No abstract

SUrMARY1. The load bearing capacity during relaxation of ventricular cardiac muscle from various animal species was investigated.2. The effect of load on the time course of relaxation was analysed either by comparing afterloaded contractions against various loads or by imposing abrupt alterations in load (load clamps).3. In heart muscle from the mammalian species studied relaxation was sensitive to loading conditions, whereas in frog heart muscle relaxation was largely independent of the loading conditions. The mechanical properties of relaxation of cardiac muscle appear, therefore, governed by the interplay of a load-controlled and an activationcontrolled decay mechanism, the relative importance of which differs with species.4. Load-dependence may be the mechanical expression of the ratio of the number of force generating sites at any time during contraction and relaxation to the load to be carried; this mechanism would predominate in mammalian animal species with a well developed calcium sequestering sarcoplasmic reticulum. Activation-dependence would seem to predominate in animal species, such as frog, in which calcium sequestration appears to be the rate limiting step during relaxation.

A new non-destructive method based on x-ray microtomography (micro-CT) was developed to measure calcium density in bone. X-ray micro-CT was used as a quantitative approach to acquire and reconstruct virtual cross-sections through the sample. Accurate beam-hardening correction was implemented. Grey values in the virtual cross-sections were calibrated as calcium mineral density in bone. From these cross-sections, three-dimensional models were created. Calcium content was calculated directly from images and expressed as percentage per volume and per weight. Calcium mineral density was studied by this method in a unique set of bones isolated from newts (Pleurodeles waltlii Michah) that had travelled into space. A demineralization of 10% was shown as a consequence of sustained micro-gravity.

Two-dimensional micro-computed tomography (micro-CT) slices can be reconstructed into three-dimensional (3D) models that demonstrate capillary beds. This study focused on the acquisition of data necessary to create scaffolding that directly mimics the unique structural patterns of a microvascular tree system. The Microfil vascular contrasting method was compared to the Baston's methylmethacrylate corrosion casting (BMCC) method to determine which provided the most accurate and high-resolution results for 3D micro-CT reconstruction derived from the two-dimensional micro-CT slices of the capillary beds. It was determined that the BMCC, a method traditionally used in the scanning electron microscopic analysis of the microvasculature, was the best method for representing capillary lumina for micro-CT scanning. The removal of tissues from the BMCC cast resulted in samples that eliminated background material, thus increasing the X-ray contrast levels of the CT images. This provided for a more complete and more distinguishable high-resolution image of the represented capillary lumina. Images created with this BMCC method were reconstructed in a stereolithography file format as 3D mesh structure for later importing into computer-aided design (CAD) software. The resulting Bio-CAD, then, can be used to guide the more accurate fabrication of the microvascular scaffolding and then serve as the framework for tissue engineering of microvascular structures. Results from this study clearly indicated that the BMCC method is superior to the Microfil method for accurate and complete high-resolution imaging of capillary beds.

Key words. 3D rendering, emphysema, in vivo scanning, X-ray microtomography. AbstractHigh resolution X-ray microtomography (micro-CT) was used for the detection of emphysema in live mice. Emphysema was induced in C57BL/6 J mice by intratracheal instillation of different amounts of porcine pancreatic elastase. This emphysema could be clearly detected by micro-CT seven weeks posttreatment: analysis of the whole data set of virtual cross-sections showed the presence of a dose-dependent level of emphysema.