Biophysical properties of cells such as intracellular mass density and cell mechanics are known to be involved in a wide range of homeostatic functions and pathological alterations. An optical readout that can be used to quantify such properties is the refractive index (RI) distribution. It has been recently reported that the nucleus, initially presumed to be the organelle with the highest dry mass density (ρ) within the cell, has in fact a lower RI and ρ than its surrounding cytoplasm. These studies have either been conducted in suspended cells, or cells adhered on 2D substrates, neither of which reflects the situation in vivo where cells are surrounded by the extracellular matrix (ECM). To better approximate the 3D situation, we encapsulated cells in 3D covalently-crosslinked alginate hydrogels with varying stiffness, and imaged the 3D RI distribution of cells, using a combined optical diffraction tomography (ODT)-epifluorescence microscope. Unexpectedly, the nuclei of cells in 3D displayed a higher ρ than the cytoplasm, in contrast to 2D cultures. Using a Brillouin-epifluorescence microscope we subsequently showed that in addition to higher ρ, the nuclei also had a higher longitudinal modulus (M) and viscosity (η) compared to the cytoplasm. Furthermore, increasing the stiffness of the hydrogel resulted in higher M for both the nuclei and cytoplasm of cells in stiff 3D alginate compared to cells in compliant 3D alginate. The ability to quantify intracellular biophysical properties with non-invasive techniques will improve our understanding of biological processes such as dormancy, apoptosis, cell growth or stem cell differentiation.

中文翻译：

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3D机械限制条件下细胞内质量密度和细胞力学的光学定量

已知细胞的生物物理特性，例如细胞内质量密度和细胞力学，涉及广泛的稳态功能和病理改变。可以用来量化此类属性的光学读数是折射率（RI）分布。最近有报道说，最初被认为是细胞内具有最高干质量密度（ρ）的细胞器，实际上其RI和ρ低于周围的细胞质。这些研究要么是在悬浮细胞中进行的，要么是粘附在2D基质上的细胞，但均未反映体内情况细胞被细胞外基质（ECM）包围。为了更好地近似3D情况，我们将细胞封装在具有不同刚度的3D共价交联藻酸盐水凝胶中，并使用组合光学衍射层析成像（ODT）-荧光显微镜对细胞的3D RI分布进行成像。出乎意料的是，与2D培养相比，3D中的细胞核显示出比细胞质更高的ρ。随后，使用布里渊荧光荧光显微镜我们发现，与细胞质相比，除具有更高的ρ外，核还具有更高的纵向模量（M）和粘度（η）。此外，增加水凝胶的硬度会导致更高的M刚性3D海藻酸盐中细胞的细胞核和细胞质都比顺应性3D海藻酸盐中细胞的细胞核和细胞质高。用非侵入性技术量化细胞内生物物理特性的能力将增进我们对诸如休眠，凋亡，细胞生长或干细胞分化等生物学过程的理解。