The arguments for pre-existing early and late endosomes

Abstract

The past decade has seen the elucidation of many of the events and processes responsible for receptor-mediated endocytosis. However, a fundamental question about the endocytic pathway remains unresolved: do early endosomes mature into late endosomes, or are these two distinct and pre-existing cellular organelles? General opinion tends to favour the former possibility, to the point where one poster session at the recent American Society for Cell Biology meeting was entitled 'Maturation of Endosomes'. This article draws together new data arguing in favour of pre-existing early and late endosomes, between which transport occurs by vesicle budding and fusion.