Supplementary Figure 1: Synchronicity of [Ca²⁺]_i transients in different cell regions

(a-c) Charts: Fraction of time (Probability) that the number of processes on the abscissa (N) had a simultaneous [Ca²⁺]_i elevation, in (a) pre-OLs, (b) early-OLs and (c) OLs from 5, 6 and 6 animals respectively (mean values, shown as horizontal lines with SEM bars, that are below all visible individual data points reflect some data points being zero and hence not plotted on the logarithmic scale charts). Inset tables: Observed distribution of probability as in main graphs, and predicted distribution calculated as follows. If the probability of a single observed process having a high [Ca²⁺]_i is denoted p ₁ (this was derived experimentally for each process and averaged over all the processes of each individual cell), and there are N processes, the probability of k processes exhibiting calcium transients simultaneously is: \(p\left(k\right)=\frac{N!{p}_{1}^{k}{(1-{p}_{1})}^{N-k}}{k!\left(N-k\right)!}\). These are the predicted values in the inset tables, when averaged over all cells. The observed values for the fraction of time that 2 or more processes simultaneously showed a high [Ca²⁺]_i were greater than predicted for pre-OLs and early-OLs, implying cooperativity of transient generation between processes or propagation of transients from one process into another, but not for OLs (see 2-sided Chi-squared p values for inset tables). (d-f) Probabilities of N (abscissa) processes having high [Ca²⁺]_i simultaneously and having the soma [Ca²⁺]_i low (black bars) or high (i.e. during a soma [Ca²⁺]_i transient, grey bars), for each of the 3 classes of cell. (g-i) Cumulative probability distributions for the data in d-f, with p values from Kolmogorov-Smirnov tests comparing the distributions for each cell class. When the soma [Ca²⁺]_i was high in early-OLs, there was a larger probability for 1 or more processes to have an elevated [Ca²⁺]_i, than was the case when the soma [Ca²⁺]_i was low. Soma transients occurred with a probability of 0.10, 0.08 and 0.02 for pre-OLs, early-OLs and OLs, respectively, and so the majority of [Ca²⁺]_i transients in processes occurred when soma [Ca²⁺]_i was low. In OLs in particular, processes only showed [Ca²⁺]_i elevations when the soma [Ca²⁺]_i was low, and the data in panel c imply that [Ca²⁺]_i transients in different myelin sheaths are independent of each other. (j-l) Probability of the soma [Ca²⁺]_i being high when the number of processes on the abscissa simultaneously exhibited high [Ca²⁺]_i for each of the 3 classes of cell. For pre-OLs the probability of the soma [Ca²⁺]_i being high is increased when more processes have a simultaneously high [Ca²⁺]_i (p=0.0024, ANOVA) indicating that gene expression might be controlled by somatic [Ca²⁺]_i elevations triggered by simultaneous activity-evoked transients in several processes on the same developing oligodendrocyte. For early-OLs the increase of probability with number of processes simultaneously exhibiting high [Ca²⁺]_i did not reach significance (p=0.86, Kruskal-Wallis test). For OLs, the soma [Ca²⁺]_i was never high when process [Ca²⁺]_i was high. All data are mean±SEM.