How does DiI reach internal membranous organelles in the fixed cells?

DiI’s presence in internal membranous organelles is influenced primarily by two factors  -  lateral diffusion and its differential affinity for different types of membranes. 

In order to reach internal membranous organelles in fixed cells, the hydrocarbon chain of the DiI molecule first embeds itself within the cell membrane. Once within the fluid membrane, the DiI molecule diffuses laterally and flip-flops between the outer and inner layer of the membrane bilayer more frequently than phospholipids. This rapid flip-flopping movement makes it easy for DiI molecules to rapidly diffuse through tight junctions and reach internal membranous organelles in fixed cells.