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Department of Physiology, The University, Southhampton.

1. The blood-brain, blood-CSF and CSF-brain barriers to protein have been studied in post-natal tammar wallabies (newborn to 70 days) using morphological and physiological techniques. 2. Endogenous and exogenous plasma proteins, and in some experiments horseradish peroxidase, were used as indicators of barrier integrity or permeability. 3. Immunocytochemical studies of endogenous tammar proteins showed that these (e.g. albumin) were present in the lumen of vessels in the brain, in cerebrospinal fluid (CSF) and within some cells in the choroid plexus and brain. No staining of the brain extracellular space was obtained; in particular there was no perivascular staining. Possible artifacts that could account for this lack of staining are discussed. 4. Ultrastructural studies showed the presence of well-formed tight junctions between cerebral endothelial cells and between choroid plexus epithelial cells, even as early as the day of birth. A membrane specialization between adjacent neuropendymal cells that had the same ultrastructural appearance as the 'strap junction' previously described in human and sheep fetuses was observed. These junctions may act as a barrier (CSF-brain barrier) to the passage of protein from CSF into brain in these immature animals, as has previously been described in eutherian fetuses. 5. In experiments in which exogenous plasma proteins or horseradish peroxidase were injected intravenously, care was taken to limit both the volume and protein load injected. These proteins penetrated into CSF. The naturally occurring steady-state CSF/plasma ratio for several proteins was approached by several of the injected (human) proteins within a few hours of I.V. injection, suggesting that much of the protein in CSF, at least when sampled from the hindbrain, originates from plasma in this species. No penetration across cerebral vessels was observed. Uptake of some proteins (e.g. albumin), occurred into neuroependymal cells at some ages. 6. These results suggest that the very immature brain of the newborn tammar is protected from protein present in the circulating plasma even at an embryonic stage of development by a combination of a well-formed blood-brain barrier to protein in the cerebral vessels and a CSF-brain barrier to protein at the level of the neuroependyma. The adult-type blood-CSF barrier to protein (tight junctions between adjacent choroid plexus epithelial cells) is present but appears to be bypassed in the immature brain, probably by a transcellular route across the choroid plexus.