EFFECTS ON BLOOD-BRAIN BARRIER, CEREBRAL BLOOD VESSELS 413 



after the systemic administration of the radioactive ion, and one can com- 

 pare the amount of radioactivity in brain tissue with respect to other 

 oroans in the body such as liver, spleen, or kidney. Thus, it is generally 

 accepted today that the phenomenon of a blood-brain barrier exists. 



How the blood-brain barrier functions and the anatomic locus of the 

 blood-brain barrier are more hypothetical. These two cjuestions are, by their 

 nature, related. At present two groups of structures are considered by various 

 investigators to be the possible anatomic sites of the blood-brain barrier: 

 the capillary endothelium of the central nervous system and the perivascular 

 sheath comprised of the membranes of the neuroglial cells. 



The proponents of the capillary wall theoiy point to many experiments 

 performed with vital dyes. At first it was felt that the endothelial cells of 

 cerebral vessels were different in their permeability characteristics because 

 they remained unstained with vital dyes (Spatz, 1933). Broman (1937, 

 1940a,b) also emphasized the vascular walls as being the site of the blood- 

 brain barrier and noted that \itally stained vessels in the choroid plexus 

 were in communication with branches within the brain which did not stain. 



Hauptmann and Gartner i 1932 ) . Hoff ( 1933 ) , and Tschirgi ( 1952) have 

 expressed a preference for the perivascular glial membrane (Figs. lA and 

 IB) as the more probable locus of the selective vascidar-permeability char- 

 acteristics encountered in the brain. Although the capillary wall enthusiasts 

 today still far outnumber the neuroglial membran-e supporters. Tschirgi 

 (1958) offers a most interesting theory involving an active mechanism of 

 transport of substances from blood to brain and metabolites from brain to 

 blood, rather than a passive mechanism of diffusion. For an excellent review, 

 sec the chapter written by Tschirgi > 1960) in the "Handbook of Physiology." 



Radiation and Blood-Brain Barrier 



It is well known that certain permeability changes in the cerebral \ascula- 

 ture can occur following direct trauma to the brain, during simple exposure 

 of the nervous system ( Macklin and Macklin. 1920; Prados et al., 1945; 

 Grenell and McCawley, 1947), or following intravenous injection or topical 

 application of certain chemical substances such as Diodrast ( Broman and 

 Olssen. 1948, 1949; Bassett ct al., 1953), epinephrine ( Friedemann and El- 

 keles, 1932). and hydrogen peroxide (Gi\re and Rexed. 1948). Thus, it is 

 not surprisino- that another mode of physical injury to the brain, exposure 

 to ionizing radiation, is also capable of producing permeability changes in 

 cerebral \essels. This fact has been recognized since Rachmanow (1926) 

 and Mogilnitzsky and Podljaschuk (1930) showed that ionizing radiations 

 caused the perivascular neuroslia of mice and rabbits to be stainable with 

 trypan blue. 



