Plasma membrane specializations in the cells of the kidney distal segment of the lamprey, Lampetra japonica (von Martens).

The unique and highly specialized structural features of the plasma membrane in the cells of the kidney distal segment of the lamprey, Lampetra japonica, were studied by electron microscopy. The cells of the distal segment are largely filled by a continuous network of cytoplasmic tubules which are derived from the basolateral plasma membrane. Thin sections and freeze-fracture replicas of the membrane of the cytoplasmic tubules show spirally wound parallel rows of particles. The rows are approximately 17 nm apart and are wound at a pitch of approximately 45 degrees with respect to the major axis of the tubules. Another type of membrane specialization was found in the freeze-cleaved surface of the basolateral plasma membrane. It consists of large square aggregations of membrane particles containing 100-400 cuboidal subunits. The distribution of these particles in this cell, as well as in other systems in which they have been noted, suggests a polarization of membrane activity.