MUSCULOSKELETAL PATTERNING IN THE PHARYNGEAL SEGMENTS OF THE ZEBRAFISH EMBRYO

The head skeleton and muscles of the zebrafish develop in a stereotype d pattern in the embryo, including seven pharyngeal arches and a basic ranium underlying the brain and sense organs. To investigate how indiv idual cartilages and muscles are specified and organized within each h ead segment, we have examined their early differentiation using Alcian labeling of cartilage and expression of several molecular markers of muscle cells, Zebrafish larvae begin feeding by four days after fertil ization, but cartilage and muscle precursors develop in the pharyngeal arches up to 2 days earlier, These chondroblasts and myoblasts lie cl ose together within each segment and differentiate in synchrony, perha ps reflecting the interdependent nature of their patterning, Initially , cells within a segment condense and gradually become subdivided into individual dorsal and ventral structures of the differentiated arch. Cartilages or muscles in one segment show similar patterns of condensa tion and differentiation as their homologues in another, but vary in s ize and shape in the most anterior (mandibular and hyoid) and posterio r (tooth-bearing) arches, possibly as a consequence of changes in the timing of their development. Our results reveal a segmental scaffold o f early cartilage and muscle precursors and suggest that interactions between them coordinate their patterning in the embryo, These data pro vide a descriptive basis for genetic analyses of craniofacial patterni ng.