Somites Without a Clock

Abstract

The formation of body segments (somites) in vertebrate embryos is accompanied by molecular oscillations (segmentation clock). Interaction of this oscillator with a wave traveling along the body axis (the clock-and-wavefront model) is generally believed to control somite number, size, and axial identity. Here we show that a clock-and-wavefront mechanism is unnecessary for somite formation. Non-somite mesoderm treated with Noggin generates many somites that form simultaneously, without cyclic expression of Notch-pathway genes, yet have normal size, shape, and fate. These somites have axial identity: The Hox code is fixed independently of somite fate. However, these somites are not subdivided into rostral and caudal halves, which is necessary for neural segmentation. We propose that somites are self-organizing structures whose size and shape is controlled by local cell-cell interactions.

Building the backbone: the development and evolution of vertebral patterning

“…A plausible scenario is that it evolved in the presence of somite polarity, under the regulation of the somite clock (Dias et al, 2014), so facilitating construction of diverse types of centra from sclerotome sub-components. The amniote centrum typically develops from a single ossification centre, but that of stem tetrapods comprises two alternating anteroventral (A-V) and posterodorsal (P-D) components, creating so-called 'rachitomous' vertebrae.…”

Shared rules of development predict patterns of evolution in vertebrate segmentation

“…Our results contribute to increasing evidence that activator-inhibitor interactions are involved in limb, digit and somite segmentation, and could reflect a universal design principle 4,[30][31][32][33][34][35] , but while confirmatory these results do not clarify the identity of the molecule(s) or the exact developmental mechanisms involved. We argue that the value of using the IC model is that, while previous models describe how segments form, they make no predictions about how they vary in size, and imply that elements are either independently formed or that segment proportions are largely the result of selection on later developmental events such as growth.…”

“…15), which propose that individual segments initially form as a function of time balanced by inhibitory signals from the distal limb tip. Somitogenesis has likewise been conceived of proceeding via a 'clock' that interacts with an inhibitory 'wavefront' 16 , and newer studies suggest that the clock period changes with shortening of the unorganized (presomitic) mesoderm 32 and is partly selforganizing 33 . In both cases, the clock determining condensation formation can be conceived of as an auto-regulatory activator process that is balanced by an auto-regulatory inhibitory signal, each of which presumably can be varied.…”

A time space translation hypothesis for vertebrate axial patterning

“…Dias et al [17,Fig. 4] found that explanted chicken posterior primitive streak (=very ventral NOM mesoderm + embryonic ectoderm (EE)) is stabilised to different positional identities by noggin applied at different times after the beginning of gastrulation.…”