Biomedical Engineering Reference
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suggests that the growing contents of LDs in the differentiating cells could have
affected the measured adipocyte stiffnesses, but in the Young-Nam's study, the
tested cells were all immature, and without any optically-visible LDs. The elastic
moduli of adipose-derived adult stem cells and MSCs were significantly greater
than those of adipocytes in the Darling et al. [
13
] study, where AFM nanoin-
dentation experiments were conducted as well—but indentations were at the center
of the examined cells, a region that would likely be occupied by (stiff) nuclei. In
contrast to the aforementioned work, Yu et al. [
81
] found that adipocytes are stiffer
than MSC, but they used micropipette aspirations which was performed deliber-
ately away from the nucleus. Considering again the diversity in measurement
techniques as reported in these articles, the results are overall incomparable and
inconclusive, but again, they do point to the technical challenges in measuring
cell-level or subcellular-level mechanical properties, particularly in maturing
adipocytes. Taken together, the above results also indicate that structure-function
relationships exist at the cellular level as well, and in particular, the numbers and
sizes of LDs and the potentially-related remodeling of the cytoskeleton should
influence cell stiffness and hence also reflect on tissue stiffness at the macro-scale.
We have recently used AFM-based and interferometric phase microcopy (IPM) for
determining whether adipocytes soften or stiffen during differentiation, by tar-
geting intracellular organelles (LDs and nucleus) [
4
,
64
,
67
] rather than locations
on the top of cultured cells (e.g. cell centers) as done in the aforementioned studies
[
13
,
80
,
81
]. Correlating stiffness changes in maturing adipocytes with the relative
contributions of (developing) organelles, rather than measuring at fixed sites, is
important since e.g. in adipocytes the nucleus tends to move to the cell periphery
with maturation [
27
,
67
].
In the AFM studies, localized stiffnesses of the lipid and nucleus regions within
the adipocytes were calculated by means of nanoindentation measurements and by
applying the Hertz model to analyze the results [
67
]. We found that the mean ratio
of stiffnesses of the LDs over the nucleus was 0.83 ± 0.14, from which we further
evaluated the ratios of LDs over cytoplasm stiffness, as being in the range of
2.5-8.3. Considering that the volume of LDs relative to the volume of the cyto-
plasm increases considerably over time with differentiation [
52
], our results
indicated that adipocytes structurally stiffen with progress of the differentiation
process [
67
,
69
]. We verified these results by means of finite element (FE) mod-
eling, which simulated the AFM experiments, and which provided good agreement
between empirical and model-predicted structural behaviors [
66
,
67
]. In the IPM
studies, we were further able to detect rotational fluctuations of the LDs in the
surrounding cytoplasm [
67
]. Since LDs were found to mechanically distort their
intracellular environment, the IPM results again indicated that LDs are mechani-
cally stiffer than the surrounding cytoplasm and that the stiffness of adipocytes
increases with their differentiation [
67
].
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