View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Cell, Vol. 92, 9±16, January 9, 1998, Copyright 1998 by Cell Press Somite Development: Meeting Review Constructing the Vertebrate Body Shahragim Tajbakhsh* and Ralf SpoÈ rle² the scene for somitogenesis. Several groups investi- *Department of Molecular Biology gated these early stages using microsurgical tech- CNRS URA1947 niques. P. Tam (Children's Medical Research Institute, Pasteur Institute Wentworthville, Australia) and G. Schoenwolf (University 25 rue du Dr. Roux of Utah) reported that during chick and mouse gastrula- 75724 Paris, Cedex 15 tion, cell fates are flexible. When epiblast fragments France corresponding to prospective chick somitic mesoderm ² Institute of Mammalian Genetics (ISG) and heart cells are exchanged during mid- to late-primi- GSFÐNational Research Center for tive streak stages, cells assume the fate of their new Environment and Health location. This developmental potential is reduced as IngolstaÈ dter Landstrasse 1 cells ingress through the primitive streak, and the com- D-85764 Neuherberg petence to form somitic cells appears at the mid-streak Germany stage (G. Schoenwolf, P. Tam). Gastrulation orches- trates considerable cell movements in the posterior primitive streak. However, C. Stern (Columbia Univer- Designing a body plan is an architectural challenge. sity, NY) reported that cell movements in the PSM ap- Both invertebrates and vertebrates have addressed this pear to be severely limited, and complete disruption of problem by first establishing repeated units of equiva- the rostrocaudal pattern by dissociation of these cells lent identity (segments), and later coordinating these results in their failure to sort out to form normal somites. motifs into regionally specialized and integrated struc- Cell lineage analysis using vital dyes has been instru- tures. The most distinct feature of vertebrate mesoder- mental in determining the cellular origins of embryonic mal segmentation is the somite. Indeed, cell fate in the tissues. These studies have demonstrated that domains immature somite is flexible and dependent on local envi- constituting the presumptive notochord, medial and lat- ronmental signals. Consequently, somitogenesis has eral somite moieties and lateral mesoderm can be topo- generated considerable interest and the somite now graphically mapped in the primitive streak (C. Stern, serves as a paradigm for investigating how naive cells G. Schoenwolf). In spite of this apparent geographic adopt identity. Somites were first defined at the begin- address, experimental rotation of the immature somite ning of the last century, and much of our understanding results in cells adopting the fate of their new position, of somite development comes from morphological ob- therefore somite cell fate is also governed by local envi- servations and experimental manipulations in avian em- ronmental signals. However, polarity is already estab- bryos (see Christ and Ordahl, 1995), and more recently, lished in the PSM since its rotation along the rostrocau- from embryo culture and genetic studies in mice (see dal axis results in somites with a reversed pattern of Gossler and Hrabe de Angelis, 1997). The rapidly devel- neural crest cell migration (see below). In addition, pro- oping zebrafish model promises to unite these ap- spective somite units in the PSM are encoded with posi- proaches. tional information by the Hox code. Indeed, transplanta- Important landmarks in somitogenesis are periodicity, tion of thoracic level PSM to the cervical region results segmentation, epithelialization, and differentiation. So- in rib formation rostrally (see Keynes and Stern, 1988). mites form pairwise within the presomitic mesoderm How, and at what stage is this Hox pattern established (PSM; segmental plate in avians), and on either side of and propagated? This remains unclear since the inser- the neuraxis until midgestation (50 in chick, 65 in mouse, tion of physical barriers in the early embryo does not and up to 500 in snakes). As a consequence of gastrula- prevent rostral propagation of Hox patterning signals tion, mesenchymal somite precursor cells feed into (S. Gaunt, Babraham Institute, United Kingdom). the caudal PSM, progressively condense as they move J. F. Nicolas (Institut Pasteur, Paris) reported on an rostrally, and concomitantly somites exit as epithelial elegant genetic approach to cell lineage studies in spheres from the rostral-most portion of the PSM (Figure transgenic mice (Nicolas et al., 1996). A muscle promoter 1; see below). Subsequently, cells oriented toward the linked to a laacZ (internal duplication; inactive) reporter notochord differentiate into the sclerotome via an epi- gene, undergoes a random