Cerebrospinal fluid contacting neurons in the reduced brain ventricular system of the atlantic hagfish, Myxine glutinosa

Abstract

Cerebrospinal fluid (CSF)-contacting neurons are sensory-type cells sending ciliated dendritic process into the CSF. Some of the prosencephalic CSF-contacting neurons of higher vertebrates were postulated to be chemoreceptors detecting the chemical composition of the CSF, other cells may percieve light as "deep encephalic photoreceptors". In our earlier works, CSF-contacting neurons of the mechanoreceptor-type were described around the central canal of the hagfish spinal cord. It was supposed that perceiving the flow of the CSF they are involved in vasoregulatory mechanisms of the nervous tissue. In the present work, we examined the brain ventricular system of the Atlantic hagfish with special reference to the presence and fine structure of CSF-contacting neurons. Myxinoids have an ontogenetically reduced brain ventricular system. In the adult hagfish (Myxine glutinosa) the lumen of the lateral ventricle is closed, the third ventricle has a preoptic-, infundibular and subhabenular part that are not connected to each other. The choroid plexus is absent. The infundibular part of the third ventricle has a medial hypophyseal recess and, more caudally, a paired lateral recess. We found CSF-contacting neurons in the lower part of the third ventricle, in the preoptic and infundibular recess as well as in the lateral infundibular recesses. No CSF-contacting neurons were found in the cerebral aqueduct connecting the subhabenular recess to the fourth ventricle. There is a pineal recess and a well-developed subcommissural organ at the rostral end of the aqueduct. Extending from the caudal part of the fourth ventricle in the medulla to the caudal end of the spinal cord, the central canal has a dorsal and ventral part. Dendrites of CSF-contacting neurons are protruding into the ventral lumen. Corroborating the supposed choroid plexus-like function of the wall of the dorsal central canal, segmental vessels reach a thin area on both sides of the ependymal lining. The perikarya of the CSF-contacting neurons found in the brain ventricles are mainly bipolar and contain granular vesicles of various size. The bulb-like terminal of their ventricular dendrites bears several stereocilia and contains basal bodies as well as mitochondria. Basal bodies emit cilia of the 9+0-type. Cilia may arise from the basal body and accessory basal body as well. The axons run ependymofugally and enter--partially cross--the periventricular synaptic zones. No neurohemal terminals similar to those formed by spinal CSF-contacting neurons of higher vertebrates have been found in the hagfish. We suppose that CSF-contacting neurons transform CSF-mediated non-synaptic information taken up by their ventricular dendrites to synaptic one. A light-sensitive role for some (preoptic?) groups of CSF-contacting neurons cannot be excluded.