Osteoblasts are susceptible to the surface characteristics of bioceramics and stimulation from outside the cells. The purpose of this study was to evaluate the effects of electrical polarization on surface characteristics and osteoblastic adhesion. The surface characteristics revealed that electrical polarization had no effect on the surface roughness, crystallinity, and constituent elements. According to contact-angle measurements, electrically polarized hydroxyapatite (HA), which provides two kinds of surfaces, negatively charged HA (N-HA) and positively charged HA (P-HA), was even more hydrophilic than that of normal HA (O-HA). Morphological observations and quantitative analyses revealed that the typical adhered cells had a round shape on O-HA but had a spindle or fanlike spreading configuration on N-HA and P-HA 1 h after seeding. After 3 h of cultivation, the rate of the number of spread cells and the size of the focal adhesions on O-HA increased and approached that of N-HA and P-HA. However, the cell areas positively stained for actin, which indicates the degree of cell spreading, were distinctly larger on N-HA and P-HA than that on O-HA. The number of focal adhesions per cell was also less than that on N-HA and P-HA.
The immediate interactions of an implanted hydroxyapatite (HA) surface and blood coagulation components were detected in vivo, and the mechanism of the enhanced osteoconductivity caused by electrical polarization was discussed. Fibrin was presented as being a key protein in the early stages of osteoconduction. Scanning electron microscope observation and immunohistochemical detection indicated that fibrin adsorption is accelerated on negatively charged surfaces (N-surfaces) and on positively charged surfaces (P-surfaces) of implanted polarized HA. The acceleration of fibrin adsorption is caused by the ionic and pH changes near the N- and P-surfaces by the attraction of calcium ions. The adsorbed fibrin formed a network scaffold for subsequent cell components, such as platelets and osseous cells. Near the N-surface, the higher concentration of calcium ions than that on the conventional nonpolarized surface (O-surface) and P-surface contributed to the adhesion of the platelets to the fibrin through integrin alpha(IIb)beta(3) and platelet activation. The activated platelets release a variety of growth factors that stimulate the osseous cells. These continuous reactions from the action of the fibrin adsorption as a trigger induced the early osteoconduction near the N-surface. The coagulation components played an important role at an early stage of the osteoconductive mechanism.
Osteoblast adhesion to surfaces of implant substrates is recognized as playing a fundamental role in the process of osteoconduction. The purpose of this study was to evaluate the in vitro adhesion of osteoblasts cultured on polarized hydroxyapatite (HA), which provides two kinds of surfaces; negatively charged HA (N-HA) and positively charged HA (P-HA). Those surfaces have been proved to enhance the osteobonding capabilities. Osteoblastic cells were seeded onto normal and polarized HA; adhesion and motility of each was observed. Polarization did not affect the percentage of the spread cells against all the adhered cells, but had a significant effect on the spreading of each cell as shown by the measured elongation of the adhered cells by fluorescence observation. The elongation of each cell was especially enhanced on the N-HA and P-HA, when compared with normal HA (O-HA). In addition, the polarization affected cell motility shown by wound healing. Motility analysis showed that the same number of cells started to migrate toward the wound areas on each type of surface. However, the migration of each cell type towards the wound area was accelerated on the N-HA and P-HA. The charges induced on the HA surface accelerated the cytoskeleton reorganization of the adhered cells. The acceleration was appeared as cell shape, actin filament pattern such as stress fiber formation, and prolongation of cell motility distance.
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