The lateral musculature in the midbody region of the teleost, Brachydanio rerio, was examined by light and electron microscopy in the adult and six developmental stages. Two main divisions of the adult lateral musculature are described: (1) a superficial portion composed of small, dark fibers with high fat content and succinic dehydrogenase (SDH) activity; and (2) a deep portion composed principally of larger, pale, “deep fibers” showing little SDH activity and containing little fat. Some “intermediate fibers” are also present in the deep portion near the horizontal septum. Myofibrils of all cell types appear ultrastructurally similar.
A general outline of myotomal differentiation has been established for the midbody somites. Myogenesis begins at the medial surface of the somite between the 20- and 25-somite stages and progresses laterally. Shortly before hatching, the myotome contains two structurally dissimilar types of young muscle cells. The appearance of these two muscle cell populations in larvae and fry supports the hypothesis that they develop into the superficial and deep portions of the adult lateral musculature. The intermediate fiber population is present by 2 1/2 months. The most lateral cells of the somite form a layer of flattened cells covering the lateral myotomal surface in the 33-somite embryo, and are considered to form the dermatome in this species.