ABSTRACT
Biological applications of X-ray microscopy were studied by examining the structures of chromosomal fibers in hydrated mammalian cells in situ. Images of hydrated biological specimens are limited by radiation damage and Brownian motion (thermal diffusion). The best way to overcome these limitations for the observation of hydrated biological specimens at high resolution will be with a single shot exposure using a short pulse of intense X-rays. Using a single exposure with a short pulse of laser-produced plasma X-rays, hydrated human chromosome fibers were successfully imaged by X-ray contact microscopy, and the resolution was sufficient to observe a single nucleosome (a structural unit in chromosome fibers) with an llnm diameter. However, for imaging thick specimens such as a whole cell at high resolution, contact microscopy is not suitable. In this type of application, three dimensional imaging will be required to resolve an internal structure. X-ray holographic microscopy of a dehydrated human cell suggests that holographic microscopy may be applicable to the study of the cellular structure. X-ray holographic tomography with simultaneous exposures of coherent X-rays from several directions may be a suitable method for the three dimensional observation of structures in hydrated mammalian cells at high resolution, and X-ray lasers can provide a suitable light source for this type of application.
