Lanthanum and freeze fracture studies on the retinular cell junction in the compound eye of the housefly

Summary The retinular (R) cell junction between adjacent photoreceptor cells in the house-fly ommatidium was characterized by freeze fracture, thin section and tracer (lanthanum) studies. Focal tight junctions occur between cells, and some P face ridge-E face groove correspondences are present in this intramembranal area. When colloidal lanthanum was introduced into the extracellular space (ECS) of the peripheral retina of the housefly, this electrondense tracer moved from the ECS (extra-ommatidial space), through the R-cell junctions and belt desmosomes, into the ommatidial cavity (OC = intrarhabdomal space) of each ommatidium. In the OC, lanthanum outlined a meshwork structure that pervaded this space. The evidence of this tracer movement suggests that there may be ionic continuity between the traditional ECS and the fluid bathing the individual rhabdomeres. The volume of the OC is calculated and we suggest that this space is part of the ECS. The functional implications of this postulate are considered in the light of: (1) the different functions of the peripheral and central cells; (2) the dissimilarity of rhabdomal membrane surface facing the OC compared to the unmodified plasma membrane of the photoreceptor cell facing the extra-ommatidial cavity; (3) the permeability properties of the R cell junction; and (4) the total ECS containing an ion store capable of sustaining current for the generator potential.We gratefully acknowledge support from the N.I.H., National Eye Institute, EYO 1686 and from the College of Agricultural and Life Sciences, Hatch Project 2100. We thank Dr. Philippa Claude (UW Primate Research Center, Madison) for training in freeze fracture technique. This part of the work was supported by Grant RR00167 from the N.I.H. to the Primate Research Center. Dr. Robert Goy, Director of the Primate Research Center is acknowledged for his kind permission to use the Center's freeze fracture apparatus. Professor Stanley D. Beck, Department of Entomology, UW, Madison provided valuable advice for which we thank him heartily