Membrane structural specialization of the toad urinary bladder revealed by the freeze-fracture technique. I. The granular cell.

Examination of the toad urinary bladder by freeze-fracture electron-microscopy demonstrates structural specialization of the granular cell's luminal membrane compared to its basal membrane. Although both membranes appear to possess about 1,700 intramembranous particles per mum2, those of the luminal membrane tend to be significantly larger in size. In addition, the fracturing properties of the two membranes are markedly different: the majority of particles are found on fracture face B (outer membrane face), in the case of the luminal membrane, and the majority are found on fracture face A (inner membrane face), in the case of the basal membrane. While the two fracture faces of the basal membrane possess a similar distribution of particle sizes, in the case of the luminal membrane the B face was found to possess particles generally larger than those found on the A face. It was established that the probability of luminal membrane particles adhering to face B instead of face A is closely correlated with the size of the particle. The structural specialization of the granular cell's luminal membrane may have an important relationship to the characteristic permeability properties of this membrane and the capacity of this cell type to respond physiologically to the hormone vasopressin.