Novel “omega muscle units” in superficial body-wall myotomes during metamorphosis in the northern brook lamprey (Ichthyomyzon fossor)

Abstract

Lampreys transform from sedentary filter feeders to more mobile adults through a dramatic metamorphosis that includes remodeling of head muscle and skeletal systems. Metamorphic modifications of body-wall myotomes that could support changes in swimming behavior from larvae to adults have not been previously reported. Thus, transverse sections of northern brook lamprey (Ichthyomyzon fossor Reighard and Cummins, 1916) in larval (n = 4), metamorphosing (n = 3), and adult (n = 2) stages were used to investigate the architecture of body-wall muscle and to detect whether Pax7 and MyoD, proteins important in myogenesis, were co-localized in any muscle nuclei. In addition to myotomal complexity of muscle units composed of parietal and central fibers, there was a novel pattern of omega-shaped muscle units with curves of muscle fibers in the superficial mid-body myotome in metamorphosing lamprey. Small satellite-like cells were identified on central fibers in metamorphosing and adult lamprey muscle using routine histology and immunolocalization of Pax7 and MyoD with antibodies that specifically detect mammalian and teleost proteins. Transient “omega muscle units” may be a marker for impending myotomal growth and increasing swimming efficiency during maturation, possibly restricted to metamorphosis. Finding satellite-like cells suggests that Pax7 and MyoD may have distinctive roles in lamprey myogenesis.