UNSW Embryo- Development of the Musculoskeletal System.PubMed Selected Reference Somitogenesis |
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Curr Top Dev Biol 1998;38:225-87
Somitogenesis.
Gossler A, Hrabe de Angelis M
Jackson Laboratory, Bar Harbor, Maine 04609, USA.
We are still far from understanding "somitogenesis" as a whole, but there is an emerging picture of the tissue interactions and molecular mechanisms that underlie and govern various aspects of this essential multistep patterning process in vertebrates. The ability to form segmental units appears to be a property specific to the paraxial mesoderm (as opposed to lateral or limb mesoderm), and this ability is probably acquired during early development, when paraxial mesoderm is specified and emerges from the primitive streak. Signaling molecules expressed in the primitive streak and tail bud are prime candidates involved in specifying paraxial (as well as other mesodermal) fates. Increasing levels of signaling molecules may be required in posterior regions of the embryo, and combinatorial signals may be essential to specify the paraxial mesoderm along the entire anterior-posterior axis. However, most of the pivotal signals, and the ways in which they are integrated and interact, remain enigmatic. Once the paraxial mesoderm is formed, segmentation proceeds largely without the requirement for continuous interactions with surrounding tissues. Somitomeres represent a morphologic pattern in the mesenchymal presomitic mesoderm, but their significance for somite formation is unclear. Molecular patterns are established in the presomitic mesoderm and probably are of functional significance. Cell interactions within the paraxial mesoderm appear to be involved in forming segment borders and ensuring their maintenance during subsequent differentiation of somites. These interactions are, at least in part, mediated by components of the conserved Notch signaling pathway, which may have multiple functions during somitogenesis. Epithelial somites are clearly a result of segmentation, but epithelialization is not the mechanism to form segments, supporting the idea that the basic mechanisms that govern segmentation in the mesoderm of vertebrates are very similar in different species despite divergent types of resulting segments (i.e., epithelial somites versus rotated myotomes). Concomitantly with segmentation, segment polarity and positional specification are established. How these processes are linked to, and depend on, each other is unknown, as is how they are regulated and how segmentation is coordinated on both sides of the neural tube. In contrast to early patterning in the presomitic mesoderm, patterning of the mature somites during their subsequent differentiation is the result of extensive tissue interactions. Virtually all tissues in close proximity to somites provide signals that are involved in induction or inhibition of particular differentiation pathways, but how these pathways are initiated is less clear. Some of the molecules mediating inductive signals and tissue interactions are known, and a growing number of candidate genes are potentially involved in regulating various steps of somitogenesis. The roles of these genes have yet to be analyzed. In addition, the molecular genetic analysis of mutations affecting somitogenesis, which were collected in the mouse and more recently in the zebrafish (Driever et al., 1996; Haffter et al., 1996; van Eeden et al., 1996), promises to add important new insights into this process. Much remains to be done, but the tools are at hand to provide further understanding of the molecular mechanisms underlying somitogenesis.
Publication Types:
- Review
- Review, academic
PMID: 9399080, UI: 98061430
Anat Embryol (Berl) 1995 May;191(5):381-96
Early stages of chick somite development.
Christ B, Ordahl CP
Institute of Anatomy, University of Freiburg, Germany.
We report on the formation and early differentiation of the somites in the avian embryo. The somites are derived from the avian embryo. The somites are derived from the mesoderm which, in the body (excluding the head), is subdivided into four compartments: the axial, paraxial, intermediate and lateral plate mesoderm. Somites develop from the paraxial mesoderm and constitute the segmental pattern of the body. They are formed in pairs by epithelialization, first at the cranial end of the paraxial mesoderm, proceeding caudally, while new mesenchyme cells enter the paraxial mesoderm as a consequence of gastrulation. After their formation, which depends upon cell-cell and cell-matrix interactions, the somites impose segmental pattern upon peripheral nerves and vascular primordia. The newly formed somite consists of an epithelial ball of columnar cells enveloping mesenchymal cells within a central cavity, the somitocoel. Each somite is surrounded by extracellular matrix material connecting the somite with adjacent structures. The competence to form skeletal muscle is a unique property of the somites and becomes realized during compartmentalization, under control of signals emanating from surrounding tissues. Compartmentalization is accompanied by altered patterns of expression of Pax genes within the somite. These are believed to be involved in the specification of somite cell lineages. Somites are also regionally specified, giving rise to particular skeletal structures at different axial levels. This axial specification appears to be reflected in Hox gene expression. MyoD is first expressed in the dorsomedial quadrant of the still epithelial somite whose cells are not yet definitely committed. During early maturation, the ventral wall of the somite undergoes an epithelio-mesenchymal transition forming the sclerotome. The sclerotome later becomes subdivided into rostral and caudal halves which are separated laterally by von Ebner's fissure. The lateral part of the caudal half of the sclerotome mainly forms the ribs, neural arches and pedicles of vertebrae, whereas within the lateral part of the rostral half the spinal nerve develops. The medially migrating sclerotomal cells form the peri-notochordal sheath, and later give rise to the vertebral bodies and intervertebral discs. The somitocoel cells also contribute to the sclerotome. The dorsal half of the somite remains epithelial and is referred to as the dermomyotome because it gives rise to the dermis of the back and the skeletal musculature. the cells located within the lateral half of the dermomyotome are the precursors of the muscles of the hypaxial domain of the body, whereas those in the medial half are precursors of the epaxial (back) muscles.
Publication Types:
- Review
- Review, academic
PMID: 7625610, UI: 95351553