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The majority of the skeletal musculature of the body is hypaxial muscle, defined as those muscles ventral to the vertebral axis and innervated by the ventral rami of the spinal nerves. These muscles include those of the body wall, limbs, muscular diaphragm, and tongue. Since all skeletal muscle of the body originates from the dorsally situated somitic dermomyotomeit is necessary to develop an understanding of how the precursors of the muscular system translocate from the site of origin to their final destination. In this presentation we provide data showing that the traditional idea that muscle translocates directly from source to final anatomical site may be too simplistic a view. Whilst investigating the development of the perineal muscles of mammals, we have discovered the existence of a novel mechanism used to translocate precursors form the somites to the perineum. We show that the muscle cells first enter the hindlimb and then move to their final destination. We also present data to show that the development of upper body muscles also is dependent on (fore) limb formation. We have termed this novel mode of muscle movement the "In-Out Mechanism". [ABSTRACT FROM AUTHOR]

Ectodermal Wnt6 plays an important role during development of the somites and the lateral plate mesoderm. In the course of development, Wnt6 expression shows a dynamic pattern. At the level of the segmental plate and the epithelial somites, Wnt6 is expressed in the entire ectoderm overlying the neural tube, the paraxial mesoderm and the lateral plate mesoderm. With somite maturation, expression becomes restricted to the lateral ectoderm covering the ventrolateral lip of the dermomyotome and the lateral plate mesoderm. To study the regulation of Wnt6 expression, we have interfered with neighboring signaling pathways. We show that Wnt1 and Wnt3a signaling from the neural tube inhibit Wnt6 expression in the medial surface ectoderm via dermomyotomal Wnt11. We demonstrate that Wnt11 is an epithelialization factor acting on the medial dermomyotome, and present a model suggesting Wnt11 and Wnt6 as factors maintaining the epithelial nature of the dorsomedial and ventrolateral lips of the dermomyotome, respectively, during dermomyotomal growth. [ABSTRACT FROM AUTHOR]