Mesenchymal stem cell durotaxis depends on substrate stiffness gradient strength

Mesenchymal stem cells (MSCs) respond to the elasticity of their environment, which varies between and within tissues. Stiffness gradients within tissues can result from pathological conditions, but also occur through normal variation, such as in muscle. MSC migration can be directed by shallow stiffness gradients before differentiating. Gradients with fine control over substrate compliance – both in range and rate of change (strength) – are needed to better understand mechanical regulation of MSC migration in normal and diseased states. We describe polyacrylamide stiffness gradient fabrication using three distinct systems, generating stiffness gradients of physiological (1 Pa/μm), pathological (10 Pa/μm), and step change (≥ 100Pa/μm) strength. All gradients spanned a range of physiologically relevant elastic moduli for soft tissues (1–12 kPa). MSCs migrated to the stiffest region on each gradient. Time-lapse microscopy revealed that migration velocity correlated directly with gradient strength. Directed migration was reduced in the presence of the contractile agonist lysophosphatidic acid (LPA) and cytoskeleton-perturbing drugs nocodazole and cytochalasin. LPA- and nocodazole-treated cells remained spread and protrusive on the substrate, while cytochalasin-treated cells did not. Nocodazole-treated cells spread in a similar manner to untreated cells, but exhibited greatly diminished traction forces. These data suggest that a functional actin cytoskeleton is required for migration whereas microtubules are required for directed migration. The data also imply that, in vivo, MSCs may preferentially accumulate in regions of high elastic modulus and make a greater contribution to tissue repairs in these locations.     

Tracking,gating, free-breathing,which technique to use for lung stereotactic treatments? A dosimetric comparison

BackgroundThe management of breath-induced tumor motion is a major challenge for lung stereotactic body radiation therapy (SBRT). Three techniques are currently available for these treatments: tracking (T), gating (G) and free-breathing (FB).AimTo evaluate the dosimetric differences between these three treatment techniques for lung SBRT.Materials and methodsPretreatment 4DCT data were acquired for 10 patients and sorted into 10 phases of a breathing cycle, such as 0% and 50% phases defined respectively as the inhalation and exhalation maximum. GTV_ph, PTV_ph (=GTV_ph + 3 mm) and the ipsilateral lung were contoured on each phase.For the tracking technique, 9 fixed fields were adjusted to each PTV_ph for the 10 phases. The gating technique was studied with 3 exhalation phases (40%, 50% and 60%). For the free-breathing technique, ITV_FB was created from a sum of all GTV_ph and a 3 mm margin was added to define a PTV_FB. Fields were adjusted to PTV_FB and dose distributions were calculated on the average intensity projection (AIP) CT. Then, the beam arrangement with the same monitor units was planned on each CT phase.The 3 modalities were evaluated using DVHs of each GTV_ph, the homogeneity index and the volume of the ipsilateral lung receiving 20 Gy (V_20Gy).ResultsThe FB system improved the target coverage by increasing D_mean (75.87_(T)–76.08_(G)–77.49_(FB)Gy). Target coverage was slightly more homogeneous, too (HI: 0.17_{(T and G)}–0.15_(FB)). But the lung was better protected with the tracking system (V_20Gy: 3.82_(T)–4.96_(G)–6.34_(FB)%).ConclusionsEvery technique provides plans with a good target coverage and lung protection. While irradiation with free-breathing increases doses to GTV, irradiation with the tracking technique spares better the lung but can dramatically increase the treatment complexity.     

Genomics detects population structure within and between ocean basins in a circumpolar seabird: The white‐chinned petrel

The Southern Ocean represents a continuous stretch of circumpolar marine habitat, but the potential physical and ecological drivers of evolutionary genetic differentiation across this vast ecosystem remain unclear. We tested for genetic structure across the full circumpolar range of the white‐chinned petrel (Procellaria aequinoctialis) to unravel the potential drivers of population differentiation and test alternative population differentiation hypotheses. Following range‐wide comprehensive sampling, we applied genomic (genotyping‐by‐sequencing or GBS; 60,709 loci) and standard mitochondrial‐marker approaches (cytochrome b and first domain of control region) to quantify genetic diversity within and among island populations, test for isolation by distance, and quantify the number of genetic clusters using neutral and outlier (non‐neutral) loci. Our results supported the multi‐region hypothesis, with a range of analyses showing clear three‐region genetic population structure, split by ocean basin, within two evolutionary units. The most significant differentiation between these regions confirmed previous work distinguishing New Zealand and nominate subspecies. Although there was little evidence of structure within the island groups of the Indian or Atlantic oceans, a small set of highly‐discriminatory outlier loci could assign petrels to ocean basin and potentially to island group, though the latter needs further verification. Genomic data hold the key to revealing substantial regional genetic structure within wide‐ranging circumpolar species previously assumed to be panmictic.