Endocytosis

Phagocytosis, pinocytosis, transcytosis and potocytosis are distinct cellular functions. In phagocytosis, the cell membrane engulfs the material to be taken up. Adjacent parts of the lamellae fuse, and the particle is now en-closed in a vacuole inside the cell, the phagosome. During pinocytosis , liquids or substances in soluble form in the extracellular space are transported into the cell ( internalization). Pinocytosis starts with the formation of invaginations or valleys in the plasma membrane ( membrane vesicularization ), with subsequent pinching off as a membrane vesicle from the plasma membrane into the cell. There are two types of pinocytosis—receptor-mediated pinocytosis and nonspecific pinocytosis (endocytosis ). In receptor-mediated pinocytosis, coated vesicles (coated pits) form. At their cytoplasmic face, they are coated with clathrin, among other proteins. Clathrin-coated vesicles have a diameter of 20 0–30 0 nm.  The figure shows the part of a myofibroblasts cell that contains the nucleus.  On the cell surface, there are round or oval plasma membrane invaginations with diameters of 60–100 nm. They are lined up at the plasmalemma like pearls on a string. These are caveolae , which do not have a clathrin coat but are enveloped by the protein caveolin. Caveolae are considered to be static invaginations, because their invaginations will only sporadically pinch off as vesicles. This mechanism of uptake and transport is called potocytosis.

Electron microscopy; magnification: × 25 000

References

Kuehnel, W.(2003). Color Atlas of Cytology, Histology, and Microscopic Anatomy. 4^th edition . Institute of Anatomy Universitätzu Luebeck Luebeck, Germany . Thieme Stuttgar t · New York .