ABSTRACT
Microautophagy was first proposed by de Duve and Wattiaux1 more than 30 years ago. This term referred to the then hypothetical notion that smaller bits of cytoplasm, perhaps even below the limits of detection by electron microscopy, could be taken up by lysosomes in addition to the more morphologically obvious macroautophagy. Some of the confusion in the literature regarding microautophagy results from the same name being applied to what may be very different lysosomal processes.2'5
Microautophagy refers to the internalization of bits of cytoplasm by vesicle formation at the lysosome membrane. The size of the vesicle and its mechanisms of formation appear to be variable (Fig. 3.1).2,3 Small organelles such as ribosomes and glycogen particles can be seen in these vesicles, but larger organelles such as mitochondria are usually excluded. Ahlberg and Glaumann2,3 described the in vitro uptake of particles and proteins by isolated lysosomes as reflections of microautophagy that result in multivesicular bodies (Fig. 5.2). However, large organelles such as peroxisomes can also be taken up by the yeast vacuole in a process also called microautophagy6 that involves wrapping of the peroxisome in a flap like protrusion from the vacuole membrane (Figs. 5.1C and 5.3). Finally, a different shape of some lysosomes in rat liver has been proposed to reflect microautophagy,7,8 but these lysosomes appeared to contain cup-like contents (Figs. 5. IB and 5.4) rather than the multi vesicular body appearance.
