Clock and wavefront model

Overview

Once the cells of the pre-somitic mesoderm are in place following by cell migration during gastrulation, oscillatory expression of many genes begins in these cells as if regulated by a developmental "clock". This has led many to conclude that somitogenesis is coordinated by a "clock and wave" mechanism.

More technically, this means that somitogenesis occurs due to the largely cell-autonomous oscillations of a network of genes and gene products which causes cells to oscillate between a permissive and a non-permissive state in a consistently timed-fashion, like a clock. These genes include members of the FGF family, Wnt and Notch pathway, as well as targets of these pathways. The wavefront progresses slowly in an anterior-to-posterior direction. As the wavefront of signaling comes in contact with cells in the permissive state, they undergo a mesenchymal-epithelial transition and pinch off of the more anterior pre-somitic mesoderm, forming a somite boundary and resetting the process for the next somite.{{Cite journal

In particular, the cyclic activation of the Notch pathway appears to be of great importance in the wavefront-clock model. It has been suggested that the activation of Notch cyclically activates a cascade of genes necessary for the somites to separate from the main paraxial body. This is controlled by different means in different species, such as through a simple negative feedback loop in zebrafish or in a complicated process in which FGF and Wnt clocks affect the Notch clock, as in chicks and mice.{{Cite journal

Intrinsic expression of “clock genes” must oscillate with a periodicity equal to the time necessary for one somite to form, for example 30 minutes in zebrafish, 90 minutes in chicks, and 100 minutes in snakes.{{Cite journal

Oscillation autonomy

Gene oscillation in presomitic cells is largely, but not completely, cell autonomous. When Notch signaling is disrupted in zebrafish, neighboring cells no longer oscillate synchronously, indicating that Notch signaling is important for keeping neighboring populations of cells synchronous.{{Cite journal | doi-access = free

References

References

1. Gilbert, S.F.. (2010). "Developmental Biology". Sinauer Associates, Inc..