The blood-brain barrier and ventricular system of Myxine glutinosa

Comparison of the rate and extent of penetration of test compounds from plasma into brain and muscle of the hagfish, Myxine glutinosa, indicates that the blood-brain barrier is poorly developed or absent in this species. We examined a series of hagfish brains in the light and electron microscope in order to relate the structure of the brain to the physiology of the blood-brain barrier and cerebrospinal fluid. The ventricular system consists of an ependymal cell-lined central canal extending from the spinal cord to the midbrain and two or more ependymal cell-lined cavities located more rostrally. A preoptic and an infundibular recess were present in the diencephalons of all brains and were isolated from each other and from the primary ventricular system. Since a typical choroid plexus could not be identified, this suggests that cerebrospinal fluid must be formed entirely by brain in this species. Cerebral capillaries differ significantly from those of other vertebrates in possessing large numbers of cytoplasmic vesicles and in the relative rarity of tight junctions between endothelial cells. These capillaries do not, therefore, appear to be morphologically specialized for barrier functions.