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Fig. 4. (A) Schematic of the lysosomal system illustrating the endocytic and autophagic pathways to the (A) lysosome and the ultrastructure of (B-E) specific compartments. The major organelles of the autophagic pathway are a pre-autophagic structure (PAS), which sequesters large areas of cytoplasm within a double membrane-limited autophagosome (AP). This organelle receives hydrolases by fusing with either a lysosome to form an autophagolysosome (AL) or with a late endosome/multivesicular body (LE/MVB) to form an amphisome (Gordon and Seglen, 1998; Liou et al., 1997). Efficient digestion of substrates within these compartments in both cases yields a lysosome containing mainly acid hydrolases. (B) Internalized materials entering the endocytic pathway are directed to early (sorting) endosomes (EE), which mature to LE/MVB. (C) An autophagosome, which is hydrolase-negative, contains recognizable but partially digested organelle compartments. (D) Another type of immature AV is double-membrane limited and contains heterogeneous intraluminal materials, including other organelles. (E) Following further substrate digestion, the content of an autolysosomes is amorphous and less dense. Complete digestion of substrates within autolysosomes ultimately yields lysosomes, which are smaller, less dense vesicles containing mainly lysosomal hydrolases (E). Bar, 500 nm.

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