A clock and trail model for somite formation, specialization and polarization

We present some theoretical considerations about the initial process of pre -patterning during embryonic segmentation, with particular reference to som ite formation. We first suggest that the pre-pattern is a stable spatial si nusoidal (or, at least, periodic) wave. The periodic wave originates from a n oscillator ("clock") in the proliferative region that gives rise to the c ells. At the moment the cells leave the proliferative or "progress" zone, o r somewhat later, a permanent record is made of the current state of the os cillation, which cells then keep during their pre-semitic phase, before exp licit somite and somite boundary formation. Thus, a trail is left behind th e progress zone in the form of a spatial sine wave. Second, we also observe that the factors involved in the progress-zone clock and its wave-like tra il may form multimers, which will oscillate with higher space-time frequenc y and thus shorter wavelengths than the monomers. Whether or not our first suggestion is correct, this phenomenon may account for multiple wavelengths in somitogenesis, and may thus encompass somite formation, but also somite polarization (half-wavelength) into anterior and posterior halves, as well as the puzzling observation that expression of herl in zebrafish is in pri mordia of alternating somites, i.e. it exhibits a 2-somite wavelength. (C) 2000 Academic Press.