Abstract
The extracellular space in the developing rat cerebral cortex is quite large but varies considerably in extent depending on age, method of fixation and osmolarity of the fixative. Since Na+ is known to be extracellular in location, the potassium pyroantimonate technique of Komnick was utilized in the newborn rat in order to follow the movement of Na+ and water. However, the location and size of the pyroantimonate precipitates appeared to vary with the type of fixative solution and method of fixation. Thus, they were large and mainly intracellular with phosphate- or s-collidine-buffered glutaraldehyde after perfusion fixation, and mostly extracellular after immersion fixation. Following primary osmium tetroxide fixation, these particles were small and numerous with the aqueous solution, being diffusely present throughout the section, and large and fewer with the phosphate-buffered solution. Whether Na+ alone was being demonstrated could not be determined. Because of this marked variability it is concluded that definite statements as to the actual in vivo location of the Na+ in this preparation cannot be made.