Myofibrils, the contractile organelles from striated muscles, have been examined in the X-ray microscope to determine the effect of radiation on their function and structure. Using X-rays of energy 350-385 eV in the water window we find that after an exposure to 7.5 x 105photons/pm2 (calculated to give an absorbed dose of 20 000 Gy) the myofibrils will no longer contract. The use of the free radical scavenging agent, DMSO, gives some protection to the fibrils. It has also been found that after this much irradiation the fibrils lose up to 20% of their mass. Further substantial mass loss occurs on subsequent irradiation. After 25 times the loss-of-function exposure only 30% of the mass remains. Analysis of a series of images of the same myofibril covering this range of exposures shows that the mass is preferentially lost in some areas of the structure and consequently significant structural changes occur. 0 1993 The Royal Microscopical Society 109

The dominance of viscous forces in the generation of propulsive thrust by cilia is emphasised. Fourier analysis indicates that ciliary bends consist of circular arcs joined by linear segments; this arc-line shape appears to be a property associated with the molecular mechanism responsible for bending the cilium and is unchanged by variations in the external viscous loading on the organelle. The flexibility of a computer-generated model of axonemal structure is demonstrated by the incorporation of recent data concerning the surface lattice of the microtubules. Computer simulations using the model show that predictions based on stochastic, rather than co-ordinated, dynein arm activity provide a qualitative match to experimental observations of microtubules gliding over fields of dynein molecules.

This article reports the continuation of a series of experiments investigating the effects of soft x-ray radiation damage on the contractile elements of mammalian striated muscle (myofibrils), using their ability to contract as a functional assay. The myofibrils were exposed to 385 eV x rays. This energy is within the ‘‘water window’’ between the oxygen and carbon K edges, where the x-ray absorption coefficient of biological materials, such as protein, is about an order of magnitude greater than that for water. An exposure of 8×105 photons μm−1 was found to prevent contraction in the majority of myofibrils. Preliminary results indicate that it is possible to increase this exposure level by approximately 25% by adding the radioprotective dimethyl sulphoxide (DMSO), an OH radical scavenger to the myofibril buffer during irradiation. This suggests that OH radicals are important in the inactivation of myofibrils through irradiation.

Articles you may be interested in X-ray source combined ultrahigh-vacuum scanning tunneling microscopy for elemental analysis Elemental imaging via scanning transmission x-ray microscopy (STXM) and scanning fluorescence x-ray microscopy (SFXM) has been used to image calcium deposits in cartilage. In the case of STXM, 0.1 ym thick sections were imaged to investigate the proximity of calcium deposits in relation to chondrocyte cells. The resolution available was 0.5 pm, and field widths of up to 25 pm were used at this resolution. The resolution available in SFXM was 10 pm, and field widths of up to 2 mm were used at this resolution on 5+m thick specimens. Together these techniques were used to map calcium deposits at the cellular level, and at the full tissue size level.

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