Mosaic structure in the plasma membrane: spiral arrays of subunits in the cytoplasmic tubules of lamprey chloride cells.

The membrane architecture of the cytoplasmic tubules in lamprey chloride cells has been studied by electron microscopy using thin-section and freeze-fracture techniques. The chloride cell of lampreys (Lampetra japonica, Petromyzon marinus) is largely occupied with a continuous network of cytoplasmic tubules, which are derived from the invagination of the basolateral plasma membrane. The lumenal surface of this tubular network is covered with spirally wound parallel rows of electron-dense material, which consist of linear aggregates of particles. Freeze-fracture of the membrane of the tubules also shows spiral arrangements of particles (approximately 9 nm in diameter) on the P-face and complementary shallow grooves on the E-face. These arrangements of particles are about 17 nm apart and wound at a pitch of about 45 degrees. These complex organizations of the membrane of the tubules are probably the sites of transport of ion and water, which is essential for the maintenance of ionic homeostasis in both low- and high-salinity environments.