Pinocytosis

Pinocytosis (Greek pinein -drinking and kytos -cell) refers to the uptake of liquids into the cell, phagocytosis to the uptake of solid substances from its surrounding medium. Both processes are summarized under the generic term endocytosis.

In contrast to phagocytosis, which is only carried out in special situations and by special cells, pinocytosis is a widespread mechanism in eukaryotic cells.

The uptake of fluid into the cytoplasm occurs in the form of small vesicles that are only 150 nm in diameter. The liquid or solute components are taken up into the cytosol of the cells. Membrane extensions enclose the substance to be taken up. They are taken up as vesicles into the cytosol of the cell and are involved in the metabolism of the cells. For example, pinocytosis plays an important role in the uptake of protein-bound lipids in hepatocytes and enterocytes.

In the reversal process, the absorbed fluids are released from the cell again (exocytosis). This cell process allows a controlled uptake of fluids and solids.

Furthermore, pinocytosis plays an important role in cell and tissue development, immune response, cell communication and signal transduction. It is also involved in signal transduction in neuronal areas. However, pathogenic microorganisms can also enter the cells via pinocytosis.

The eukaryotic cells know four different types of pinocytosis:

• clathrin-dependent endocytosis (this is the best understood form of endocytosis to date, characterized by its first step, the internalization of extracellular material at specialized areas of the plasma membrane, the so-called clathrin-coated pits)
• macropinocytosis,
• caveolae-mediated endocytosis...
• and the clathrin-independent pathways in which material is taken up neither via clathrin-coated pits nor via caveolae (nonclathrin noncaveolae-mediated endocytosis).

Some specialized cells are also able to take up large amounts of extracellular fluid via macropinocytosis or large particles via phagocytosis (Gruenberg 2001).

A special form of pinocytosis is receptor-mediated endocytosis, in which only specific molecules are taken up. Here, the ligands first bind to specific receptors in the cell membrane. The ligand-receptor complexes concentrate in special regions of the cell membrane, so-called " pits ". A lattice of clathrin molecules forms on the inside of the pits, which is why these pits are also called "clathrin-coated pits". With the reversal process of pinocytosis, exocytosis, the vesicle is released from the cell again. The membrane of a vesicle fuses with a cell membrane, allowing the substance taken up by the vesicle to exit. This process is stimulated by certain membrane receptors.

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2. Gruenberg J (2001). The endocytic pathway: a mosaic of domains. Nat Rev Mol Cell Bi 2: 721-730.
3. Gruenberg J et al (1989). Characterization of the early endosome and putative endocytic carrier vesicles in vivo and with an assay of vesicle fusion in vitro. J Cell Biol 108: 1301-1316.
4. Huth E (2005) Cellular uptake and intracellular fate of particulate drug delivery systems. Inaugural dissertation for the award of the doctorate of the Faculty of Chemistry, Pharmacy and Geosciences of the Albert-Ludwigs-University Freiburg.