Abstract: BACKGROUND: At present, hydroxyapatite has been used more and more widely in the field of facial fillers due to its good biocompatibility and low immunity. The main sources are divided into natural extraction and artificial synthesis. However, there are few comparative studies on natural extraction and artificial synthesis of hydroxyapatite, especially the difference in stimulating collagen secretion between the two. 
OBJECTIVE: To investigate the physicochemical differences between naturally derived and commercially available synthetic calcium hydroxyapatite particles and promoting collagen secretion. 
METHODS: (1) Natural hydroxyapatite particles from pig bones and two kinds of commercially available synthetic hydroxyapatite particles (denoted as SYN1 and SYN2) were used to characterize the microstructure and surface element content of the three kinds of materials. (2) The three kinds of materials with different mass concentrations (1, 5, and 10 mg/mL) were co-cultured with human skin fibroblasts. Cell proliferation was detected by CCK-8 assay. The three kinds of materials at 5 mg/mL were co-cultured with human skin fibroblasts. Cell adhesion was observed by CCK-8 assay and scanning electron microscopy. The expression of type I collagen was detected by RT-PCR, ELISA, and western blot assay. (3) Twelve New Zealand rabbits were selected, of which six were subcutaneously injected with a mixture of natural hydroxyapatite and 2% sodium carboxymethylcellulose gel on the back. The remaining six rabbits were subcutaneously injected with a mixture of SYN2 and 2% sodium carboxymethylcellulose gel on the back. The samples were collected 1 and 3 months after injection and stained with Masson and Sirius red.
RESULTS AND CONCLUSION: (1) Scanning electron microscope showed that the crystal grains of natural hydroxyapatite particles were more regular and uniform, with holes evenly distributed between particles. The SYN1 grains were smaller and densely arranged, while the SYN2 grains were irregular. The median particle sizes of natural hydroxyapatite particles, SYN1, and SYN2 were 38, 24, and 40 μm, respectively. Natural hydroxyapatite contained Mg and Zn, SYN1 did not contain Mg and Zn, and SYN2 did not contain Zn. (2) CCK-8 assay showed that 1 and 5 mg/mL of natural hydroxyapatite and SYN2 promoted the proliferation of human skin fibroblasts, while 5 and 10 mg/mL of SYN1 inhibited the proliferation of human skin fibroblasts. The number of cells adhered to the surface of natural hydroxyapatite particles was more than that of SYN1 and SYN2, and the cells on the surface of natural hydroxyapatite particles spread well, with visible filopodia. RT-PCR, ELISA, and western blot assay results showed that the expression of type I collagen in the natural hydroxyapatite particle group was higher than that in the SYN1 group and the SYN2 group. (3) The results of Masson and Sirius red staining showed that the amount of type I collagen in the subcutaneous tissue of the natural hydroxyapatite particle group was greater than that of the SYN2 group 1 and 3 months after injection. (4) The results show that compared with synthetic hydroxyapatite particles, natural hydroxyapatite particles have a richer distribution of trace elements and can better promote fibroblast adhesion, proliferation and collagen regeneration.

Key words: hydroxyapatite, natural extracts, synthetic, cell proliferation, collagen regeneration, injectable filler material, engineered skin material