The vertebrate mesoderm contributes the structural bulk of the vertebrate body plan, as well as the material for remarkable innovations that characterize many different taxonomic lineages. The evolutionary history and developmental role of this prolific germ layer has long motivated curiosity across a spectrum of phenomena. From one perspective, the structural derivatives themselves are of intrinsic interest and their development is one critical and fascinating component. From another vantage, the developmental process commands the focus of interest, and the embryonic mesoderm is a laboratory in which to explore these mechanisms.

This special issue on the vertebrate mesoderm brings together a series of reviews that spread across this spectrum. The problem of how specific morphology is generated from simple and uniform beginnings is a central fascination for developmental and evolutionary biologists. Molecular genetic studies continue to identify more and more players involved in complex pattern formation enabling directed experimental manipulations to test hypotheses about function. There is also a growing awareness of the context in which the molecular players perform their functions; bringing us from the Eppendorf tube and computer back to the embryo itself, the context of its own molecular and cellular processes. The standard array of organisms used to explore development are all descendants of a common ancestor, a common ancestor with a mesoderm full of potential.

The eight reviews collected here all touch on one or more of three subjects. First, what is the nature of pattern formation in the vertebrate embryo? This question is addressed at the local level of molecular receptors, ligands, and cell-to-cell communication in the differentiation pathways of mesodermal cell populations, particularly the two major lineages in the somites: sclerotome and dermomyotome. The question of patterning is also approached at a global level, where regionalization of the body plan, between head and trunk or limb and flank for instance, occurs through the long-term influence and interactions of mesodermal cell lineages, and the distribution of particular molecular players like the Hox genes. The hypotheses generated by addressing global patterning lead back to questions at the cellular level, and both lead inevitably back to the embryo.

The embryo can serve as a laboratory and these reviews demonstrate how each of our common model species offers its own unique strengths and opportunities to address general questions about development and the broad arena of patterning. Xenopus has a long, illustrious history as a model for early tissue interactions and the zebrafish now provides a genetic model for “simple” vertebrate anatomy (although hardly such to the ichthyologist). Detailed fate mapping and physical manipulations of avian embryos have established and continue to test the topography and timing of critical tissue interactions in amniotes. Transgenic techniques in the mouse provided our first genome-level view of a vertebrate, and are allowing evermore-directed manipulation of proximal mechanisms. Data from each of these powerhouse laboratory organisms is contributing to an increasingly detailed view of development.

Finally, several of these reviews demonstrate what we can learn from the study of nontraditional model systems. As stated above, each model system has its own advantages for specific embryonic manipulation. Together they also provide a currently sparse but broad view of how developmental systems have evolved within certain lineages. The phylogeny of vertebrates is well understood, and this knowledge is based in large part on the evidence available in the fossil record, which is often the remains of mesodermal derivatives. Originally based on skeletal data, and now confirmed at a molecular level, the relationships between typical model species is quite clear. However, the isolated comparison of two amniotes (say, bird and mammal) provides no definitive information about the evolution of their similarities or differences; only that they are the same or different as a result of modifications over the 300 plus million years since the two shared a common ancestor. By adding developmental data from an anamniote out-group, like a frog, or a deeper branch represented by zebrafish, the direction of change may also be exposed. By continuing to add lineages for comparison, we get a more complete picture of how developmental processes are modified between and within lineages, and how those processes have contributed to evolutionary diversification. The addition of nontraditional model systems (like postmetamorphic or direct developing frogs, or sharks, or lamprey), further exposes lineage-based conservation or innovation. To fully understand characters that our model systems have in common, seeking out a lineage that holds the appropriate phylogenetic position, and represents an alternative evolutionary and developmental trajectory, will add the necessary primitive anchor and give evolutionary weight to our comparisons. The molecular data from our standard model systems gain more specific significance and greater predictive power within a historical context.

As the frontispiece for these reviews, we have chosen a drawing restored from Kaestner's1892 volume on the trunk and tail muscles of vertebrates. It beautifully illustrates the fundamental similarities of three stages of development in four grades of mesodermal evolution. We can project onto this diagram the wealth of data accumulated over the last 115 years and described in the reviews presented here. At the same time we can see the many questions that remain. 1

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Figure 1. Illustration from Kaestner (1892). Restored and modified by J.A. Powzyk, A.C. Burke, and S.H. Deveto.

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  • Kaestner S. 1892. Uber die allgemeine Entwicklung der Rumpf- und Schwanzmusculatur dei Wirbeltheiren. Mit besonderer Beruecksichtigung der Selachier. Arch Anat Entwicklungsgesch 153222.