The segmented mesoderm in vertebrates

The patterning and differentiation of cells in the somitic mesoderm is an important step in the development of axial structures that include the vertebral column and skeletal muscle. Development of the somitic mesoderm is characterized by the progressive segmentation of mesoderm tissue directly flanking the neural tube into two bilateral rows of somites. Somites when they first appear consist of radially arranged epithelial balls of mesoderm called epithelial somites. These epithelial somites then undergo a number of morphological changes that eventually result in their dissolution and differentiation into a variety of tissues including bone, cartilage, dermis and muscle (reviewed in Keynes and Stern, 1988). An early morphological change during somite maturation occurs when cells in the ventromedial part of the somite lose their epithelial arrangement and form a loosely arranged mesenchyme from which sclerotome cells are derived. At the same time, cells in the dorsal part of the somite retain their epithelial organization and form the dermomyotome. The dermomyotome subsequently develops into two structures, an outer dermatome cell layer and an inner myotome cell layer. Both the axial and appendicular muscles and dermis are generated from the dermomyotome. In the chick embryo, two populations of myogenic precursors have been identified in somites. The striated muscles of the axial musculature derived from the medial part of the somite and limb muscles that develop from the lateral somite (Ordahl and Le Douarin, 1992). Initially both the lateral and medial somite halves are capable of generating limb or trunk muscles; however, this is lost as the somites mature (Aoyama and Asamoto, 1988; Ordahl and Le Douarin, 1992). Muscle precursor cells for the limb musculature are derived from the lateral portion of the dermomyotome adjacent to the limb bud. These cells migrate into the limb bud where they differentiate and fuse to form the primary myotubes of the limb muscles (Chevallier et al., 1977; Christ et al., 1977). Although the events governing the development of the somite and the generation of different cell types within the somite are poorly understood, embryonic manipulations in the chick embryo have provided insights into the nature of the interactions that regulate somite development. Experiments manipulating somites in the chick embryo are consistent with cell fate in the early somite being determined by the interaction of somitic cells with adjacent tissues (Aoyama and Asamoto, 1988; Ordahl and Le Douarin, 1992). Recent experiments indicate that the notochord and floor plate can influence the differentiation of cell types derived from the somite. For example, implanting an extra notochord adjacent to somites causes the loss of the dermomyotome from regions adjacent to the graft and the development of ectopic vertebrae (Pourquie et al., 1993). In contrast, removing both the notochord and neural tube from E2 chick embryos results in the loss of both sclerotome and myotome (Rong et al., 1992), suggesting that 957 Development 120, 957-971 (1994) Printed in Great Britain © The Company of Biologists Limited 1994