Tissue water diffusion

All steps in fixation and tissue processing involve exchange of fluids in the three-dimensional space of the specimen. At the start of fixation, tissue fluid (mostly water) is inside the specimen, while fixative molecules are on the outside. Ignoring for the moment the actual structure of the tissue and the effect a fixative may have upon it, assume that the specimen is like a porous sponge filled with water. To enter this system, a fixative molecule must replace a molecule of water. Diffusion is the driving force when two different liquids meet, there is a gradual equalization in the distribution of their molecules. Ideally, at the end of the process, the concentration of one in the other will be the same both inside and outside the specimen. 

Water Diffusion. Water sorption of polymethacrylate/Varcum-2217 blends was measured by Immersing dried film Into water for various time periods. The wet films were pat-dr1ed with tissue papers, followed by blowing nitrogen on the surface. The films before and after water Immersion were carefully weighed with a 4-d1g1t electric analytic balance (14). 

The q-space imaging method, which deals with signals only after long diffusion times, discards all information relevant to dynamic aspects of water diffusion and transport, especially the restriction of water transport by membrane and cell wall permeability barriers in cellular tissues. This information is contained in the functional dependence of the pulsed gradient spin echo amplitude S(q,A,x) on the three independent variables q, A, and x (x is the 90-180 degree pulse spacing) [53]. As the tool to explore the q and A dependence of S(q,A,x), generalized diffusion times and their associated fractional populations are introduced and a multiple exponential time series expansion is used to analyze the dependence [53]. 

Diffusion-weighted MRI, due to its extreme sensitivity to changes in tissue water and, therefore, to ischemia (Le Bihan et al. 1986 Moseley et al. 1990 van Gelderen et al. 1994 Warach et al. 1992,1995 Sorensen et al. 1996 Lovblad et al. 1998), makes it possible to demonstrate ischemic changes due to acute hypoperfusion (see Chap. 7). These findings correspond to those found in the early stages of laminar cortical necrosis (Figs. 16.3, 16.6, 16.7). 

A number of isotopically different forms of water can be prepared, which greatly facilitates experimental studies. Replacing both of the usual hydrogen atoms with deuterium (2H) results in heavy water, or deuterium oxide, with a molecular weight of 20. The role of water in chemical reactions can then be studied by analyzing the deuterium content of substances involved as reactants or products. Tritium (3H), a radioactive isotope with a half-life of 12.4 years, can also be incorporated into water. Tritiated water has been used to measure water diffusion in plant tissues. Another alternative for tracing the pathway of water is to replace the usual 160 isotope with lsO. This labeling of water with lsO helped determine that the O2 evolved in photosynthesis comes from H20 and not from CO2 (Chapter 5, Section 5.5A). 

In CT, contrast occurs solely because of the attenuation of the X-ray beam by its interaction with tissue. Contrast in MRI depends on how the image is acquired. Proton density images reflect the different amounts of water content in different tissues. Chemical shift imaging can distinguish water from fat, based on the different chemical shifts of these protons. Contrast also can be altered in other ways by the exploitation of differences in water diffusion in tissue, by chemical exchange, and by magnetization transfer effects, to name a few. The majority of scans used clinically derive contrast from differences in the relaxation times Ti, T2, and T2. ... 

Stratum Corneum Structure. Reviewers agree that for most compounds the rate-limiting barrier properties of skin are located within the SC (2, 10, 12, 15, 16, 21), The texture and cohesiveness of this tissue are familiar to anyone who has ever peeled bits of it from sunburned skin large sheets of SC can be separated from skin (2). Such sheets look like used polyethylene film, and their resistance to water diffusion approximates that of Mylar film of similar thickness (16). However, each sheet is a mosaic made of individual cells (2). 

Lfnionized ammonia can freely diffuse through tissue cells but forms ammonium hydroxide upon contact with tissue water. Dissolved ammonia and the less permeable ammonium ion exist in a dynamic equilibrium that serves to retard the absorption of... 

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