Variations in rat mesenteric tissue thickness due to microvasculature.

Studies of microvascular, tissue and lymphatic transport using microphotometric and microfluorometric techniques are potentially subject to artifact due to variations in tissue specimen thickness. Absorbance techniques utilize the Lambert-Beer law in which A = log I0/I = act, where A is absorbance, I0 is incident light intensity, I is transmitted intensity, a is an absorbance coefficient, c is concentration of substance, and t is path length. If differences in t are known to be present, then inferences of changes in c from changes in A become suspect. In microfluorometry the amount of light gathered is proportional to the number of fluorochromes in the effective cuvette, which is determined by the microscope's numerical aperture and the sample thickness. If variations in thickness are known to occur, the effective cuvette volume may be changing; therefore, inferences of changes in fluorochrome concentration from changes in intensity become suspect. Existing data suggest that rat mesentery is 15-30 microns thick, but variation over a tissue region is unknown. Our goals are to determine thickness variation in avascular, fat-free mesenteric tissue regions; thickness variation near blood vessels; and average tissue thickness. Sprague-Dawley rats were anesthetized with Inactin. Mesenteric tissue from a loop of small intestine was draped over a platform for observation; thickness was measured with an oblique microscope and a microgravimetric technique. The average variation in avascular fat-free tissue was 1.1 micron/100-microns distance, and average thickness was 17.4 microns. There was a significant increase in thickness over the microvasculature.