Tissue viability

By far, the most widely used and most empirically studied tissue viability imaging techniques are those that study tissue perfusion, and discussion of perfusion imaging techniques will dominate this section. We will also mention a few emerging techniques that currently are not as widely used in the acute stroke setting, but show promise for the future. 

Modern sensitive chromatographic and voltammetric techniques now make it possible to estimate the release of unlabelled endogenous transmitter from slices of brain tissue (commonly the hippocampus and striatum) or spinal cord (Fig. 4.4). However, whatever analytical method is used, the thickness of the slice is paramount. It is important to maintain the balance between preserving the integrity of the tissue (the thicker the slice, the better) against maintaining tissue viability by perfusion with oxygenated aCSF (the thinner the slice, the better). 

The situation concerning a possible role for free radicals in fatigue of skeletal muscle therefore remains unclear. Antioxidants may have some inhibitory role in fatigue of a diaphragm preparation but our experiments using a similar intact skeletal muscle system have not supported these conclusions. It is therefore possible that antioxidants are only beneficial where tissue viability is compromised. Further work is required to clarify this area. 

Because of the possible effects of active and carrier-mediated processes and metabolic biotransformation, the issue of tissue viability is important for in vitro buccal mucosal experiments. The barrier nature of the buccal mucosa resides in the upper layers of the epithelium, where unlike in the stratum corneum, the cells contain a variety of functional organelles [119, 122, 125, 150], and so tissue viability may be an important component of the barrier function of the tissue. Various methods have been employed to assess the viability of excised buccal mucosa, including measurement of biochemical markers, microscopic methods, and linearity of transport data [42], While biochemical methods, including measurement of adenosine 5 -triphosphate (ATP) levels and utilization of glucose, provide information on the metabolic activity of the tissue, this does not necessarily relate to the barrier function of the tissue. In excised rabbit buccal mucosa, levels of ATP were measured and found to decline by 40% in 6 h, and this correlated well with transmission electron microscopic evaluation of the tissue (intact superficial cells) [32], In addition, the permeability of a model peptide was unaltered up to 6 h postmortem, but at 8 h, a significant change in permeability was observed [32], These investigators therefore claimed that excised rabbit buccal mucosa could be used for diffusion studies for 6 h. 

Recently it has been claimed that the tissue can be considered viable if the drug permeability does not change over the course of the experiment, and thus the actual permeability experiments themselves may provide insight into the viability of the tissue [109, 157], This method was employed in permeation experiments using porcine buccal mucosa, where the permeability of compounds was assessed in two consecutive permeability experiments to ensure the nature of the barrier was not compromised [111, 112]. While this demonstrates that the barrier nature of the tissue was unaltered between the permeation experiments, the tissue may have already undergone tissue death in the time between the excision and the commencement of the initial permeation experiment, and thus the permeability rate obtained in vitro may not be representative of the in vivo situation. Therefore, more studies assessing the dependence of the barrier nature of the buccal mucosa on tissue viability are... 

Imbert D, Cullander C (1999) Buccal mucosa in vitro experiments I. Confocal imaging of vital staining and MTT assays for the determination of tissue viability. J Control Release 58 39-50... 

The ex vivo techniques of drug permeability using the Ussing chamber has extensively developed in the last few decades. It is very easy to monitor and therefore to maintain tissue viability throughout the study. This is ensured by monitoring the transepithelial potential difference and short-circuits current... 

Ussing chambers Good screening model Tissue viability... 

The more viable the segment is the better is the quality of the results. Based on the tissue viability data, extremes in permeability values can be discarded from the data set, resulting in better and more reliable results and a better overall understanding of drug transport [108],... 

Involvement of the pleura, i.e. formation of pleural plaques (fibrotic masses on the pleura) may accompany asbestosis or occur independently, that is, as lesions with no obvious causal relationship (Whitwell, 1978). Pleural plaques only occasionally cause symptoms, as when they restrict the motion of the lung by thickening the membrane (pleura) around the lung or disrupting tissue viability by calcifying. 

Rhodes CG, Camici PG, Taegtmeyer H, Doenst T. Variability of the lumped constant for [18F]2-deoxy-2-fluoroglucose and the experimental isolated rat heart model clinical perspectives for the measurement of myocardial tissue viability in humans. Circulation 1999 99 1275-1276... 

Nasal tissue from animals can be mounted in Ussing chambers, permitting experiments similar to those performed in cell cultures to be performed in intact tissues. The nasal tissues of rabbits, dog, sheep, and cattle have been used and such experiments provide the reassurance that the ex vivo system is representative of the nasal mucosa in vivo. The limitations of this technique are the requirement for the use of fresh tissue, the limited duration of tissue viability, and interspecies variation in tissue permeability and metabolic capacity. 

Thomas, S., Andrews, A., Hay, P and Bourgoise, S. (1999). The antimicrobial activity of maggot secretions Results of a preliminary study. /. Tissue Viabil. 9, 127-132. 

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