Cells have a clever way to transport cargos like growth factors across the cell membrane and into the cell. It is called clathrin-mediated endocytosis. Molecules of the protein clathrin gather on the inside of the cell membrane, and they deform the membrane to make what looks like a pit as seen from the outside.
Click here for original story, Fluorescence microscopy shows how living cells form vesicles to transport cargo like growth factors
Source: Phys.org