BRAIN EXTRACELLULAR FLUID
Read also: The History of Artificial Cerebrospinal Fluid (ACSF)
The brain is protected by a rigid bony case so it cannot expand in the case of fluid imbalance. Because of that, the brain needs to tightly control the flux across the cerebral capillaries and this line of defence or restriction of chemical communications between blood and brain, called blood-brain barrier, was introduced by the work of Erhlich et al., in nineteenth century and the classic experiment of Goldman confirmed the concept of the blood-brain barrier (reviewed in 1).
The details of its location and organization remained a subject of debate, until the results of electron microscopy of Reese and Karnovsky (2). However, the issue of the brain extracellular fluid (ECF) remained, especially after the estimation made by electron microscopists that volume of ECF is between 5 and 10% of brain tissue (3). There also were doubts about the very nature of BBB and many considered it a histological artefact until the results of Davson and Spaziani (4) on perfusion of markers into the blood and the finding that the easily achieved equilibrium in the muscles was not observed in the intact brain but demonstrated the equilibrium with the incubation medium in the sliced brain. These findings physiologically clarified the amount of space allotted to ECF – 15 % (250-300 ml in human brain), same as in any other tissue of the body.
What is now known about ECF content, sources, and control mechanisms? Molecules that are allowed to cross the BBB enter the brain ECF, diffuse into the CSF and are returned through CSF drainage mechanisms to the blood. ‘We cannot exclude the possibility that the fluid (extracellular) is secreted by the cells of the capillary endothelium, however revolutionary such a hypothesis may seem; if this were true, the rate of formation would be rigorously controlled; a defect in the secretory system for example, the development of numerous and large leaks in the capillaries — would circumvent this control, permitting a much more rapid loss of fluid into the nervous tissue than normally occurs, and this would give rise to the phenomenon of oedema,’ (5).
References
- Bradbury MWB (1979) The Concept of a Blood-Brain Barrier. Chichester: Wiley
- Reese TS, Karnovsky MJ (1967) Fine structural localisation of a blood-brain barrier to exogenous peroxidase. J Cell Biol 34:207 217
- Wyckoff RWG, Young JZ (1956) The motor-neurone surface. Proc Roy Soc B 144: 440-450
- Davson H, Spaziani E (1959) The blood-brain barrier. J Physiol 149: 135-143
- Davson H, Welch K, Segal MB (1987) The Physiology and Pathophysiology of the Cerebrospinal Fluid. Edinburgh: Churchill Livingstone
