Gap junctions in the cardiac muscle cells of the lamprey
Electron microscopy of both thin sections and freeze-fracture replicas has demonstrated the occurrence of gap junctions (nexuses) in the cardiac muscle cells of the lamprey. These gap junctions are identical in basic structure with those found in the mammalian heart. However, they are much smaller (less than 0.5 μ in diameter), and more irregularly distributed than the typical gap junction in the mammalian heart. These small gap junctions seem to provide a structural basis for the electrical coupling between cardiac muscle cells in the lower vertebrates.
In addition, the well developed sarcoplasmic reticulum and subsurface cisternae, which contain an electron dense spheroidal cast, are frequently observed in the cardiac muscles of the lamprey.