Figure 6. The number of endosomes first rose, then decayed to a steady state during continuous stimulation at 30 Hz.

Data points averaged from preparations stimulated in FM1-43 for the times indicated and then fixed. A, typical endosomes (punctate spots) and vesicles (haze) internalized at various times during stimulation. Brightness of each image was adjusted to optimize visualization of endosomes. Endosomes contributed substantially to overall brightness initially (1–36 s) but most dissipated over time (60–240 s); those remaining often appeared fuzzy (arrowheads). B, mean size of endosomes seemed to increase slightly, then decrease with duration of stimulation (differences not significant). C and D, mean number of endosomes (C) and area occupied (D) increased substantially, then decreased with duration of stimulation. A steady state in number and total area was reached at ∼120 s.