INTERCELLULAR BARRIERS TO AND TRANSCELLULAR TRANSFER OF ALBUMIN IN THE FETAL SHEEP BRAIN

The nature of the barriers that keep proteins out of the developing br ain has been studied in tissues obtained from fetal sheep in experimen ts conducted under controlled physiological conditions. In anaesthetis ed pregnant ewes, 60 day gestation fetuses (term is 150 days) were exp osed to human albumin injected intravenously for periods up to 6 h. Th e immunocytochemical distribution of exogenous human albumin was compa red with that of endogenous sheep albumin at both the light and electr on-microscopical level. Immunogold labelling of ultracryosections sugg ests that a tubulocisternal endoplasmic reticulum system in immature c horoid-plexus epithelial cells is the route by which albumin crosses f rom blood to cerebrospinal fluid (CSF) in the developing brain. The in tegrity of the blood-brain barrier, the blood-cerebrospinal fluid barr ier and the cerebrospinal fluid-brain barrier to protein, was confirme d. In addition, at the outer surface of the developing brain there als o appears to be a restriction on the passage of albumin from CSF into the brain. These observations support earlier proposals that the immat ure brain develops within an internal environment from which proteins in plasma and CSF are largely excluded.