## 3 Results and Discussion
### More Irregular Particle Clusters Are Equally Likely to Be Internalized as the Less Irregular Ones  
Particle shape is an essential determinant of the cellular uptake and of the overall response of the cells to the nanoparticle treatment. A number of studies have demonstrated that nanoparticles with different morphologies can be uptaken, accumulated, and/or eliminated from the cells at thoroughly different rates [74; 75; 76]. For example, displaying a high level of semblance with biogenic apatite platelets, HAp flakes upregulated the expression of genes and augmented the production of proteins that initiate osteogenic differentiation more efficiently than HAp needles or rods [77]. Another study contrasted these findings by showing that HAp spheres enhance the osteogenic differentiation of MSCs more than HAp needles or flakes [78]. However, a study showing that the uptake of gold nano-spheres by RAW264.7 and HeLa cells is inversely proportional to their size in the 22-66 nm range, while the uptake of gold nano-triangles is directly proportional to their size in the 46-94 nm range [79] may be particularly illustrative here, by its demonstrating that shape can outweigh size as a factor determining the uptake efficiency. Inspired by the rod-shaped nanoparticle cluster shown in Figure 3b, and its ambiguous localization halfway between the inside and the outside of a cell, an analysis was conducted based on measuring the deviation of the intracellular and extracellular HAp nanoparticle clusters in hMSC cultures from the perfect circularity, as defined by Equation (4). Against expectations, the circularity of internalized and external clusters was practically identical at \(80.14\pm 0.08\) for the intracellular clusters, and \(80.22\pm 0.09\) for the extracellular ones (Figure 6a). This indifference of the cell to the cluster geometry when it comes to the decision to internalize it or not, however, applies to a sample composed of spherical nanoparticles constituting the clusters. When the primary particles comprising separate samples differ in morphology, the situation might be different. To test this, two distinct powders were synthesized, differing in the primary particle shapes, both being in the micrometer size range, but one being highly elongated and uniaxial (Figure 6c), and the other being planar (Figure 6d). The fact that the uptake was more pronounced for particles with more isotropic shapes compared to those that looked more needle-like in morphology (Figure 6b) suggests that the cells do have a way of distinguishing between more and less preferential particle geometries. These findings may be read as a pointer toward the ability of cells to sense the fine, nanostructured topography of clusters and thereupon discard any geometric differences between them, which they do take into account when larger particles, containing no such nanoscale substructure, are brought into an interaction with them. These results combined suggest that in a sample where nanoparticles of uniform morphologies form a broad distribution of aggregates of different shapes, the uptake process is stochastic and all shapes have an equal probability of uptake, assuming that their sizes are identical. However, when the elementary particles differ in morphology, the uptake is bound to be shape-dependent. A lateral conclusion may be that the oval shape of vesicles internalizing the particles upon the uptake is not exclusive when it comes to the internalization of particle geometries deviating from perfect circularity. In fact, as shown in Figure 7, although spherically shaped lysosomes present a great majority of the lysosomal population in both control cells and cells undergoing the nanoparticle uptake, a somewhat higher percentage of liposomes exhibits asymmetric shapes in the latter cells. This insight coincides with the earlier findings, where discoid vesicles were transformed into tubular vesicles under pH changes,such as those facilitated by the partial dissolution of HAp inside the lysosomal compartments [80].