Which of the following statements about signaling pathways associated with somit

Question

Which of the following statements about signaling pathways associated with somitogenesis is NOT true? Select one

: a. Somites form when FGF levels are low.

b. The notch-delta pathway activate genes that are expressed in an oscillatory fashion that mirrors the time it takes to form a somite.

c. The Harry-1 gene is expressed only in the anterior portion of mature somites.

d FGF expression is highest in the posterior presomitic mesoderm.

e: Sonic hedge hog (Shh) expression in the notochord induces Pax1 expression in the sclerotome.

Explanation / Answer

The initiation and regulation of somitogenesis occurs due to cascade of gene and their products that leads oscillation (to and fro) movement of cells. The important cellular signaling pathways involved are FGF , Wnt, retinoic acid and Notch pathways that regulate the sequential formation of newer somites along the anterior-posterior axis and thus leads to the paraxial mesoderm differentiation that leads to formation of new somites.

1.         Wnt and FGF signaling pathways shows highest activities in the posterior, presomitic mesoderm or PSM while as retinoic acid pathway is shows highest activity in the somites.

2.         The notch signaling is the central mechanism which is important for making neighboring cells to oscillate and trigger further cellular cascade to form somites

3.         Another signaling pathway associated with the somitogenesis is reported to be Sonic hedgehog (Shh) which leads the formation of SHH protein within pre-somitic mesoderm. It is reported that Shh are expression in the notochord which further leads induction of Pax1 expression.

4.         The formation of somites is also determined by a lower critical threshold level of FGF that causes the expression of Mesp2 for the formation of somitomeres.

Therefore, based upon the signalling cascades involved in the somitogenesis, the correct option to the given question is:

c. The Harry-1 gene is expressed only in the anterior portion of mature somites.

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