In Silico Biology of Bone Regeneration Inside Calcium Phosphate Scaffolds - Tissue Engineering: Computer Modeling, Biofabrication and Cell Behavior - page 33

proposed in literature [ 2 ]. However, the release of calcium and phosphate ions by
dissolution, is believed to be the main origin of the bioactivity of CaP biomaterials
[ 1 , 2 , 10 , 19 ]. The dissolution properties of CaP biomaterials are influenced by the
exposed surface area, the composition and the pH. Pioletti et al. [ 34 ] showed that
small CaP particles ( < 10 µm) can induce phagocytosis. This process could then, in
turn, produce an accumulation of Ca 2 + in the mitochondria, which can cause lysis
of the mitochondria and cell death. Phagocytosis also alters the pH of the surround-
ing body fluids. This pH-change subsequently alters the dissolution properties of
the CaP particles. The size of the particles is not only critical because it can induce
phagocytosis, it also determines the reactivity of the particles. The smaller the parti-
cles, the larger the exposed surface to the environment and the faster the biomaterial
will dissolve. The dissolution rate will increase, simply because larger quantities of
exchange can take place [ 1 ]. The composition of the calcium phosphate biomate-
rials is another important characteristic that determines the dissolution properties.
A change in the calcium to phosphate ratio means a change in phase composition,
which directly affects the ionic exchange mechanisms [ 1 ].
Experimental evidence clearly indicates the key role of calcium. Yuan et al. [ 53 ]
observe more bone formation in scaffolds made up of biphasic CaP than of hy-
droxyapatite, the latter having a lower dissolution rate. The effect of calcium ion
implantation in titanium on bone formation was investigated by Hanawa et al. [ 20 ].
They found a larger amount of new bone on the Ca 2 + -treated side than on the un-
treated side. Eyckmans et al. [ 14 ] noticed that the CaP granule remnants in a de-
calcified scaffold serve as anchoring points for cell attachment. Titorencu et al. [ 44 ]
report that osteoblasts respond to changes in Ca 2 + concentration in the bone micro-
environment. Moreover, differentiation of MSCs towards osteoblasts is accompa-
nied by the expression of Ca 2 + binding-proteins and the incorporation of Ca 2 + into
the extracellular matrix [ 44 ]. Chai et al. [ 9 ] observed a significant Ca 2 + -induced
cell proliferation and upregulation of osteogenic gene expression in a dose- and
time-dependent manner. It also appears that osteoblasts sense and respond to the
extracellular Ca 2 + concentration independently of systemic calciotropic factors in
a concentration-dependent manner [ 13 ]. Bootman et al. [ 7 ] report that the extracel-
lular calcium concentration could control the frequency of the intracellular calcium
spiking, which encodes specific cellular information according to Sun et al. [ 43 ].
The release of PO 3 4 also plays a key role by regulating the cell cycle and prolif-
eration rate, influencing gene expression [ 3 ] and the secretion of bone-related pro-
teins [ 23 ]. However, several in vitro studies showed that the addition of high levels
of exogenous PO 3 4 (5-7 mM) induced osteoblast apoptosis and non-physiological
mineral deposition [ 27 ]. Nevertheless, PO 3 4

is believed to play a critical role in

bone matrix mineralization [ 32 ].
Despite of the vast in vitro research findings, the influences of Ca 2 + and PO 3 4
differ from cell type to cell type. This implies that there will be not one optimal
Ca 2 + and P i concentration that could universally drive all cell types towards suc-
cessful osteogenesis. Moreover, the optimal concentration may vary according to
the cellular stage, e.g. proliferation and differentiation. Therefore, specific windows