Fig. 5: Longitudinal tissue microrheology in vivo.
a, Sketch of an animal with the intestinal tissue highlighted in green and the pharynx in red. The close-up sketch shows a pair of posterior intestinal cells with lipid droplets in blue. The lipid droplets were isolated from adult animals, purified and tested under various conditions for their suitability as optical tweezer probes (b and c; Methods). For in vivo application, individual droplets were trapped to measure the rheological response of the material in its vicinity (d–g). b, Refractive-index matching with varying concentrations of iodixanol. Bright-field micrograph of a lipid droplet in buffer B on the left (Methods; representative for N = 6 droplets) and in 48% of iodixanol (right; N = 12) (i). The graph shows the intensity profile along the dotted line indicated in the photograph. Scale bar, 2 µm. Force profile on a droplet in the matched conditions, for 0%, 48% and 54% of iodixanol (ii). c, Force scan across the lipid droplet for particle radius estimation (Methods). The lipid droplets vary in size from the trapping-force Rayleigh (dark red) and Mie (light red) limits. N = 2 droplets, representative for all the measurements. d, Fluorescence of GFP::lmn-1 and bright-field images demonstrating nuclear deformation with a trapped lipid droplet on contact during a tweezer experiment (i). δ indicates the deformation of the nucleus during the test, and the arrowhead points to the trapped lipid droplet. Scale bar, 2 µm. Kymograph of two consecutive step indentations of an intestinal nucleus using a lipid droplet as the force probe (ii). Force and displacement during the same step of the indentation protocol (iii). e,f, Frequency-dependent shear modulus for two different ages of wild-type (e) and age-matched (f) lem-2 mutants. The median and ±25% quantiles are represented by lines and shadows, respectively. g, Viscosity (Cβ) of the cytoplasm as extracted from the high-frequency component derived from the fit of the fractional Kelvin–Voigt model to the rheological spectrum of day 1 and day 8 adults for four different genotypes, as indicated. The red circle indicates median ± bootstrapped 95% confidence interval. P = 0.003 derived from a non-parametric Kruskal–Wallis test, followed by a pairwise comparison using a one-sided Dunn test without adjustment, as indicated above the horizontal brackets (for details on statistics and number of measurements, see Supplementary Data Table 2 and Extended Data Fig. 10). For wild type in D1, N = 35, n = 3, m = 10; in D8, N = 35, n = 3, m = 7. For GFP::lmn-1 in D1, N = 32, n = 4, m = 9; in D8, N = 24, n = 4, m = 8. For lem-2 in D1, N = 28, n = 3, m = 8; in D8, N = 20, n = 2, m = 6. For emr-1 in D1, N = 25, n = 3, m = 10; in D8, N = 21, n = 2, m = 6.
