| Popis: |
Despite the efforts of many workers over recent years the mechanism of the interaction between ultrasound and biological systems is not well understood. The nature of this interaction has been studied employing the root tip of Vicia faba, the broad bean, as the biological system in the experimental work. Several different mechanisms have either been postulated or established as the cause of ultrasonic damage to biological systems. Each of these mechanisms is described and, where appropriate, a derivation of the equations governing the interaction of that mechanism with the medium in which the ultrasound wave is travelling, is presented. These equations are then used to calculate the magnitude of those parameters most likely to cause damage, for example shearing stress, temperature rise, pressure etc., under specific conditions. Work in which ultrasonic damage has been attributed to particular mechanisms is included in this review. Recently a number of workers have investigated the effect of ultrasound on the root tip of Vicia faba, and similar plant systems, following the successful use of these plants over the last 140 years in investigations into the biological effects of ionising radiation. This work has been extended with a series of complementary experiments designed to determine the mechanism of low megahertz ultrasonic damage to the root tip of Vicia faba. For comparison, in conjunction with each megahertz ultrasonic irradiation, thermal and low frequency [special characters omitted] ultrasonic irradiations were also carried out. This experimental work followed two main lines of approach. Firstly the effect of megahertz ultrasound on the growth rate of the primary root was investigated; various parameters such as ultrasound intensity, exposure time, irradiation temperature and ultrasound frequency were varied in turn. Secondly light and electron microscopic techniques were employed to determine the effect of ultrasound on the structure of the root tip. The light microscopy included both the examination of stained thin sections and the use of a technique which permitted the direct observation of the interaction of ultrasound with the cells of the root tip. Many of the characteristics of the mechanism of low megahertz ultrasonic damage to the root tip of Vicia faba have been established. It is shown that these characteristics are consistent with the theory that the damage is a consequence of the pulsation of intercellular gas channels under the influence of the ultrasound field. When these gas channels pulsate adjoining cell walls are set into vibration producing streaming within the cells. The amplitude of vibration will depend on how close the frequency of the ultrasound field is to the resonant frequency of the gas channels. Many of the experimental observations are explained by considering the variation in the difference between these two frequencies either under particular conditions or for gas channels in different parts of the root tip. It is further shown that both elongating and meristematic cells are damaged during low megahertz ultrasonic irradiation of the root tip. The implications of these findings for investigations into the effects of megahertz ultrasound on mammalian systems are discussed. It is suggested that in the absence of firm evidence of gas cavities or dissolved gas in mammalian tissue the role of work with plant tissue in the establishment of safety limits for the medical uses of ultrasound may, in the long term, be quite limited. |
