Variation of cerebral cortex fluid spaces in vivo as a function of species brain size

Abstract

Sizes of cerebral cortex fluid spaces accessible to chloride, thiocyanate-C14, sucrose-C14, and inulin-C14 were investigated in nine mammalian species. Indicator solutes were injected intracisternally, and after suitable intervals subarachnoid CSF and subadjacent cerebral cortex were sampled. Factors of site, time, and reproducibility of sampling were studied in detail. Despite influences of nonhomogeneous distribution in CSF of injected solutes and other factors on space measurements, appropriate attention to experimental conditions provided consistent, reproducible results. For a given species, there was identity of cortical chloride and thiocyanate spaces and identity of sucrose and inulin spaces, with the latter slightly but significantly smaller than the former. From smaller to larger species, all spaces exhibited parallel, significant increases (e.g., average sucrose-inulin spaces for guinea pig: 20.8%; cat: 27.4%; chimpanzee: 39.4%), varying as a function of the logarithm of the average, species brain weight. Findings are discussed relevant to CSF solute distribution and efflux and to electron microscope observations, glial and extra-cellular distribution of solutes, and abnormal conditions such as edema following circulatory arrest.