Fig. 3.

Some growth cone filopodia contain endocytic organelles. A, Determination of pH of 35 endocytic organelles in filopodia revealed a bimodal frequency distribution of pH. A majority of the organelles (60%) were slightly acidic, whereas a substantial portion (40%) remained neutral. B, The positions of all 30 organelles observed in filopodia longer than 5 μm were measured as a distance from the growth cone lamella and plotted against organelle pH. There was virtually no overlap between the neutral and acidified populations: all neutral organelles were located >5 μm from the lamella, whereas nearly all of the acidified organelles were confined to the proximal 5 μm of filopodia.