FURTHER STUDIES ON SWELLING OF BRAIN SLICES

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Abstract
The fluid in incubated brain tissue slices which is permeable by inulin from the surrounding medium, the "inulin space", consists mostly of fluid taken up in swelling but also of a little of the initial water. Unless the tissue is kept cold until aerobic conditions have been fully established, some fluid is taken up which is not penetrated by inulin and is therefore intracellular. Inulin and sucrose, previously absorbed by slices incubated aerobically in their presence, diffuse quickly from slices when these are placed in inulin-free or sucrose-free medium. This indicates that pinocytosis is not involved in the uptake of swelling fluid by incubated slices. When protein labelled with a fluorescent dye is present in the incubation medium, its distribution in the water of tissue slices is similar to that of inulin. This makes possible microscopic visualization of the water compartment in question. Freezedried preparations of slices of grey or white matter that had been incubated with labelled protein revealed under the fluorescent microscope an area of intense fluorescence in the margins. This indicates that the protein space and, by inference, the inulin space represent a damaged region of the slice and are not a measure of definite extracellular or intracellular compartments. This finding is in agreement with the electron-microscopic evidence for the absence of extensive extracellular space in brain. However, the demonstration that the uptake of the greater part of the swelling fluid is in damaged peripheral areas of the slice is in disagreement with conclusions, drawn from electron-microscopic observations, that swelling occurs exclusively in glial cells. Swollen glial cells may account for a small but significant intracellular swelling in incubated slices. Present studies on effects of fixation of cerebral cortex slices indicate that the swelling which we observe by weighing unfixed tissue should also be observable by electron microscopy.