Electron micrographs of nascent (0-hour) reticulocytes. Nascent reticulocytes were placed in culture medium and incubated for 15 minutes at 37°C prior to fixation and processing for transmission electron microscopy. Images of 2 reticulocytes demonstrating endocytic vesicles (arrowheads), mitochondria (m), and intracellular vacuoles (v). Inserted images in lower left corner of each panel show lower-power magnification of the same respective cells. Image in the lower center of the top panel shows a higher-power magnification of the coated pit in the upper right of the same reticulocyte. The clathrin coating the pit is shown by white arrows. Magnification, × 23 000.