Analyzing endosomes in nonsectioned cells by transmission electron microscopy

Cells take up plasma membrane proteins and lipids, together with plasma membrane-associated and fluid phase-endocytosed constituents, via distinct endocytic mechanisms. After having pinched off from the plasma membrane, endocytic vesicles fuse with endosomes; from there endocytosed molecules are sorted and transported by means of vesicular transport intermediates to distinct destinations, including lysosomes, the trans-Golgi network, and the plasma membrane. To study endosomal sorting processes, both light microscopic and electron microscopic techniques have been employed. A novel technique, involving 3,3'-diaminobenzidine (DAB) cytochemistry-based whole-mount immunoelectron microscopy, can be used to study the structure of endocytic compartments as well as the subendosomal/lysosomal distribution of relevant proteins in nonsectioned cells at high resolution.