Variability of intercellular spaces between macula densa cells: a transmission electron microscopic study in rabbits and rats.

The macula densa in rabbits and rats was studied by transmission electron microscopy (TEM). After different experimental procedures, the kidneys were fixed by direct perfusion with the fixative without prior flushing of the blood. With this technique, the renal cortex is consistently well preserved. The lateral intercellular spaces of all proximal and distal nephron segments were constantly found to be closed, whereas those of the macula densa varied: depending on the functional situation of the kidney, they were found to be either dilated or closed. Dilated intercellular spaces in the macula densa were encountered in control, sodium-rich, and sodium-deficient rabbits and rats and, in addition, in rats with hypotonic and isotonic hypervolemia. In contrast, in rats with hypertonic hypovolemia and in rats undergoing furosemide or mannitol diuresis, the lateral intercellular spaces of the macula densa were closed. Whether these findings reflect the in vivo state of the macula densa interspaces remains uncertain. The association with specific functional stages demonstrates, at least, a specific behavior of the macula densa cells that is different from that of all other proximal and distal nephron segments and appears to be similar to that of the collecting duct epithelium. The findings suggest that the macula densa is a water-permeable cell plaque within the otherwise water-impermeable thick ascending limb.