FIG. 4.

Bidirectional cross talk in developing co-cultures. (a) Representative brightfield images and overall time course of the ratio of active vs quiescent muscle strips in (i) co-cultures and (ii) muscle-only cultures. (b) Spontaneous contraction forces in muscle-only and co-culture samples at days 3, 5, and 7. Values are spontaneous contraction force averaged over a 30 s recording per muscle strip at each day, and box plots represent the 25th, 50th, and 75th percentiles with whiskers representing 1.5×IQR, n = 27 muscle strips for co-culture, n = 20 muscle strips for muscle-only at each day, ^*p < 0.05, and ^**p < 0.005 (Mann Whitney U test). (c) Comparison of the percentage of cross-striated muscle fibers between muscle-only and co-culture groups. Bars represent mean ± SD, n = 6 muscle strips for each group, and ^**p < 0.005 (Mann Whitney U test). Confocal images at the right show sample muscle fibers with and without cross-striations. (d) MEA raster plots of representative samples from (i) control and (ii) CM groups at day 9. Black dashed lines represent the individual spikes, blue dashed lines represent the bursts, and pink boxes outline the synchronous bursts. (e) Time evolution of the MEA burst rate of neurons in control and muscle CM groups. Box plots represent the 25th, 50th, and 75th percentiles with whiskers representing 1.5×IQR, the values are average burst rates per electrode over 10 min recording from the entire well, n = 12 wells each for control and CM at each day, ^**p < 0.005, and ^***p < 0.0005 (student's t-test). (f) Conceptual illustration of bidirectional cross talk and its functional outcomes. Scale bars: (a) 500 μm and (c) 10 μm.