A glial blood-brain barrier in elasmobranchs.

The blood-brain interface in elasmobranchs was examined using physiological and anatomical techniques. Electrolyte analyses of plasma and cerebrospinal fluid (CSF) in control fish and in fish with elevated plasma cation concentration (Ca, Mg or K) demonstrated homeostasis of CSF calcium and magnesium concentrations in dogfish and skates and of CSF potassium concentration in dogfish. In contrast, CSF potassium concentration was not regulated independent of plasma potassium concentration in skates. Ultrastructural examination of the blood-brain barrier in skates showed that brain capillaries are fenestrated and intravenously injected horseradish peroxidase penetrates readily across this endothelium into the basement membrane. Further penetration of tracer into the neuropil is prevented by tight junctions between perivascular glial cells. Results indicate that two hallmarks of the blood-brain barrier--impermeability and homeostatic transport systems--can be localized to the perivascular glial sheath in elasmobranchs. The functional and structural similarity of this glial membrane to a transporting epithelium is discussed.