The majority of tissue related diseases are known to alter tissue structure. During the last 10 years, increasing efforts have been put into the development of new techniques that could provide in vivo information on tissue morphology. Optical coherence tomography (OCT) is known to provide structural information in situ. However, there is also a strong demand to evaluate the mechanical properties of biological tissues in vivo. To address this need, we combined microindentation with non-invasive OCT imaging to determine spatiotemporal distributions of mechanical properties of in vivo and formaldehyde fixed chicken embryos. The use of OCT allows us to quantify changes in tissue morphology and to localize indentation at specific regions. To measure viscoelastic properties of living tissue, indentation tests are simultaneously performed on in vivo HH8-HH12 chicken embryos using a cantilever based force transducer. After performing live tissue indentation, we probed the properties of formaldehyde fixed embryo. The same general contrasts of elasticity between different histological regions were found, but the average value was found to be higher for the fixed sample. Furthermore, with this technique it is possible to follow the remodeling of tissue elastic properties during embryonic development, measure viscoelastic properties of living tissue, and investigate correlations between local mechanical properties during cell migration and differentiation. This method is applicable to a wide variety of biological samples and can provide new insight to better understand the link between the mechanical response of tissue and its biological structure, and to compare diseased tissues with healthy one.
There is a significant interest in characterizing mechanical properties of the brain tissue due to the role of mechanics in neurodevelopment and neurological disorders. Previous Scanning Force Microscopy studies have reported that brain tissue has highly heterogeneous mechanical properties. Yet, it is not known how the structural components of the brain such as neurons, glial cells, their axons and dendrites, and extracellular matrix contribute to these stiffness variations. To investigate the structure-stiffness relation in brain tissue and thus solve this issue, we have employed dynamic indentation-controlled mapping with a spatial resolution of ~50 µm to measure viscoelastic properties of hippocampus and cortex on isolated horizontal mouse brain sections. Nonlinear-viscoelastic nature of the brain tissue was observed by oscillatory ramp testing, where stiffness increased linearly with the strain (strain < 10 %); frequency sweeps revealed frequency-stiffening (1-10 Hz) with the phase delay in the range of 15-30˚. Viscoelasticity maps showed that regions with distinct mechanical properties correspond to morphological layers with the mean storage modulus varying from 779±77 Pa for granular layer to 3260±74 Pa for stratum lacunosum-moleculare (mean ± SE). Density of the nuclei was estimated for the measured regions and was found to negatively correlate with the stiffness, except for alveus, mostly composed of axonal fibers, being significantly softer than all other high-stiffness low-cell-density regions. Taken together, our study shows that our novel indentation method is able to map mechanical differences of the brain at the cellular level, leading to a better understanding of the relation between tissue composition and stiffness.