The newly made proteins leave the endoplasmic reticulum wrapped in a layer of
membrane called a vesicle. They travel toward the Golgi apparatus (on the right)
where the proteins are modified and sorted for transport. The Golgi is busy with
protein traffic moving in and out. The vesicle fuses with the membrane at one
end of the Golgi and a new vesicle containing the modified proteins is pinched
off the other side. The proteins are transported through the cytoplasm and delivered
to where they are needed. Some proteins are used inside the cell. Others, like
these growth factors, must be exported to function. The vesicle fuses with the
cell membrane, dumping the proteins outside the cell. The released proteins will
signal surrounding cells, or, in some pathways to cancer, will coax this cell
into further action.