TEM of a clathrin-coated pit (heart shape, pink) at the inner leaflet of the plasma membrane (dark blue) of a human cell. Cytoskeletal elements, the cell's 'rail tracks' are also visible (light blue). These play an important role in transport and cell division. A bird's eye view of the inner leaflet was obtained by growing HeLa cells on glass coverslips and then removing the tops of the cells by sonication. The basal membranes left on the coverslip were then processed for imaging by TEM.
Clathrin coated pits are responsible for receptor-mediated uptake of extracellular molecules, such as iron and growth factors, a process called clathrin-mediated endocytosis. After cargo recruitment these pits eventually pinch off from the plasma membrane as vesicles, which are transported into the interior of the cell to release the cargo. These pits have a morphologically characteristic crystalline coat composed of different proteins that are associated with the protein clathrin, which forms a distinct lattice that is seen in this image. Interestingly cholesterol plays a role in the correct formation of these clathrin coated pits and reduction of cholesterol in the plasma membrane results in irregular shaped pits which cannot pinch off and hence block endocytosis over time. The clathrin coated pits have a diameter of 150-200 nm. For original black and white TEM see B0008808.
