Fig. 1.

Somite formation in the oscillator-growth scenario. a a vertical embryo is depicted, with anterior head above and posterior tail region below, where the growth zone is located. The presomitic mesoderm (PSM) is the region from the last already-formed somite to the growth zone at the posterior PSM boundary. The time evolution of newly formed cells at the growth zone is depicted in (b) as the horizontal black bar. Initially, a gene expression, depicted as a grey time box, oscillates in time (with a period of three time-boxes, left part of black bar). Eventually (right part of the horizontal bar), the period increases to more than six time boxes, (and even more, see Fig. 5a) and the oscillation thus comes almost to a halt. A bistable system freezes this gene expression in the (almost) stopped phase, somite boundaries start to form and a new mature somite is created, depicted as a circle. Thus somites are added in an anterior to posterior direction, with control from the posterior part of the embryo. Reproduced from [58]