Somite pattern regulation in the avian segmental plate mesoderm

Cellular aspects of somite formation in vertebrates

Vertebrate segmentation, the process that generates a regular arrangement of somites and thereby establishes the pattern of the adult body and of the musculoskeletal and peripheral nervous systems, was noticed many centuries ago. In the last few decades, there has been renewed interest in the process and especially in the molecular mechanisms that might account for its regularity and other spatial-temporal properties. Several models have been proposed but surprisingly, most of these do not provide clear links between the molecular mechanisms and the cell behaviours that generate the segmental pattern. Here we present a short survey of our current knowledge about the cellular aspects of vertebrate segmentation and the similarities and differences between different vertebrate groups in how they achieve their metameric pattern. Taking these variations into account should help to assess each of the models more appropriately.

Sonic hedgehog in temporal control of somite formation

Vertebrate embryo somite formation is temporally controlled by the cyclic expression of somitogenesis clock genes in the presomitic mesoderm (PSM). The somitogenesis clock is believed to be an intrinsic property of this tissue, operating independently of embryonic midline structures and the signaling molecules produced therein, namely Sonic hedgehog (Shh). This work revisits the notochord signaling contribution to temporal control of PSM segmentation by assessing the rate and number of somites formed and somitogenesis molecular clock gene expression oscillations upon notochord ablation. The absence of the notochord causes a delay in somite formation, accompanied by an increase in the period of molecular clock oscillations. Shh is the notochord-derived signal responsible for this effect, as these alterations are recapitulated by Shh signaling inhibitors and rescued by an external Shh supply. We have characterized chick smoothened expression pattern and have found that the PSM expresses both patched1 and smoothened Shh signal transducers. Upon notochord ablation, patched1, gli1, and fgf8 are down-regulated, whereas gli2 and gli3 are overexpressed. Strikingly, notochord-deprived PSM segmentation rate recovers over time, concomitant with raldh2 overexpression. Accordingly, exogenous RA supplement rescues notochord ablation effects on somite formation. A model is presented in which Shh and RA pathways converge to inhibit PSM Gli activity, ensuring timely somite formation. Altogether, our data provide evidence that a balance between different pathways ensures the robustness of timely somite formation and that notochord-derived Shh is a component of the molecular network regulating the pace of the somitogenesis clock.

Avian somitogenesis: translating time and space into pattern

Vertebrates have a metameric bodyplan that is based on the presence of paired somites. Somites develop from the segmental plate in a cranio-caudal sequence. At the same time, new material is added from Hensen's node, the primitive streak and the tailbud. In this way, the material residing in the segmental plate remains constant and comprises 12 prospective somites on each side. Prospective segment borders are not yet determined in the caudal segmental plate. Prior to segmentation, the cranial segmental plate undergoes epithelialization, which is controlled by signals from the neural tube and ectoderm. The bHLH transcription factor Paraxis is critically involved in this process. Formation of a new somite from the cranial end of the segmental plate is a highly controlled process involving complex cell movements in relation to each other. Hox genes specify regional identity of the somites and their derivatives. In the chicken a transposition of thoracic into cervical vertebrae has occurred as compared to the mouse. Transcription factors of the bHLH and homeodomain type also specify the cranio-caudal polarity and that of particular cell groups within the somites. According to segmentation models, somitogenesis is under the control of a "segmentation clock" in combination with a morphogen gradient. This hypothesis has recently found support from molecular data, especially the cycling expression of genes such as cHairy1 and Lunatic Fringe, which depend on the Notch/Delta pathway of signal transduction. FGF8 has been described to be distributed along a cranio-caudal gradient. The first oscillating gene described shown to be independent of Notch is Axin2, encoding a negative regulator of the canonical Wnt pathway and a target of Wnt3a. Wnt3a and Axin2 show a similar distribution as FGF8 with high levels in the tailbud. The chick embryo has recently become accessible to molecular approaches such as overexpression by electroporation and RNA interference which can be expected to help elucidating some of the still open questions concerning somitogenesis.

Mathematical models for somite formation

Somitogenesis is the process of division of the anterior-posterior vertebrate embryonic axis into similar morphological units known as somites. These segments generate the prepattern which guides formation of the vertebrae, ribs and other associated features of the body trunk. In this work, we review and discuss a series of mathematical models which account for different stages of somite formation. We begin by presenting current experimental information and mechanisms explaining somite formation, highlighting features which will be included in the models. For each model we outline the mathematical basis, show results of numerical simulations, discuss their successes and shortcomings and avenues for future exploration. We conclude with a brief discussion of the state of modeling in the field and current challenges which need to be overcome in order to further our understanding in this area.

Is the somitogenesis clock really cell-autonomous? A coupled-oscillator model of segmentation

A striking pattern of oscillatory gene expression, related to the segmentation process (somitogenesis), has been identified in chick, mouse, and zebrafish embryos. Somitogenesis displays great autonomy, and it is generally assumed in the literature that somitogenesis-related oscillations are cell-autonomous in chick and mouse. We point out in this article that there would be many biological reasons to expect some mechanism of coupling between cellular oscillators, and we present a model with such coupling, but which also has autonomous properties. Previous experiments can be re-interpreted in light of this model, showing that it is possible to reconcile both autonomous and non-autonomous aspects. We also show that experimental data, previously interpreted as supporting a purely negative-feedback model for the mechanism of the oscillations, is in fact more compatible with this new model, which relies essentially on positive feedback.

