Mesoderm and segmentation Flashcards

(12 cards)

1
Q

what are the four types of mesodermal tissue

A

Axial mesoderm: forms the notochord
Paraxial mesoderm: forms the somites
intermediate mesoderm: forms the kidneys and gonads
lateral mesoderm: forms the heart, blood peritoneum

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2
Q

what tissues are derived from somites

A

axial skeleton, skeletal muscles and part of the dermis
Somites are also important in spinal chord formation

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3
Q

why were somites originally evolved

A

they originally formed muscles called myotomes that are used in side to side movement such as in fish

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4
Q

what are somites decided into

A

dermomyotome: holds the precursors for dorsal dermis and muscle
myotome: froms contractile muscles
scleromyotome: forms the vertebral column and ribs

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5
Q

describe the process of paraxial segmentation

A

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

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6
Q

what is the signalling process behind paraxial segmentation

A

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

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7
Q

what roles does Wnt signalling play in somite formation

A

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.

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8
Q

what genes are expressed in somite patterning

A

Pax3: expressed within the dermomyotome
MyoD: expressed within the myotome
Pax1: expressed within the scleromyotome
Lbx1: expressed within the lateral dermomyotome

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9
Q

how are somites patterned ventrally

A

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

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10
Q

how are somites patterned dorsally

A

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

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11
Q

how does antagonistic signalling affect somite pattering

A

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

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12
Q

how does synergistic signalling affect somite patterning

A

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

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