Question

What can prevent the passage of molecules and ions between cells, such as in the blood-brain barrier?

Answer 1

tight junctions

In addition to holding cells together, tight junctions play a role in the transport of materials. Tight junctions prevent the passage of molecules and ions through the space between cells. So these molecules and ions must actually enter cells  either by diffusion or active transport  in order to proceed through a tissue. This allows tight junctions to indirectly play a role over what substances are allowed into a specific cell. Tight junctions play this role in maintaining the blood-brain barrier.