## 3 Results and Discussion
### Uptake of HAp Nanoparticles Can Facilitate the Cell Adhesion and Proliferation on the Surface  
The observed corrugations on the cell surface visualized under AFM (Figure 1) were likely to have originated from the uptaken solid entities, which are considerably stiffer than the cytoplasm. This inspired the question of whether such topographic irregularities might cause an improved cell adhesion onto the surface in analogy with the way the introduction of similar topographic irregularities to the surface of biomaterials promotes the cell adhesion and proliferation, especially when the cells are boney in nature [83]. Albeit scarce, prior literature reports were tentatively not in favor of this hypothesis. For example, an earlier study showed that the osteoblast detachment force actually dropped after the uptake of HAp [84]. Moreover, the treatment of suspended hepatoma cells with HAp diminished their ability to adhere to the cell culture plates, and resulted in cell apoptosis [85]. Yet another study reiterated this diminishment of the cell adhesion onto the surface consequential to the uptake, by showing that the internalization of 40-80 nm sized HAp nanoparticles by hepatoma cells suspended the proliferation and weakened the adhesion of the cells onto the bottoms of the cell culture flasks [86]. Since the interaction between integrins and nanoparticles has been shown to favor endocytosis [87], it can be expected that integrin-mediated mechanotransduction and integrin-mediated endocytosis may compete with one another, thus rendering the uptake and the adhesion inversely proportional to one another. This scenario, however, need not apply to osteoblastic cells that MSCs can transition to. Namely, these cells are migratory, but the activation of mineralization binds them, at least temporarily, to a particular locus, where the intensified binding onto the surface is expected. This effect was first indirectly evidenced by the increased intracellular and pericellular collagen production by cells treated with HAp (Figure 8b), than by the control cells (Figure 8a). Collagen was used here instead of HAp as an immunofluorescent proxy for gauging the mineralization activity, because the endogenous and the administered HAp are indistinguishable in a stain assay such as Alizarin Red. Furthermore, both the cell density as an indicator of the proliferation rate (Figure 8c), and the viability measured as the extent of the mitochondrial metabolic activity (Figure 8d) were higher in the population of cells treated with HAp than in the control population. Here it should be noted that compared to the statistically significant increase in cell viability following the uptake of HAp, the uptake provided some, but not a fully statistically significant advantage for cell proliferation (Figure 8c-d). Finally, the estimation of the  
Figure 7: Lysosomal-associated membrane protein 1 (LAMP1) red stains achieved by the transient transfection of control cells (**a**) and cells challenged with nanoparticles (**b**) with LAMP1-YFP. Nuclear DNA was stained in blue using DAPI. Yellow arrows in (**b**) indicate the slightly elongated or misshaped lysosomes coexisting with their spherical counterparts and mostly prominent in the cells undergoing the nanoparticle uptake. change in the cellular adhesion force with the uptake of HAp nanoparticles was made on the basis of the methodology postulated by [PERSON] et al [88]. As described in Figure 9a,b, this method correlates the adhesion force with the specific parameters of the cell geometry, including the circumference and the opening angles at the cell corners as discrete sites of adhesion.