Laminin chains in developing and adult human myotendinous junctions

In addition to being the specialized site for transmission of force from th e muscle to the tendon, the myotendinous junction (MTJ) also plays an impor tant role in muscle splitting during morphogenesis. An early event in the f ormation of the MTJ is a regional deposition of basement membranes. We used immunocytochemistry to investigate the distribution of laminin chains duri ng the development of MTJs in human limb muscle at 8-22 weeks of gestation (wg) and in adult MTJs. We used polyclonal antibodies and a new monoclonal antibody (MAb) against the human laminin alpha 1 G4/G5 domains. At 8-10 wg, laminin alpha 1 and laminin alpha 5 chains were specifically localized to the MTJ. Laminin alpha 1 chain remained restricted to the MTJ at 22 wg as t he laminin beta 2 chain had appeared, whereas the laminin alpha 5 chain bec ame deposited along the entire length of the myotubes from 12 wg. In the ad ult MTJ, only vestigial amounts of laminin alpha 1 and laminin alpha 5 chai ns could be detected. On the basis of co-distribution data, we speculate th at laminin alpha 1 chain in the forming MTJ undergoes an isoform switch fro m laminin 1 to laminin 3. Our data indicate a potentially important role fo r laminin alpha 1 chain in skeletal muscle formation.