Somite formation and patterning

As a consequence of their segmented arrangement and the diversity of their tissue derivatives, somites are key elements in the establishment of the metameric body plan in vertebrates. This article aims to largely review what is known about somite development, from the initial stages of somite formation through the process of somite regionalization along the three major body axes. The role of both cell intrinsic mechanisms and environmental cues are evaluated. The periodic and bilaterally synchronous nature of somite formation is proposed to rely on the existence of a developmental clock. Molecular mechanisms underlying these events are reported. The importance of an antero-posterior somitic polarity with respect to somite formation on one hand and body segmentation on the other hand is discussed. Finally, the mechanisms leading to the regionalization of somites along the dorso-ventral and medio-lateral axes are reviewed. This somitic compartmentalization is believed to underlie the segregation of dermis, skeleton, and dorsal and appendicular musculature.

Dynamic expression of lunatic fringe suggests a link between notch signaling and an autonomous cellular oscillator driving somite segmentation

The metameric organization of the vertebrate trunk is a characteristic feature of all members of this phylum. The origin of this metamerism can be traced to the division of paraxial mesoderm into individual units, termed somites, during embryonic development. Despite the identification of somites as the first overt sign of segmentation in vertebrates well over 100 years ago, the mechanism(s) underlying somite formation remain poorly understood. Recently, however, several genes have been identified which play prominent roles in orchestrating segmentation, including the novel secreted factor lunatic fringe. To gain further insight into the mechanism by which lunatic fringe controls somite development, we have conducted a thorough analysis of lunatic fringe expression in the unsegmented paraxial mesoderm of chick embryos. Here we report that lunatic fringe is expressed predominantly in somite -II, where somite I corresponds to the most recently formed somite and somite -I corresponds to the group of cells which will form the next somite. In addition, we show that lunatic fringe is expressed in a highly dynamic manner in the chick segmental plate prior to somite formation and that lunatic fringe expression cycles autonomously with a periodicity of somite formation. Moreover, the murine ortholog of lunatic fringe undergoes a similar cycling expression pattern in the presomitic mesoderm of somite stage mouse embryos. The demonstration of a dynamic periodic expression pattern suggests that lunatic fringe may function to integrate notch signaling to a cellular oscillator controlling somite segmentation.

Barrier inhibition of a temporal neuraxial influence on early chick somitic myogenesis

Skeletal myogenesis in the chick embryo first occurs in the somite. Somites are transient, paired mesodermal structures adjacent to the neural tube. Somites form from the segmental plate mesenchyme at approximately 90-min intervals. We identify somitic myogenic cells by using confocal microscopy to detect the muscle specific intermediate filament protein, desmin, in whole mount chick embryo preparations. The appearance of desmin in somitic cells does not occur at a constant interval after the somite has formed. The rate of chick somitic myogenic onset, as evidenced by detection of desmin, is approximately 1.5 times faster than the rate of somitogenesis (Borman and Yorde [1994] J. Histochem. Cytochem. 42:265-272). Somitic myogenesis does not appear to be directly linked to somitogenesis but instead may be regulated by some influence external to the somite. Here we have specifically addressed the issue of whether an impermeable barrier placed between the neuraxis and the somites can prevent the onset of somitic myogenesis. When tantalum foil barriers are placed medial to the caudalmost 3-5 somites of embryos having up to 20 somites total (stage 13), the predominant result is an inhibition of myogenic cells lateral to the barrier. Conversely, when the tantalum foil is placed medial to the caudal somites of an embryo having 21 somites (stage 14) or more, desmin is detected lateral to the barrier in most cases. There is a temporal influence originating in the neuraxis which plays a role in the onset of somitic myogenesis. Although the nature of this interaction between the neuraxis and the somites is not yet clear, we have defined a precise temporal location within the developing embryo at which this tissue interaction is taking place.

Hox homeobox genes and regionalisation of the nervous system

The Hox family of homeobox-containing genes are intimately associated with the processes of axial patterning in vertebrate embryos. This family of transcription factors is widely conserved in evolution and by analogy with their Drosophila counterparts, the HOM-C homeotic genes, may play a role in establishing regional identity in a number of embryonic systems, including the CNS. The patterns of expression of these genes are linked with the generation of rhombomeres and neural crest in the developing hindbrain, and suggest that they provide a molecular system for generating a combinatorial patterning mechanism. Analysis of mouse Hox mutants generated by homologous recombination have clearly demonstrated that the genes have important roles in normal regionalisation of the hindbrain and branchial arches, and this has lead to interest in how their early patterns are established in the nervous system. The Hox genes and their relation to hindbrain segmentation therefore provide a means of examining the cascade of events which regulates pattern formation in early neural development.