Membrane specializations in flagellar ribbons of elasmobranch fish

Specialized epithelial cells lining the elasmobranch nephron bear numerous flagella which are organized into closely-packed, parallel rows forming ribbons (Lacy et al., 1989a). The compact arrangement of the adjacent flagella comprising each ribbon suggests they are structurally bound together, forming a single unit which functions to force urine along the nephric tubule. In the present study, the structural basis of the interflagellar connections was investigated by scanning electron microscopy (SEM) and by transmission electron microscopy (TEM) of thin sections and freeze fracture replicas. Various fixatives and histochemical stains were used to elucidate the structure and composition of the interflagellar adhesive material. SEM of the luminal cell surface showed the organization of the flagella in ribbons. In TEM, fixation in a solution containing glutaraldehyde and tannic acid, Ruthenium red or Alcian blue, or postfixation in reduced OsO(4) revealed that the plasma membrane of each flagellum of a ribbon was surrounded by a thin layer of surface coat composed of very short filaments more prominent at sites where adjacent flagella were in close apposition. In comparable locations, freeze-fracture replicas disclosed small aggregates or plaques of particles arranged in an irregular, discontinuous line on both faces P and E of the flagellar membrane. In areas where the flagella were not arranged into ribbons (most frequently after immersion fixation), the surface coat was thick and expanded and, in replicas, the intramembranous particles were randomly scattered. All of these plasma membrane specializations appear to function in binding adjacent flagella and thus facilitate a coordinated flagellar ribbon beat.