P. G. T. Howell

Monte Carlo simulations of electron scattering in bone

Relative changes in the mineralization level within bone can be studied using backscattered electron (BSE) imaging in a scanning electron microscope (SEM). We calculated the size and shape of the volume element studied, choosing conditions which are typical for practical experimental work with polymethylmethacrylate (PMMA)-embedded bone. Absolutely flat surfaces of embedded bone blocks cannot be generated, and a further aim was to examine the effect of the surface topography on the detected BSE signal level. For normal beam incidence, 20 kV, and modeling an annular detector by collecting BSE with take-off angles of between 45 degrees and 75 degrees to the flat sample surface, it was found that the collectable BSE signal intensity peaks for electrons which leave the specimen surface at a radial distance of approximately 1 micron from the beam impact point. The layered structure of the bone generates topographic relief on polishing. Modeling this by a sinusoidal profile of wavelength 5.0 and amplitude 0.5 micron and again for 20 kV, it was found that the signal derived from the troughs is reduced by 14.4% and that from the crests is increased by 17.2%. The two effects may add constructively to generate the frequently observed strong contrast correlating with the distribution of bone lamellae. No net change in mineral packing density would be expected from a change in collagen orientation, and the lamellar contrast observed in practice can be explained solely by the topographic contrast.

Activated Invariant NKT Cells Regulate Osteoclast Development and Function

Invariant NKT (iNKT) cells modulate innate and adaptive immune responses through activation of myeloid dendritic cells and macrophages and via enhanced clonogenicity, differentiation, and egress of their shared myeloid progenitors. Because these same progenitors give rise to osteoclasts (OCs), which also mediate the egress of hematopoietic progenitors and orchestrate bone remodeling, we hypothesized that iNKT cells would extend their myeloid cell regulatory role to the development and function of OCs. In this study, we report that selective activation of iNKT cells by α-galactosylceramide causes myeloid cell egress, enhances OC progenitor and precursor development, modifies the intramedullary kinetics of mature OCs, and enhances their resorptive activity. OC progenitor activity is positively regulated by TNF-α and negatively regulated by IFN-γ, but is IL-4 and IL-17 independent. These data demonstrate a novel role of iNKT cells that couples osteoclastogenesis with myeloid cell egress in conditions of immune activation.

Simple SEM stereophotogrammetric method for three-dimensional evaluation of features on flat substrates

A semi‐automated method was developed for the measurement of the volume of a calcified tissue substrate which can be resorbed by single, isolated resorptive cells (osteoclasts). The procedure would, however, be applicable to the study of any pit in, or projection from, a flat surfaced specimen. Stereo‐pair images are recorded with an SEM. 3‐D co‐ordinates of relevant surface features are derived using a stereo‐comparator which outputs data directly to a microcomputer. Algorithms adopted have been chosen to give the best compromise between accuracy and speed of data acquisition.

Volumes From Which Calcium and Phosphorus X-Rays Arise in Electron Probe Emission Microanalysis of Bone: Monte Carlo Simulation

Monte Carlo simulations of trajectories for electrons with initial energy of 10 keV through 30 keV were used to map the 3D location of characteristic x-ray photon production for the elements C, P, and Ca until the electrons either escaped as backscattered electrons (BSE) or had insignificant energy. The x-ray production volumes for phosphorus slightly exceed those for calcium, but both greatly exceed the volume through which BSE travel prior to leaving the sample. The x-ray volumes are roughly hemispherical in shape, and the oblate spheroid from which BSE derive occupies only the upper third to half the volume of x-ray generation. Energy-dispersive x-ray emission microanalysis (EDX) may not be secure as a method for the quantitation of BSE images of bone in the scanning electron microscope (SEM). Ca:P elemental ratios from EDX analyses may also be imperfect.

A model of temperature transients in dental implants.

Dental implants provide a continuous interface between the oral environment and the deep core structures of the jaws. Implants and trans-mucosal superstructures are primarily metal and heat conduction occurs readily. A hypothetical heat conduction model is investigated to determine the ranges of temperature gradients that might occur in implants. This model showed that a 60 degrees C heat source will cause a heat front of > or = 47 degrees C to advance > 3 mm down an implant within one second. Oral temperature transients may be a factor in implant pathology.

Measurement of lacunar volume in bone using a stereological grid counting method evolved for the scanning electron microscope

A stereological grid method has been evolved for scanning electron microscopy using a self‐generated superimposed grid which compensates for the non‐linearity of the CRT image. The method was applied to determine the volume proportion of lacunae in monkey humerus cortical bone. Flat cut specimens of bone were prepared and photographed with a superimposed grid, and the number of lacunae on intersections of the grid were counted and compared with the total number of intersections on bone matrix. The lacunar volume proportion was 1.06% ± 0.50%.