Cell movement and contraction in somite development.

During somite formation the segmental plate mesoderm, lying on either side of the axial organs, reorganizes into roughly spherical pairs of epithelial structures. This segmentation process includes changes in cell shape and position, cell-cell and cell-substratum adhesive properties and accumulation of extracellular matrix material which proceed down the anterior-posterior axis. Later in somite development the sclerotome region "disperses", migrating around the spinal cord where it produces the cartilage model of the vertebral column. Experimental manipulation of segmentation and sclerotome dispersal with drugs affecting microfilaments, microtubules and calcium-dependent contraction suggest that cells migrate into position, elongate, and undergo apical contraction as part of the segmentation process. This process of calcium-dependent, possibly calmodulin-mediated, contraction can be both stimulated precociously and inhibited, showing similarities with contractile morphogenetic events in epithelial organ systems such as eye and thyroid. Similar experiments with drugs affecting contractile microfilaments demonstrate that active cell movement, along with extracellular matrix production, is involved in sclerotome dispersal.