1.3 Somitogenesis Flashcards
What are adult derivatives of somites?
Only relevant posterior to the head
- axial skeleton, vertebrate, ribs = not limb bones
- skeletal muscle: body, limb = not smooth or cardiac
Dermis
Which germ layer gives rise to somites?
Paraxial mesoderm
Gastrulation - paraxial mesoderm - presomitic mesoderm - somitomeres - somites
What are adult vertebrate deriviatives of somites and the intermediate embryonic derivatives?
Somite = sclerotome (axial skeleton, vertebrate, ribs) + dermomyotome (myotome, skeletal muscle: body + limb), (dermotome, dermis)
What are important processes in somitogenesis?
Cell division
Mesenchymal - epithelial transition
Fate restriction
Cell division in somitigenesis
In presomitic mesoderm provides more tissue to differentiate into somites
Length of somitic (pre) mesoderm stays constant
No. + frequency of somites generated depends on organism - affected by temperature but normal number still generated
How are somites formed at regular intervals?
hairy is expressed at corresponding periodic intervals corresponding to appearance of new somite
each cell expresses hairy 12x as it moves to anterior presomitic mesoderm
How are somites formed a set distance from the node?
Fgf8 - expressed in cells in and around the node
As these cells move in rostral direction during gastrulation, Fgf8 mRNA is degraded = gradient
- low Fgf8 = rostral
-high Fgf8 = caudal
At specific threshold level, somite formation occurs
What factors mean presomitic mesoderm is predetermined to generate the right number of somites?
Adjacent tissues dont affect somitogenesis - isolating presomitic mesoderm in culture still develops somites
Number of cells in presomitic mesoderm has no effect - if you remove a slice of presomitic mesoderm somites still develop just smaller in size
What is the relevance of the HOXC8 mutation in somitiogenesis?
Detection in mouse transforms first lumbar vertebrate to thoracic character - homeotic transformation - conversion of one body part to another normally more anterior
How is A-P patterning controlled after gastrulation?
HOX genes expressed responding to earlier signals
HOX proteins are transcription factors
HOX genes are differentially expressed along A-P axis of mesoderm
Each somite expresses a different set of HOX genes
How are somites numbered?
Counting from posterior tells you the age of each somite
What are young somites (I-III)?
Somite is epithelial ball surrounding remaining mesenchymal cells - somitocoll cells
Cell division occurs in epithelial and somitocoll cells
What are slightly older somites (IV-V)?
Ventral-medial portion of somites undergoes epitheial-mesenchymal transition becomes sclerotome
Somitocoll cells join mesenchymal cells forming part of the sclerotome - these cells migrate surrounding notochord and neural tube
Dorsal aspect remains epithelial - dermomyotome
What are the even older somites (VI-X)?
Dermomyotome cells ingress from dorsal medial tip and ventral lateral tip to form a new myotome layer between dermomyotome and sclerotome
Dorsal-medial myotome gives epaxial muscle (back)
Ventral-lateral myotome gives hypaxial myotome (body wall + limb)
Myotomal cells never divide, cells are continually recruited from dermomyotome
What are the oldest somites (XI onwards)?
Sclerotome surrounds notochord and breaks it down - resegments, fissure develops in middle along A-P axis
Posterior half of one sclerotome fuses with anterior haf of adjacent sclerotome
Myotome doesn’t resegment
Sclerotome encases neural tube, differentiates first into cartilage then bone then vertebrate
