Biomedical subjects

Alexandra E Porter

Publications and source records attributed to Alexandra E Porter.

9 recordsLinked to original sources

PubMed2006

Uptake of C60 by human monocyte macrophages, its localization and implications for toxicity: studied by high resolution electron microscopy and electron tomography.

Despite great interest in the engineering applications of carbon-based nanoparticles, recent studies have raised concerns about their potential toxicity and safety. The release of C(60) into the environment has been suggested to be a potential risk with possible ecological implications. Here we evaluate energy-filtered transmission electron microscopy (EFTEM) and scanning transmission electron microscopy (STEM)-based electron tomography as techniques for imaging the three-dimensional (3-D) distribution of nanoparticles within cells. Our aim was to establish if human monocyte macrophages internalise nanoparticles and to assess whether nanoparticles are modified by cells following uptake. Using these techniques we were able to show a marked increased in the amount of information gained from 3-D imaging. 3-D electron tomography revealed several sub-cellular compartments containing C(60) within the cell: secondary lysosomes, along the outer and nuclear membrane and most notably inside the nucleus of the cell. Using EFTEM and STEM-based techniques we were able to visualize cell structures such as membranes, the mitochondria, ribosomes and the nucleus, without the need for traditional staining techniques. In particular we demonstrate the potential of electron tomography for whole cell studies to enable 3-D distributions of particles within cells. The concentrations of C(60) used in this study were not toxic and were chosen to study which sub-cellular compartments accumulated C(60). Knowledge of the sites of accumulation of nanoparticles will allow us to predict vulnerability if the nanoparticles can generate free radicals.

Cell Culture Techniques↗

PubMed2006

Nanoscale characterization of the interface between bone and hydroxyapatite implants and the effect of silicon on bone apposition.

Silicon plays an important role in bone mineralization and formation and is therefore incorporated into a wide variety of medical implants and bone grafts used today. The significance of silicon (Si) can be understood through an analysis of the mechanisms of bone bonding to calcium containing biomaterials and through comparisons of hydroxyapatite (HA) and silicon-substituted hydroxyapatite (Si-HA). The addition of Si to HA causes a decrease in grain size that subsequently affects surface topography, dissolution-reprecipitation rates and the bone apposition process. Through the use of high-resolution transmission electron microscopy (HR-TEM) studies, the interactions between bone and silicon hydroxyapatite (Si-HA) at interfaces are reviewed and related to their impact on bone apposition and ultimately the performance of medical implants.

Animals↗

PubMed2006

The structure of the bond between bone and porous silicon-substituted hydroxyapatite bioceramic implants.

The significance of micrometer-sized strut porosity in promoting bone ingrowth into porous hydroxyapatite (HA) scaffolds has only recently been noted. In this study, silicon-substituted HA (0.8 wt % Si-HA) with approximately 8.5% of the total porosity present as microporosity within the struts of the implant was prepared for high-resolution transmission electron microscopy (HR-TEM) via both ultramicrotomy and focused ion beam milling. Between the struts of the porous Si-HA, pores with varying shapes and sizes (1-10 microm in diameter) were characterized. Within the struts, the Si-HA contained features such as grain boundaries and triple-junction grain boundaries. Bone ingrowth and dissolution from a Si-HA implant were studied using HR-TEM after 6 weeks in vivo. Minor local dissolution occurred within several pores within the struts. Organized, mineralized collagen fibrils had grown into the strut porosity at the interface between the porous Si-HA implant and the surface of the surrounding bone. In comparison, deeper within the implant, disorganized and poorly mineralized fibers were observed within the strut porosity. These findings provide valuable insight into the development of bone around porous Si-HA implants.

Animals↗

PubMed2005

Ultrastructural observation of single-crystal apatite fibres.

Hydroxyapatite (HAp) has been widely used as a biomaterial for substituting human hard tissues such as bone. By altering the morphology of HAp crystals, novel properties may be produced by controlling the orientation of the crystal planes. Apatite fibres were successfully synthesized by precipitation from aqueous solutions containing Ca(NO(3))(2), (NH(4))(2)HPO(4), urea and HNO(3). The products were composed of carbonate-containing apatite fibres with preferred orientation along the {h00} planes. Examination of individual fibres using transmission electron microscopy showed that the as-synthesized apatite fibres were highly strained single crystals with the c-axis orientation parallel to the long axis of the fibre. The crushed fibres consisted of domains that were preferentially oriented with the c-axis parallel to the long axis of the fibres. When the apatite fibres were heated to 800, 1000 and 1200 degrees C for 1h, the domains were removed and grain boundaries, dislocations and voids were formed.

Biocompatible Materials↗

PubMed2005

A transmission electron microscopy study of mineralization in age-induced transparent dentin.

It is known that fractures are more likely to occur in altered teeth, particularly following restoration or endodontic repair; consequently, it is important to understand the structure of altered forms of dentin, the most abundant tissue in the human tooth, in order to better define the increased propensity for such fractures. Transparent (or sclerotic) dentin, wherein the dentinal tubules become occluded with mineral as a natural progressive consequence of aging, is one such altered form. In the present study, high-resolution transmission electron microscopy is used to investigate the effect of aging on the mineral phase of dentin. Such studies revealed that the intertubular mineral crystallites were smaller in transparent dentin, and that the intratubular mineral (larger crystals deposited within the tubules) was chemically similar to the surrounding intertubular mineral. Exit-wave reconstructed lattice-plane images suggested that the intratubular mineral had nanometer-size grains. These observations support a "dissolution and reprecipitation" mechanism for the formation of transparent dentin.

