So in this case it was a ligand that was produced by one cell and moved to a second cell. In this case, those cells actually physically interact. So what happens is a protein on one cell interacts with the protein on the membrane of the other cell and when they bind by binding, that initiates the creation of some type of internal signal. And then what that signal does is it alters the gene expression of that cell and that ultimately determines what type of pathway that cell will follow and what type of cell it will actually produce. And finally, we have gap junctions. So some cells are actually physically connected to one another via special type of junction known as a gap junction.