## Materials and Methods
### Immunofluorescence Staining  
Cell cultures on coverslips were treated with 250 ug/mL of HAp nanoparticles for 24 h before being washed, fixed, permeabilized, and stained. Cells were fixed with 4% paraformaldehyde, then washed with the wash buffer (0.1 wt.% Triton-X and 0.1 wt.% bovine serum albumin in phosphate buffered saline, PBS) and stained with 0.2 mL per well of the staining solution containing 1:4000 _v/v_ AlexaFluor 568 phalloidin as an actin-staining reagent and 1:100 _v/v_ Lonza (Basel, Switzerland) OsteoImage as HAp-staining reagent, along with one drop of NuclBlue ReadyProbes as a cell nucleus-staining reagent. Fixed cells were incubated at room temperature for 2 h before being mounted and cured with ProLong Diamond antifade mounting agent. Control cells that did not contain HAp were stained using the same protocol and reagents, but excluding OsteoImage.  
Cells evaluated for their proliferation in response to nanoparticle uptake were detached from the polymeric growth surfaces after the exposure to HAp nanoparticles being embedded in them, resuspended, and reseeded on glass coverslips. After 7 days of incubation, as a measure of the proliferation degree, the cell number was counted and compared for different sample groups, including the untreated control cells and the cells treated with the nanoparticles. A portion of the cells was stained after 7 days of incubation for collagen type I and nuclei. The staining procedure began with washing the cells with PBS (pH 7.4) and fixing them for 15 min in 3.7% paraformaldehyde. The cells were then washed with PBS \(3\times 5\) min and then with the aforementioned blocking solution \(2\times 5\) min. The cells were then blocked and permeabilized in the blocking solution for 48 h, and then incubated in 500 \(\mu\)L/well of the primary antibody, 10 ug/mL rabbit anti-collagen-type-1 (_Abcam_, Cambridge, MA, USA) in the blocking solution for 1 h. The cells were then washed with PBS \(3\times 10\) min and incubated with 500 \(\mu\)L/well of the secondary antibody, 10 ug/mL AlexaFluor 555 goat anti-rabbit IgG (Invitrogen, Carlsbad, CA, USA), and 20 ug/mL 4',6-diamidino-2-phenylindole dihydrochloride nuclear counterstain (DAPI, Invitrogen, Carlsbad, CA, USA), all in the blocking solution for 1 h and then washed with PBS \(3\times 5\) min. The coverslips containing the fixed and stained cells were mounted onto glass slides using hard set vectashield and nail polish. To visualize the lysosomes, the cells were transfected transiently with LAMP1-YFP (Addgene plasmid #1816) [32] using jetPRIME transfection reagent (Polyplus-transfection, Illkirch-Graffenstaden, France). Immunofluorescence images were collected on Nikon T1-S/L100 and C1 si confocal optical microscopes at 60 \(\times\) magnification in oil.  
Circularity (\(\varphi\)) of the nanoparticle clusters was defined based on Cox's formula [33; 34], where A is the surface area of the 2D projection of the particle, and P is the particle perimeter, where a perfect circle is indicated by \(\varphi=1.0\), while the particle increasingly resembles an elongated polygon as \(\varphi\) approaches 0.0:  
\[\varphi=4\pi\text{A/P}^{2} \tag{4}\]