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Fig. 5. Hes genes are molecular oscillators. (A) Oscillatory
expression of Hes1 is regulated by negative feedback. Promoter
activation (green), as induced by Notch, for example, induces the production
of Hes1 protein, which represses expression of its own gene (red). Then, both
Hes1 mRNA and protein disappear rapidly because they have very short
half-lives, allowing the next round of expression. In this way, Hes1
expression autonomously oscillates. (B) Hes7 oscillation in somite
segmentation. (Ba) Ventral view of a mouse embryo at the five-somite
stage. Somites form periodically by segmentation of the anterior region of the
presomitic mesoderm (PSM, shown in blue). (Bb) Hes7 expression
is periodically propagated, like a wave, from the posterior end to the
anterior region of the PSM (shown by blue arrow, classified into three
phases), and each wave leads to the generation of a pair of somites (buff).
(Bc) This dynamic change is elicited by oscillatory expression in each
PSM cell with a slight delay from the posterior to the anterior direction.
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