Studies on cellular recovery from injury. II. Ultrastructural studies on the recovery of the pars convoluta of the proximal tubule of the rate kidney from temporary ischemia.

The pars convoluta of the proximal tubule of the rat kidney was studied by light and electron microscopy during the recovery phase from transient ischemia. The left kidney was made ischemic by clamping the aorta just above the left renal artery leaving the blood supply to the right kidney and the intestine intact. The pars convoluta (P1 and P2 segments) of the proximal tubule was examined both immediately after various periods of ischemia (15, 30, 60 and 120 min) and after the same ischemic periods followed by 3, 6, 12 and 24 h of blood reflow (= recovery phase). It was found that ischemia for periods up to 60 min were compatible with cell survival whereas 120 min of ischemia gave rise to irreversible cellular changes. Before regaining a normal conformation during the recovery phase, cells made ischemic for 15 min were characterized by slightly decreased cell height, dispersed nuclear chromatin, mitochondria in orthodox conformation and increased numbers of digestive vacuoles and of lipid droplets (stage A2). Most cells made ischemic for 15 min appeared normal when examined after 24 h of reflow. Cells made ischemic for 30 min were also characterized by stage A2 changes after 3 h of reflow and remained in this stage during all recovery periods studied. Cells made ischemic for 60 min first passed into a stage designated A1 and then later during the recovery phase into stage A2. Stage A1 cells were characterized by decreased height, condensed mitochondria, apparently increased numbers and sizes of secondary lysosomes and slightly dilated rough surfaced endoplasmic reticulum; 120 min of ischemia followed by reflow was not compatible with cell survival. Cells made ischemic for 120 min showed the following alterations denoted as stages C and D: shrunken with pyknotic nuclei, swollen mitochondria with large flocculent densities, and filled with different sized vesicles in the apical portion (stage C). Stage D included cells which had undergone necrosis, i.e., phasma membranes and organelles were fragmented and occurred as debris in the tubule lumens.