Extended Data Fig. 1: Electron microscopy and biomechanics.
From: Circadian control of the secretory pathway maintains collagen homeostasis
a, Collagen fibril diameter distributions measured from transverse TEM images of tail tendons sampled through a circadian period. Mice were housed in 12-hour dark/12-hour light cycles. ZT, zeitgeber time (hours into light). Fibrils (n = 1400) were measured for each panel. Black lines show 3-Gaussian fit curves. b, Typical scanning transmission electron microscopy image of fibrils from mechanically disrupted tendon. A set of 50 similar STEM images were acquired for each of 30 tendon samples. Bar, 500 nm. c, Representative mass-per-unit-length distribution measured from scanning transmission electron microscopy images of mechanically disrupted whole tendons. Time point shown is ZT12. The data shown is for n- = 997 fibrils from a single Achilles tendon. All mass-per-unit-length distributions from 30 tendon samples showed a characteristic prominent D1 peak. Percentage of D1 fibrils at all time points over 24 h is shown in Fig. 1e. d, Elastic modulus of Achilles tendon is not time-of-day dependent. Bars show mean ± s.e.m., n = 4 biological replicates, unpaired t-test, p = 0.46. e, Representative hysteresis curves from cyclic loading show the energy loss is greater in tendons taken at ZT15 compared with ZT3, n = 4 biological replicates, n = 5 hysteresis cycles for each displacement rate for each tendon sample. f, Time constants derived from stress–relaxation from an initial 1 N load of Achilles tendon sampled at ZT3 compared to ZT15 (T1, T2 and T3). Bars show mean ± s.e.m., n = 4 biological replicates; the p-values were 0.114, 0.026 and 0.75 for the time constants T1, T2 and T3, respectively. g, Comparison of energy loss values of Achilles tendons at ZT3 versus ZT15 for a 5-fold range of strain rates showing a consistent ~ 40% greater energy loss at ZT15 compared with ZT3. Bars show mean ± s.e.m., n = 4 biological replicates, p-values using the unpaired t-test are 0.27, 0.26, 0.21, 0.004, 0.01 for displacement rates of 1, 2, 3, 4 and 5 mm/min, respectively. See also Statistical Source Extended Data Fig. 1.
