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Fig. 2. Cytoplasmic structures visualised by transmitted light are dynamic but move at a lower speed than chromaffin granules. Time-lapse high magnification images of chromaffin granule fluorescence (B) and transmitted light structures (A) acquired at 1 Hz over a 1-minute period in basal medium. Arrows depict disruptions in the peripheral ring and vesicles accessing this region. (C) Dynamics were analysed by measuring the time-dependent variations of averaged intensity in a region of interest (ROI, see boxes in A,B). Vesicles present short cycle oscillations in fluorescence (2-5 seconds), whereas visible light intensity changed in cycles of time ranging from 10-20 seconds. (D) Distribution of X-Y displacements for vesicles and particle structures obtained after threshold of the transmitted light images. The average displacements were 0.79±0.09 µm over 20 seconds (for 60 threshold particles in 16 cells) for the visible light structures and 3.75±0.50 µm/20 seconds (n=135 vesicles corresponding to 11 cells) for the vesicle displacement. (E) Mean square displacement (MSD) as a function of time obtained for vesicles (n=27) and particle structures (n=18). Also plotted were the best linear fits for the different data, used to estimate the diffusion coefficient according to Qian et al. (Qian et al., 1991) (continuous lines), and the best fit to the equation MSD(t) = r_c² [1 – A₁ exp (–4 A₂ D t /r_c²)], defining movement restricted by a cage (dashed line).

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