P Grandjean and P J Landrigan1 provide a valuable and timely review of industrial chemicals that are thought to have damaging effects if they reach the developing brain. In discussing possible mechanisms that might account for the vulnerability of the developing brain to these agents, Grandjean and Landrigan state “The blood—brain barrier is not completely formed until about 6 months after birth”, citing a paper by Adinolfi.2 This perpetuates the long-standing misconception that “the” blood—brain barrier in the fetus and newborn is immature.
The term blood—brain barrier is used to describe a series of physiological mechanisms (inward and outward transport mechanisms) that control the composition and stability of the internal environment of the brain. Underlying these physiological mechanisms is the presence of intercellular tight junctions between cerebral endothelial cells (blood—brain barrier) and between choroid plexus epithelial cells (blood—CSF barrier). The presence of tight junctions is essential for the functional effectiveness of all the physiological barrier mechanisms. Tight junctions between cerebral endothelial and between choroid plexus epithelial cells have been known for decades to exclude the intercellular passage of proteins from blood into brain and CSF.3
The tight junctions at both blood—brain and blood—CSF interfaces are well formed from early embryonic life.4 However, there is a transcellular mechanism in a subpopulation of choroid plexus epithelial cells that transfers proteins from blood to CSF.5 This protein transfer mechanism might be relevant for the entry of some neurotoxic agents into the immature brain. For example, some of the heavy metals listed by Grandjean and Landrigan bind to proteins in plasma and could thus be transported into the brain via the choroid plexus. However, this is a reflection of a developmental specialisation rather than of immaturity.
2AdinolfiM. The development of the human blood-CSF-brain barrier. Dev Med Child Neurol1985; 27: 532-537. CrossRef | PubMed
3SaundersNR. Ontogenetic development of brain barrier mechanisms. In: Bradbury MWB, ed. Physiology and pharmacology of the blood brain-barrier. Handbook of experimental pharmacology. Berlin: Springer-Verlag, 1992: 327-369.
4EkCJ, HabgoodMD, DziegielewskaKM, SaundersNR. Structural characteristics and barrier properties of the choroid plexuses in developing brain of the opossum (Monodelphis domestica). J Comp Neurol2006; 460: 451-464. CrossRef | PubMed
5JohanssonPA, DziegielewskaKM, EkCJ, et al. Blood-CSF barrier function in the rat embryo. Eur J Neurosci2006; 24: 65-76. CrossRef | PubMed
a University of Melbourne, Parkville, VIC 3010, Australia