Platelet plays a central role in hemostasis and thrombus formation. In biorheological sense, platelet is rather a minor blood component, but once activated, it shows various characteristic reactions, namely adhesion, aggregation, shape change, release of its contents and so on.
In most cases, platelet function is assessed only under existence of an anticoagulant agent like cit-rate. Therefore, apparent platelet reaction in vitro might differ from that in blood stream. Some fundamental items on platelet and technical problems are reviewed. In addition, the effects of various agents on platelet aggregation and clinical appreciation are briefly discussed.
Nowadays fluidity or viscosity of blood is often discussed in relation to various pathological states. In some cases, incorrect concept overruns our society. The progress of technology, especially in medical science, should be transmitted to general public precisely. We should share the knowledge with people who are interested in their health and also support them properly.

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A dominant inorganic substance in bones is known to be a biological apatite (BAp) nano-crystal which crystallizes in an anisotropic hcp lattice, and the c-axis of BAp is parallel to extended collagen fibrils. We applied the microbeam X-ray diffractometer system with a beam spot of 100μm in diameter to evaluate bone function of original, regenerated or pathological bones focusing on the texture of BAp crystallites.
Preferential alignment of anisotropic BAp crystallites in typical biological calcified tissues changes depending on the bone shape and stress condition in vivo. The c-axis of BAp crystallites, for example, is distributed dominantly as one orientation along the longitudinal direction in the rabbit ulna and along the head and tail direction in the monkey vertebra, and as two-dimensional (2D) orientation in the flat bone of rabbit skull. The c-axis of BAp of a dentulous mandible basically aligns along the mesiodistal direction in the flat bone, but this alignment changes parallel to the biting direction near the tooth. The preferential distribution of BAp is therefore closely related to the stress distribution in vivo and the corresponding mechanical function which is the most important as a biological structural component. The similar analysis of the BAp texture is also applicable to the regenerated bones and pathological hard tissues.
We can finally conclude that crystallographic information of BAp such as orientational distribution is a new measure to understand the bone microstructure and the related mechanical function.

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Small-Angle X-ray Scattering (SAXS) is caused by inhomogeneity in materials and is due to shape, size and spatial distribution of the scattering particle. Bone that consists of hydroxyapatite-like mineral deposited in a hydrated organic matrix is well described as two-phase structure. Thus, the SAXS can provide information on mean size, predominant orientation and typical shape of mineral crystals in bone. At first, the mean sized has determined by Guinier's analysis. It has been known that the intensity curve by isotropic three-dimensional materials having two-phase structures decreases as q^-4 at large qs (q=4πsinθ/λ), according to the theory of Porod. Perret and Ruland have extended Porod's theory to one applicable to isotropic two-dimensional two-phase system. We applied their theory for the intensity curve from oriented bone. The SAXS pattern was also found to show a diffuse scattering that develops in a fan-like fashion perpendicular to the long axis of the oriented bone. Recently, Fratzl et al have extensively studied bone mineralization using the theory of Perret and Ruland. The thickness of the mineral crystals was found to be 3-4 nm.The SAXS results suggested the existence of needle-shaped mineral crystals and plate-shaped crystals. Recently, microfocus SAXS has been developed.

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Bone tissue is a composite material composed of hydroxyapatite (HAp) and collagen. As HAp in bone tissue exists in the form of crystalline particles, an X-ray diffraction method is useful to determine a lattice strain of HAp, which can be a measure of strain distribution in bone tissue. However, HAp crystals in bone have been known to have the low degree of crystallization. A quantitative determination of the lattice strain of HAp in bone seems to be difficult. We have proposed a new method to determine the lattice strain of crystals with the low degree of crystallization. The method improved the accuracy and the reproducibility of the strain measurement of HAp in bone tissue. In this work, in order to investigate the relationship between the macroscopic strain of bone and the strain of HAp lattice, the X-ray diffraction measurements were performed under tensile loading. The cortical bone specimens of 50×6×0.8 mm in average were cut from a shaft of bovine femur. The stepwise tensile load was applied to the specimen during the X-ray irradiation. The lattice strain value of HAp was lower than the macroscopic strain value detected by a strain gauge, though the relationship of both values were almost linear.

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We aimed to investigate anisotropic viscoelastic properties of bone, which had been expected from an orthogonal anisotropy in Young's modulus of cortical bone. Relaxation Young's modulus of cortical bone was investigated for two different directions with respect to the longitudinal axis of bone (bone axis, BA) : the modulus parallel (P) and normal (N) to the BA. Stress relaxation for both specimens manifests two relaxation processes, referred to as f-process in a short time region and s-process in a long time region. The two processes were decomposed by fitting the relaxation data to the empirical equation previously proposed for cortical bones. The f-process of the P sample almost completely coincided with that of the N sample. On the other hand, there was a difference in the modulus value of s-process between the P sample and the N sample. The results indicate that the f-process represents the relaxation in the collagen matrix in bone and that the s-process is related to a higher-order structure of bone that is responsible for the anisotropic mechanical properties of bone.

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