Adult↗

PubMed2004

Ultrastructural comparison of dissolution and apatite precipitation on hydroxyapatite and silicon-substituted hydroxyapatite in vitro and in vivo.

Recent histological studies have demonstrated that the substitution of silicate ions into hydroxyapatite (HA) significantly increases the rate of bone apposition to HA implants. The enhanced bioactivity of silicon-substituted HA (Si-HA) over pure HA has been attributed to the effect of silicate ions in accelerating dissolution. In the present study, high-resolution transmission electron microscopy (HR-TEM) was employed to compare dissolution of HA and Si-HA in an acellular simulated body fluid (SBF) to dissolution in an in vivo model. HR-TEM observations confirmed a difference in morphology of apatite precipitates in vivo and in SBF: apatite deposits were platelike in vivo and nodular in SBF. Compositional mapping suggested that preferential dissolution of silicon from the implant promotes the nucleation of carbonate apatite around the implant. The in vivo findings illustrated an absence of dissolution at the bone-HA or Si-HA interface, whereas dissolution was extensive from within the implant. The amount of dissolution in acellular SBF was similar to dissolution from within the implant, although the site at which the dissolution nucleates was different: dissolution predominates at the crystallite surfaces in SBF, whereas grain boundary dissolution predominates in vivo. These findings suggest that proteins in the in vivo milieu modify the processes of dissolution from the implant.

Biocompatible Materials↗

PubMed2004

Ultrastructural comparison of hydroxyapatite and silicon-substituted hydroxyapatite for biomedical applications.

Silicon-substituted hydroxyapatite (Si-HA) has been shown to lead to significantly increased rates of bone apposition when compared with phase-pure hydroxyapatite (HA) bioceramic implants (Patel N, et al. J Mater Sci Mater Med 2002;13:1199-1206). However, uncertainty remains about the mechanism by which Si increases the in vivo bioactivity. In this study, defect structures in Si-HA were observed and characterized for the first time using high-resolution transmission electron microscopy. Using tilting experiments and the g. b = 0 criterion for invisibility, the Burgers vectors of dislocations in phase-pure HA and 0.8 wt % Si-HA were characterized to be screw and mixed in character. Dislocations were observed in both pure HA and 0.8 wt % Si-HA with no significant difference in dislocation density between HA and Si-HA. However, our findings suggest that an increased number of triple junctions in Si-HA may have a significant role in increasing the solubility of the material and the subsequent rate at which bone apposes Si-HA ceramics.

Biocompatible Materials↗

PubMed2004

Effect of sintered silicate-substituted hydroxyapatite on remodelling processes at the bone-implant interface.

Phase pure, sintered granules of hydroxyapatite (HA) and silicon-substituted hydroxyapatite (Si-HA) were implanted for 6 and 12 weeks in an ovine model. Samples containing the bone-implant interface were prepared for ultramicrotomy and transmission electron microscopy (TEM) using an anhydrous sample preparation procedure. The results demonstrate that the morphology of apatite deposits and the sequence of events at the interfaces of bone with pure HA and with Si-HA implants, were different. Organised collagen fibrils were first found at the bone/Si-HA interface after 6 weeks, whereas they were found only after 12 weeks around the pure HA implant. Many more nodular aggregates comprised of plate-like apatite crystallites were observed in the vicinity of Si-HA than around the pure HA after 12 weeks in vivo. These findings suggest that the incorporation of silicate ions into HA promotes processes of bone remodelling at the bone/HA interface. TEM observations suggested that the trabecular bone weaves over the Si-HA and that the collagen fibrils form a mechanical interlock with the Si-HA ceramic implants. High-resolution lattice imaging illustrated apatite crystallites contiguous with the Si-HA ceramic and revealed a direct relationship between the bone mineral and the Si-HA ceramic.

Animals↗

PubMed2004

Bone bonding to hydroxyapatite and titanium surfaces on femoral stems retrieved from human subjects at autopsy.

The success of clinical results obtained with many hydroxyapatite (HA)-coated prosthetic designs has deflected attention from the need to extend the life of the HA coating on the device. In the current study the percentages of HA and titanium surfaces to which bone was bonded, on HA-coated and non-coated titanium femoral stems retrieved from human subjects, were evaluated. Plasma-sprayed hydroxyapatite (PSHA)-coated devices demonstrated wide variability in the percentage of the PSHA coating remaining on the stems. The coating was missing from a substantial portion of a stem after only about 6 months of implantation. The percentage of revealed metal to which bone was bonded was significantly less than the percentage of the HA coating demonstrating such bonding. The revealed metal to which bone was bonded was comparable to the same value for a separate group of non-PSHA-coated titanium stems. If HA-coatings degrade over time precipitous decline in performance may occur even after several functional years. Many ultrastructural features of the bone bonded to the HA coatings on these implants from human subjects were comparable to those found on HA-coated devices implanted in a canine model.

Autopsy↗