Extracellular compartments of the blowfly eye: Ionic content and topology

To analyze the elemental composition and topology of the extracellular comp artments of the compound eye, the eyes of blowflies Calliphora vicina were rapidly frozen and ultrathin cryosections were freeze dried. Three zones of an ommatidium, peripheral cytosol of visual cells, rhabdomeres, and ommati dial cavities were analyzed by X-ray microprobe analysis. The ommatidial ca vity was found to contain sodium and potassium in proportion similar to tha t in the blowfly hemolymph. Potassium-to-sodium ratio in a cytosol was typi cal for a cytosol. The rhabdomeres displayed an electrolyte content interme diate between the above compartments. Three topologically connected extrace llular compartments were characterized by the experiments with tracers, mon astral blue and lanthanum: (1) common intercellular space of ommatidia incl uding peripheral clefts between the visual cells, both tracers entered this compartment; (2) the ommatidial cavity, which is not accessible for monast ral blue, however, as revealed by our X-ray microanalysis, it was reachable for lanthanum; (3) rhabdomeric loops, which were accessible for lanthanum entering either via the cavity or from the common intercellular clefts. The above characteristics of the ionic content and topology of ommatidial comp artments might suggest higher sodium and lower potassium content in the mic rovilli as compared with the cytosol. The rhabdomeric and "cavital" plasma membranes are assumed to be permeable for these ions so that a voltage of o nly 25-30 mV, negative inside, is probably formed across them, much lower t han the known resting potential -60 mV across the peripheral plasma membran e of a visual cell.