IDENTIFICATION OF SEPARATE SLOW AND FAST MUSCLE PRECURSOR CELLS IN-VIVO, PRIOR TO SOMITE FORMATION

We have examined the development of specific muscle fiber types in zeb rafish axial muscle by labeling myogenic precursor cells with vital fl uorescent dyes and following their subsequent differentiation and fate . Two populations of muscle precursors, medial and lateral, can be dis tinguished in the segmental plate by position, morphology and gene exp ression. The medial cells, known as adaxial cells, are large, cuboidal cells adjacent to the notochord that express myoD. Surprisingly, afte r somite formation, they migrate radially away from the notochord, bec oming a superficial layer of muscle cells, A subset of adaxial cells d evelop into engrailed-expressing muscle pioneers. Adaxial cells differ entiate into slow muscle fibers of the adult fish. We have named the l ateral population of cells in the segmental plate, lateral presomitic cells. They are smaller, more irregularly shaped and separated from th e notochord by adaxial cells; they do not express myoD until after som ite formation, Lateral presomitic cells remain deep in the myotome and they differentiate into fast muscle fibers. Thus, slow and fast muscl e fiber types in zebrafish axial muscle arise from distinct population s of cells in the segmental plate that develop in different cellular e nvironments and display distinct behaviors.