Intramembrane aspects of myotendinous and myomuscular junctions: a freeze-fracture study of the gill sac-muscle of the Atlantic hagfish, Myxine glutinosa.

The intramembrane organization of the sarcolemma at the sites of myotendinous and myomuscular junctions was studied in the gill sac-muscle of the Atlantic hagfish, Myxine glutinosa, by using freeze-fracture replicas. At these sites rows of irregularly shaped particles (diameter approximately 6 nm) and short fibrils are present on the P face and a complementary pattern of grooves is present on the E face of the split plasma membrane. The center-to-center distance between adjacent rows of particles and grooves ranges from 12 to 20 nm. Rodlike projections being in register with the rows of particles and grooves, respectively, extend from the plasma membrane toward the extracellular space. These rodlike projections are also recognizable in thin sections, where they appear as spinelike projections (cross sections) or linear arrays (grazing sections) located in the lamina lucida of the basal lamina. The intramembrane particles are considered to be integral membrane proteins and to represent transmembrane links in a series of molecules by which intracellular actin filaments and extracellular collagen fibrils are connected across the plasma membrane. The rodlike projections are probably peripheral membrane proteins possibly connecting the plasma membrane with structural components of the basal lamina.