When many ligands bind to cell-surface receptors, ligand-receptor complexes are 
internalized via clathrin coated pits by a process called receptor-mediated 
endocytosis. The cytoplasmic fate of ligands internalized within endocytic 
vesicles or endosomes is variable. For example, maternal immunoglobulins are 
transported through the cytoplasm of neonatal intestinal epithelial cells and 
are exocytosed at the basolateral surface. However, other ligands are degraded 
as a result of their delivery to the lysosomal compartment of cells. Although 
the translocation of endosomes to the Golgi region in the cell centre seems to 
be a general phenomenon presumably coupled to ligand degradation by lysosomes 
and endosomes and lysosomes undergo saltatory movements within the cytoplasm, 
the spatial control of interaction between the two structures is not understood. 
To address this problem we have begun to examine the spatial and temporal 
intracellular distribution of endosomes and lysosomes. Utilizing a new 
fluorescent microscopic approach, we have now been able simultaneously to 
visualize endosome and lysosome populations in living cells. Our results suggest 
that a specific relocation of lysosomes is rapidly induced upon binding of 
different types of ligands to the cell surface; this migration of lysosomes to 
the Golgi region of the cell precedes the translocation of endosomes into the 
same area.