1 Department of Biological Chemistry, Harvard Medical School, Boston, Massachusetts
It is, of course, well-known that the newborn guinea pig has a relatively mature central nervous system but this may have little to do with the functional characteristics of the blood-brain barrier for bilirubin and other organic compounds. For example, although the maturing brain of guinea pigs, mice, rats, cats, and humans shows marked metabolic differences, in these mammals a number of substances are regularly excluded from the brain at all stages of development. Thus, despite species differences and "the changing metabolic pattern of a given specie" the blood-brain barrier for certain compounds is functionally intact in the fetus, newborn, and adult.