CHEMICAL ARCHITECTURE OF THE CENTRAL NERVOUS SYSTEM 



'795 



WHOLE BRAIN GRAY 



(ECS) (ICF) (SOLIDS) 



30% / 47 "/. 



table 2. Components of Cerebral Lipids* 



fig. 1. The gross composition of whole brain and of gray 

 and white matter. Percentages of total solids accounted for as 

 proteins (/ J ) and lipids I/.) arc shown at the right of each block. 

 For whole brain, the dotted tine indicates the approximate por- 

 tions of the extracellular space {ECS) occupied by cerebro- 

 spinal fluid (on the left) and interstitial fluid (on the right). 

 The ionic composition of whole brain fluids are expressed in 

 milliatoms per kg of extra- and intracellular HjO, respectively. 

 For gray and white matter, the dotted line represents the esti- 

 mated division of tissue water into extracellular ion the left) 

 and intracellular (on the right), the former being about 34 

 per cent for gray and 28 per cent for white. (References: 29, 

 42, 65, 96, [12, 115-118, [32, 138, 145, 149, 150, 153, [55, 

 164, 174-176, 182, 212, 221-223, 234, 241, 346, 247, 251, 

 252.) 



generally assumed that the chloride ion is essentially 

 extracellular and may therefore be used as a measure 

 of the extent of the extracellular space. 1 The basis for 



1 For calculation of the chloride space in brain, cerebro- 

 spinal fluid CI - concentration is taken to represent that of 

 extracellular fluid CI - and the space is calculated as follows 

 (i49> : 



Extracellular Ff.O (gm/kg tissue) = 



Total tissue chloride (mEq/kg) 



Extracellular fluid chloride (mEq/1.) 



X 100 



Where extracellular fluid CI" concentration is not known, this 

 has been calculated by correction for the Donnan equilibrium 

 effect from serum chloride concentrations ( 1 45 ) . 



The disagreement between electron microscopists, who have 

 observed little or no space between cells in electron micrographs 

 of brain (248), and biochemists and physiologists, who have 

 maintained that a functional extracellular space must exist to 

 account for observed phenomena, are gradually being resolved. 

 Fixation and embedding procedures employed in the prepara- 

 tion of electron micrograph sections are now recognized to 

 involve shrinkage and often collapse or compression of less 

 well 'suspended' structures. Both the biochemical studies on 

 the distribution of ions in cerebral tissues and the electrochemi- 

 cal data from microelectrode recordings require the presence 



* References: 21-23, 29, 66, 68, 69, 71, 74-76, 78, 115-118, 

 128-132, 134, 153, 182. 



t After Folch & Sperry 71 1 



J Estimated from Mcllwain 1 153) and Korey (134)- 



§ Strandin is a high molecular weight complex of gan- 

 glioside-type molecules. 



this assumption has been reviewed by Lowry & 

 Hastings (145). The problems encountered with the 

 myelin sheath (146, 234), with studies on brain slices 



of a functional extracellular space. The recent electron micro- 

 scope studies by de Robertis and co-workers (86) and by Luse 

 1 147 1 demonstrate that one type of glial cell, probably the 

 astrocyte (86, 126), behaves as if it is part of the biochemically 

 definable extracellular (chloride) space. In addition, the prob- 

 ability that the lumina of the endoplasmic reticulum of neural 

 cells communicate with the extracellular fluid, either per- 

 manently or intermittently (as a type of pinocytosis), is gaining 

 wide acceptance (209). Thus, it appears that the brain con- 

 forms to other body tissues in regard to functional divisions of 

 its fluid spaces, but it may utilize somewhat specialized struc- 

 tural features in the discreet organization of these spaces. 



