lec 23- somite formation Flashcards
where do somites form from?
from mesoderm locate don either side of the notochord
what do somites give rise to?
give rise to skeletal muscles, bone and cartilage of the body including the spinal column
how do somites form?
-they arise in pairs from mesoderm, they form behind the Hensens node as it moves towards the posterior pole of the embryo
-changes in cell shape and intercellular contacts result in the formation of blocks of cells called somites
what gives rise to the facial muscles and bones?
the mesoderm that lies anterior of the somite-generating region
what do the first 5 somites contribute to?
the posterior part of the skull, rest of the skull is derived from the neural crest
what do the remaining smites produce?
the muscles and bones of the body and limbs
the cells that give rise to the somites arise on either side of the primitive streak when:
it has reached the anterior end of the embryo
what do the cells that form the somites do once the primitive streak is at the anterior end?
-the cells move through the node and form a population of stem cells located around the Hensens node
what happens to the cells as the node regresses? what do they form?
the cells are left behind, they form presomitic mesoderm and progenitor neural cells that will form the spinal cord
how do we know if somites are specified?
-when a block of pre-somitic mesoderm is turned upside down, the somites develop the same way as originally intended, somite 6 forms first
-this means that the presomitic mesoderm tissue is not responding to positional ques and that it is specified to become a somite before somite formation begins
what is the clock and wavefront model when it comes to somite specification?
-proposes that cells in the pre-somitic mesoderm becomes committed to form a somite when a determination wavefront passes through the tissue and the cells then receive a signal from the clock
explain the clock and wavefront model:
-FGF/Wnt signalling molecules form a gradient, with the highest levels at the posterior, there is also an opposing gradient of retinoic acid
-the anterior pre-somitic mesoderm differentiates below a critical threshold of FGF/Wnt, this threshold is the determination wavefront
-the opposing gradient of retinoic acid has an antagonistic effect on FGF action and prevents the pre-somitic region from getting too long
-as the axis elongates towards the posterior, the source FGF/Wnt signals move and the wavefront also advances through the mesoderm towards the posterior
what is the clock characterized by in the clock and wavefront model?
cyclic expression of genes
how does the clock work?
-at the beginning of the cycle, gene expression occurs in the most recently specified pair of somites, it is not expressed in the pre-somitic mesoderm
-second, gene expression in the new somites has ceased and now gene expression is in the most posterior cells of the mesoderm and it progresses towards the anterior
-next gene expression occurs in anterior and ceases in more posterior cells
-the next cycle begins again with gene expression in recently specified pair of somites, cycle is repeated
what does the clock and wavefront model propose?
-cells in pre-somitic mesoderm becomes committed to form a somite when the determination wavefront passes through them and then they receive a signal from the clock in the form of oscillating gene expression
how does retinoic acid signaling occur?
-retinol is taken up by cells and converted into retinoic acid by enzymes in cytoplasm
-another enzyme in the cytoplasm may break the retinoic acid into metabolites for other uses
-retinoic acid diffuses through the plasma membrane due to being Lipid soluble
-retinoic acid then binds to receptors that act as transcriptional regulators
-retinoic acid allows for transcription to occur when it binds to its receptors as the receptors repress transcription when no retinoic acid is present
can vertebrates synthesize vitamin A from scratch?
no
how do vertebrates get vitamin A?
they convert carotenoids from food into retinol
can deficiencies in vitamin A and too much of it cause birth defects?
yes
how do somites aquire their identities?
based on their position along the AP axis
what do anterior somites form?
posterior regions of the skull
what do the next somites after the anterior ones form?
vertebrae along the spine
how do we know somite fate is determined?
-presomitic mesoderm from thoracici vertebrae is grafted to the somite region where cervical vertebrae forms
-the grafted tissue forms its original fate
-this indicates that it is determined
-ultimately meaning that pre-somitic mesoderm is specified and determined to form a specific structure before the somite forms
what patterns the nervous system and somites?
Hox genes
