Freeze-fracture study of the pavement cell in the lamprey gill epithelium. Analogy of membrane structure with the granular cell in the amphibian urinary bladder.

Freeze-fracture studies of the lamprey gill epithelium reveal structural differences of the luminal and basolateral plasma membrane of the pavement cells. The luminal membrane is characterized by only a few intramembrane particles on the P face and numerous large (10-13 nm) particles on the E face, whereas the basolateral membrane shows the majority of intramembrane particles (6-8 nm) on the P face. The structural specialization of the luminal membrane and the differences between the luminal and basolateral membranes of the pavement cell are similar to those previously demonstrated for the unstimulated granular cell of the amphibian urinary bladder. Because of this similarity, it is suggested that the 2 cell types are analogous and that the luminal membrane of the pavement cell in the lamprey gill epithelium is functionally characterized by a low water permeability. A possible role of sodium uptake by the pavement cells from freshwater and putative differences of osmoregulatory mechanisms in the gills of lampreys and teleosts in freshwater environments are discussed.