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.
source https://www.lifetechnology.com/blogs/life-technology-science-news/fluorescence-microscopy-shows-how-living-cells-form-vesicles-to-transport-cargo-like-growth-factors