intramolecular recombina- thelial±mesenchymal transition. Underneath the surface tion event (active lacZ; optionally triggered by Mitomycin ectoderm, the remaining epithelium forms the dermo- C in utero) and marks the myotomal descendants of a myotome, which later contributes to skeletal muscle clone. This retrospective analysis predicts that 100±150 and dermis. Current progress in the understanding of cells constitute the myotomal stem cell population in the somitogenesis was discussed at a recent meeting on PSM and that somites form with a calculated periodicity. Somite Development organized by O. Pourquie (Devel- This is in good agreement with lineage studies in chick opmental Biology Institute of Marseille, France) on the using vital dyes (128 cells; C. Stern). mediterranean island of Les Embiez (France; Oct., 1997). M. George-Weinstein (Philadelphia College of Osteo- pathic Medicine, PA) raised the possibility that in the Somites Are Born after Gastrulation PSM and epithelial somites, ªfounder cellsº may recruit Gastrulation, which begins with the formation of the uncommitted cells into myogenesis, a process stimu- primitive streak, leads to mesoderm formation and sets lated by the cell adhesion molecule N2cadherin. In Cell 10 Figure 1. Scanning Electron Micrographs and Schema Portraying Somite Development The schematic (center) indicates relative maturity at each axial level. A chick embryo is illlustrated for convenience due to development via a flat germinal disc, whereas mouse embryos exhibit a curvature. Somites advance through each stage indicated by roman numerals (Christ and Ordahl, 1995), as seen from transverse views: (A) mesenchymal presomitic mesoderm (PSM). (B) somite I, most recently formed epithelial somite (S) containing a mesenchymal core (somitocoele). An extracellular matrix and basement membrane surrounds the immature somite except for its connection to the intermediate mesoderm (IM; Christ and Ordahl, 1995). (C) Mature stage XII somite (shown from hindlimb level, therefore the hypaxial somitic bud is reduced, see text), dermomyotome (DM), myotome (M), and sclerotome (SC) are distinguishable. (D) Approximate expression patterns (mouse E8.5±9.5) indicate metameric expression prefiguring somites (see text). The most recently forming somite (somitomere) in the rostral PSM is indicated. The chick PSM is approximately double the length of that of a mouse embryo at a comparable stage. Gastrulation takes place in the primitive streak, which initially forms by a thickening of the epiblast cell layer. See Johnston et al. (1997) for lunatic fringe expression. Dll1 expression persists in all somites while Dll3 appears only in the caudal-most somites (Dunwoodie et al., 1997). EphA4, formerly Sek1 (Orioli and Klein, 1997). C, caudal; DRG, dorsal root ganglion; E, endoderm; LM, lateral mesoderm; NC, notochord; NT, neural tube; R, rostral; SE, surface ectoderm. Electron micrographs (A±C) courtesy of K. Tosney. contrast, whenepiblasts (prior to gastrulation) are disso- may trigger differentiation. Indeed, the phenomenon of ciated into single cells in serum-free medium, predomi- lateral inhibition operates in vertebrates since retroviral nantly muscle cells are observed. This competence is mediated overexpression of Delta1 results in ectopic repressed when epiblasts are cultured as intact tissue apteria (nude skin) in chick (D. Dhouailly, Institut Albert (M. George-Weinstein). Therefore cell±cell interactions Bonniot, La Tronche, France). may also be necessary to inhibit differentiation in vivo. Interestingly, the Notch-Delta signaling pathway impli- Somite Segmentation cated in selecting distinct cell fates from a group of How are somite segmentation and periodicity defined equivalent cells (lateral inhibition) in Drosophila, has also in the vertebrate embryo? The remarkably dynamic ex- been shown to repress myogenesis (see Cossu et al., pression pattern of c-hairy1 in the chick PSM, reported 1996). Importantly, these genes are strongly expressed by O. Pourquie , now provides us with some clues. Cyclic in the primitive streak and PSM (Figure 1D). Therefore, waves of expression move rostrally with a periodicity uncoupling of a pathway necessitating cell±cell contact corresponding to the time required to form one somite Meeting Review 11 (90 min; Figure 1D). This pulsed expression/degradation somite segmentation from the PSM (L. K. Durbin, King's of c-hairy1 is an intrinsic property of the PSM and it is College, London). What factors govern segmentation? not altered by blocking protein synthesis, thus ruling out Using an in vitro culture system of mouse PSM, R. Con- negative feedback mechanisms implicated in circadian lon discussed the signaling requirements from adjacent