Clock and wavefront model
The clock and wavefront model is a model used to describe the process of somitogenesis in vertebrates. Somitogenesis is the process by which somites, blocks of mesoderm that give rise to a variety of connective tissues, are formed.
The model describes the splitting off of somites from the paraxial mesoderm as the result of oscillating expression of particular proteins and their gradients.
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
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.[2][3] Generally speaking, the segmentation clock model is highly evolutionarily conserved.[4]
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.[5]
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.
See also
References
Further reading
- Cooke, J.; Zeeman, Christopher (1976). "A clock and wavefront model for control of the number of repeated structures during animal morphogenesis". Journal of Theoretical Biology. 58 (2): 455–476. PMID 940335.