Membrane fluidity plays an important role in many cell functions such as cell adhesion, and migration. In stem cell lines membrane fluidity may play a role in differentiation. Here we report the use of viscosity-sensitive fluorophores based on a BODIPY core, termed "molecular rotors", in combination with Fluorescence Lifetime Imaging Microscopy, for monitoring of plasma membrane viscosity changes in mesenchymal stem cells (MSCs) during osteogenic and chondrogenic differentiation. In order to correlate the viscosity values with membrane lipid composition, the detailed analysis of the corresponding membrane lipid composition of differentiated cells was performed by timeof-flight secondary ion mass spectrometry. Our results directly demonstrate for the first time that differentiation of MSCs results in distinct membrane viscosities, that reflect the change in lipidome of the cells following differentiation. Membrane fluidity is considered a key parameter influencing biological function of cells, such as cell adhesion, migration and differentiation 1. These properties are of particular importance in stem cell lines, where small modifications in membrane parameters have the potential to either promote a lineage commitment or a selfrenewal 2. The plasma membrane is the interface between a cell and its environment, it directly interacts with the matrix outside the cell and is responsible for many important tasks such as signaling and mass transfer. In stem cells, its composition and properties are likely to reflect their differentiation status. However, little is known on how the viscosity parameters of different stem cell lineages can change depending on the direction of differentiation. There is evidence that the membrane fluidity substantially changes during induced pluripotent stem cells (iPS) differentiation. Generalized polarization monitoring was previously used to detect the rise of membrane rigidity during iPSC differentiation 1. Furthermore, the results in 1 potentially suggested that membrane rigidification could be transmitted to neighboring cells, resulting in the acceleration of a cells differentiation, in a wave-line fashion. It was also reported that the viscoelastic properties can predict which subpopulations of undifferentiated mesenchymal stem cells (MSCs) differentiate into osteocytes, and which would turn into adipocytes or chondroblasts. The stiffest cell populations produced more bone cells; the softest cells predominantly produced fat cells; the cells with the highest viscosity became cartilage cells 3. While cell stiffness measured in 3 is a distinctly different property to the cell membrane viscosity, both depend on membrane lipid compositions. There is evidence that differentiation of human mesenchymal stem cells (MSCs) into osteoblasts, chondrocytes or adipocytes produces specific membrane compositions and biophysical properties,

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Brain diseases including Down syndrome (DS/TS21) are known to be characterized by changes in cellular metabolism. To adequately assess such metabolic changes during pathological processes and to test drugs, methods are needed that allow monitoring of these changes in real time with minimally invasive effects. Thus, the aim of our work was to study the metabolic status and intracellular pH of spheroids carrying DS using fluorescence microscopy and FLIM. For metabolic analysis we measured the fluorescence intensities, fluorescence lifetimes and the contributions of the free and bound forms of NAD(P)H. For intracellular pH assay we measured the fluorescence intensities of SypHer-2 and BCECF. Data were processed with SPCImage and Fiji-ImageJ. We demonstrated the predominance of glycolysis in TS21 spheroids compared with normal karyotype (NK) spheroids. Assessment of the intracellular pH indicated a more alkaline intracellular pH in the TS21 spheroids compared to NK spheroids. Using fluorescence imaging, we performed a comprehensive comparative analysis of the metabolism and intracellular pH of TS21 spheroids and showed that fluorescence microscopy and FLIM make it possible to study living cells in 3D models in real time with minimally invasive effects.

Scaffold biocompatibility remains an urgent problem in tissue engineering. An especially interesting problem is guided cell intergrowth and tissue sprouting using a porous scaffold with a special design. Two types of structures were obtained from poly(3-hydroxybutyrate) (PHB) using a salt leaching technique. In flat scaffolds (scaffold-1), one side was more porous (pore size 100–300 μm), while the other side was smoother (pore size 10–50 μm). Such scaffolds are suitable for the in vitro cultivation of rat mesenchymal stem cells and 3T3 fibroblasts, and, upon subcutaneous implantation to older rats, they cause moderate inflammation and the formation of a fibrous capsule. Scaffold-2s are homogeneous volumetric hard sponges (pore size 30–300 μm) with more structured pores. They were suitable for the in vitro culturing of 3T3 fibroblasts. Scaffold-2s were used to manufacture a conduit from the PHB/PHBV tube with scaffold-2 as a filler. The subcutaneous implantation of such conduits to older rats resulted in gradual soft connective tissue sprouting through the filler material of the scaffold-2 without any visible inflammatory processes. Thus, scaffold-2 can be used as a guide for connective tissue sprouting. The obtained data are advanced studies for reconstructive surgery and tissue engineering application for the elderly patients.

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