the paraxial mesoderm from a mesenchymal state to epidermal tissue
-Wnt activates the transcription factor Paraxis and its cofactors Pax7 and Pax3
when these are inhibited, segmentation still occurs but no somites are formed.
what genes are expressed in somite patterning
Pax3: expressed within the dermomyotome
MyoD: expressed within the myotome
Pax1: expressed within the scleromyotome
Lbx1: expressed within the lateral dermomyotome
how are somites patterned ventrally
the notochord secretes SHH
SHH is required to activate Pax3, Pax1 and MyoD.
This means signals from the notochord are required to form the dermomyotome, scleromyotome and the myotome
how are somites patterned dorsally
Wnt is secreted by the ectoderm
-Wnt is required to activate Pax1, Pax3 and MyoD
this means Wnt is necessary to form the dermomyotome, sclerotome and myotome
how does antagonistic signalling affect somite pattering
Dorsal-Ventral patterning
Shh inhibits dermomyotome formation and promotes scleromyotome formation (Pax1)
Wnt inhibits scleromyotome formation and promote dermomyotome (Pax3)
This means that the dorsal side will have dermomyotome and the ventral side has sceleromyotome
how does synergistic signalling affect somite patterning
medial-lateral patterning
Shh and Wnt promote epaxial myotome formation
BMP4 and Wnt from the neural plate and ectoderm promote hypoxia myotome formation
This means that the medial regions will contain epaxial myotomes that will give rise to the deep muscles in the back, whilst the lateral regions will contain hypoxia mesoderm that will give rise to the limb muscles
