Formation of Clathrin Coated Vesicles.

Electron micrographs showing the sequence of events in the formation of a clathrin coated vesicle at the surface of the plasma membrane. See animation.

Initially a clathrin coated pit forms. The cargo receptors extend through the membrane and interact with cargo molecules in the cytoplasm. In this case the cells are chicken oocytes and the cargo receptors are interacting with particles containing protein and lipids that will be incorporated  into the yolk. As the process continues, the vesicles rounds up and pinches off.

Fig. 14-18b This figure is an SEM image  looking at the inside of the plasma membrane of a skin cell. It shows many clathrin coated pits and vesicles forming  on the inner surface of the plasma membrane.

Each of the polygonal rings on the surface of the pits is a complex of coat protein molecules. See animation on coat protein, vesicle formation and selection of cargo proteins.

Clathrin animation