Two types of autophagy in the podocytes in renal biopsy specimens: ultrastructural study

Two types of autophagy in the podocytes were found in renal biopsy specimens by electron microscopy. Type I autophagy (about 1 microm in diameter) was found in 10 out of the 100 cases with renal diseases, and showed a condensed ribosome area with a limiting membrane. The origin of limiting membrane appeared to be from degenerated mitochondria. During type I autophagy formation, the thickness of limiting membrane changed from 5-6 nm to about 8-10 nm thickness. Type I autophagy did not transform to autophagosomes and autophagic vacuoles. On the other hand, many cases (90 out of the 100 cases) showed type II autophagy. Type II autophagy (3-8 microm in diameter) showed that many ribosomes were aggregated, formed condensed ribosome area, which always included many aggregated lipid droplets at first. Next, during the formation of autophagosome, rough ER connected to condensed ribosome area, and partly formed limiting membranes from dilated ER membrane. Finally, the limiting membrane of autophagic vacuoles was completely formed, and this membrane changed from about 5-6 nm to 8-10 nm thickness. Ribosomes and lipid droplets were resolved in autophagic vacuoles. Thus, type II autophagy might play a significant role in clearance of proteins and lipids in comparison with type I autophagy. The occurrence of type I autophagy in the renal biopsy specimens was not clearly associated with age, sex or pathological diagnosis. However, cases with type I autophagy may show a tendency to poor prognosis.