endocytic

Endocytic describes processes by which cells internalize extracellular material through vesicle formation at the plasma membrane. Endocytosis enables nutrient uptake, regulation of surface receptors, and internalization of ligands and particles for processing or signaling.

Several endocytic pathways exist. Phagocytosis involves large particles and is prominent in macrophages and neutrophils, driven by actin remodeling to envelop the target. Pinocytosis samples the extracellular fluid non-specifically. Receptor-mediated endocytosis uses specific cell-surface receptors to bind ligands and internalize them via clathrin-coated pits; adaptor proteins (such as AP2) recruit clathrin, and dynamin mediates vesicle scission before uncoating and delivery to early endosomes. Clathrin-independent routes include caveolae-mediated endocytosis, which relies on caveolin-rich membrane domains, and macropinocytosis, which involves extensive membrane ruffling to engulf fluid and solutes.

Trafficking after internalization directs cargo to endosomal compartments. Early endosomes serve as a sorting hub, directing materials for recycling back to the plasma membrane or trafficking to late endosomes and lysosomes for degradation. The process is regulated by Rab GTPases (for example, Rab5 on early endosomes and Rab7 on late endosomes) and by endosomal acidification that influences ligand release and receptor fate.

Functional roles of endocytic pathways include nutrient uptake (such as transferrin and low-density lipoprotein), receptor downregulation and signaling modulation, and antigen processing in immune cells. Endocytosis also intersects with pathogen entry and can influence disease processes and therapeutic delivery strategies.