Somitogenesis begins with the formation of somitomeres (whorls of concentric mesoderm) marking the future somites in the presomitic mesoderm (unsegmented paraxial). The presomitic mesoderm gives rise to successive pairs of somites, identical in appearance and which differentiate into the same cell types but the structures formed by the cells vary depending upon the anteroposterior (e.g the thoracic vertebrate have ribs, the lumbar vertebrate do not). Somites have unique positional values along this axis and it is thought that these are specifed by the Hox(homeotic) genes.
The "clock and wave front" model has been proposed as the mechanism by which embryonic time is converted in spatial periodicity. Oscillating transcriptional activity with delayed negative feed back loops have been observed in somitogenesis through insitu hybridisation. This provides evidence that a molecular oscillator ticks in somitogenesis and stops when all the somites are fully formed.
Notch and Wnt transduction pathways have been shown to be active in somitogenesis.
During somitogenesis of chick embryos, a pair of somites forms every 90 minutes with a total of 50 pairs somites to generate.
The somites differentiate to form:
-Scleratome - future cartilage of the axial skeltion (vertebrate, ribs)
-Myotome - precursor cells to skeletal muscle
-Dermatome - cells that contribute to the connective tissue of the dermis.
The process of Somitogenesis is a currently active topic in research.