Which type of endocytosis involves the uptake of small extracellular fluid droplets?
Cells internalize extracellular material by mechanistically distinct endocytic routes distinguished by particle size, actin requirement, coat dependence. Phagocytosis mediates uptake large particulates above 0.5 micron such as bacteria, yeast, dead cells via extension actin-driven pseudopods triggered clustering Fc gamma receptors, complement receptors CR3, scavenger receptors signaling via Syk, PI3K, Rho GTPases prominent professional phagocytes macrophages neutrophils forming phagosome destined lysosome. Pinocytosis literally cell drinking from Greek pinein describes constitutive uptake small droplets extracellular fluid and dissolved solutes without particle binding via multiple clathrin-dependent and independent mechanisms including macropinocytosis involving Rac1 mediated ruffling producing 0.2-5 micron vacuoles and micropinocytosis via caveolae and CLIC GEEC pathways 60-80 nm vesicles. Fluid-phase markers horseradish peroxidase, fluorescent dextran accumulate linearly in pinosomes. Receptor-mediated endocytosis concentrates specific ligands LDL, transferrin using AP2 clathrin coats with high efficiency saturable. Autophagy delivers internal cytoplasmic components organelles to lysosome for self-digestion opposite direction. Therefore pinocytosis uniquely denotes non-concentrative fluid-phase small droplet uptake distinct from phagocytosis and receptor mediated.
Ref: Alberts et al., MBC Chapter 13: Pinocytosis for fluid-phase uptake.