Circulation of cerebrospinal fluid in the bullfrog, Rana catesbiana

In order to demonstrate features of the cerebrospinal fluid (CSF) path in the amphibian, 2--6 microliters of either Evans Blue-albumin (EBA) or ferrocyanide were injected into the ventricular system of anesthetized bullfrogs. The animals were sacrificed 1 to 135 minutes after injection by either quick freezing (EBA injections) or fixative perfusion (ferrocyanide injections). The contents of the cranial and vertebral cavities were then examined grossly and histologically for distribution of the tracers. In all animals, the tracers were seen throughout the ventricular cavity and in the subarachnoid space surrounding the caudal hindbrain. The site of communication between these two fluid spaces was the posterior tela of the hindbrain. Within this tela, "pores" were found between groups of pavement-like ependymal cells. In many animals, tracer was also observed in the vertebral subarachnoid and epidural spaces, adjacent to spinal nerve roots. In three EBA-injected animals, this tracer was also seen in the subcardinal lymph spaces. These findings suggest that the subarachnoid space in the bullfrog communicates functionally with the ventricular system by way of specialized "pores" in the posterior tela of the hindbrain. There is also indication of movement of fluid within the subarachnoid space which is predominately caudal in direction, with a primary absorptive path for CSF that consists of a perineural route to the lymphatic system.