Endocytosis: a review of mechanisms and plasma membrane dynamics.

Endocytosis is the process of internalization of extracellular material within an invagination of the plasmalemma. Ultimately, this invagination is thought to give rise to an independent cytoplasmic vesicle. This vesicle is given a more specific name (e.g. phagosome, pinosome, coated vesicle, primary food vacuole) depending on the cell type being studied and the mode of endocytosis involved. Historically, the multitude of morphological varieties of endocytosis has led to a plethora of confusing terminology (e.g. cytosis, endocytosis, intracytosis). However, it was de Duve in 1963 who used the term 'endocytosis' to encompass all variant forms of this basic cellular transport mechanism. Classically, endocytosis has been divided into two types: phagocytosis, or eating, and pinocytosis, or drinking. The term phagocytosis is used to describe the internalization of particulate matter visible by light microscopy and, hence, larger than a few tenths of a pm in diameter. The term pinocytosis is used to describe the uptake of all smaller substrates, ranging from insoluble particles to low molecular weight solutes and to the fluid itself. Pinocytosis can be subdivided further, based on whether the substance enters the cell in the fluid content (fluid-phase pinocytosis) or bound to the vesicle membrane (adsorptive pinocytosis). Moreover, adsorptive pinocytosis can be subdivided further into endocytosis of markers which bind in a non-specific manner to the cell surface (e.g. cationic ferritin binding to anionic sites on the plasmalemma), and endocytosis of ligands mediated via binding to specific receptors on the cell membrane. The latter mechanism has been called receptor-mediated endocytosis (pinocytosis). Molecules known to be taken up by this receptormediated mechanism include lysosomal enzymes