Comparative ultrastructure of cerebrospinal fluid-contacting neurons and pinealocytes

The pinealocytes of fishes, amphibians, reptiles, birds and mammals have been compared with cerebrospinal fluid (CSF) contacting neurons. We found that the intraventricular dendrite terminal of the latter resembles the pinealocytic inner segment and that the atypical cilium (9x2+0 tubules) of the CSF contacting neurons is analogous with the outer segment of the pinealocytes, even though the outer segment bears photoreceptor lamellae in lower vertebrates. Regular, but small-sized photoreceptor outer segments were also found on pinealocytes of the chicken. In mammals, too, primitive outer segments are present in the form of 9x2 to cilia similar to those of CSF contacting dendritic terminals. In the Golgi areas of the perikarya of both cell types there are granulated vesicles which may contain transmitter substances and/or neurohormones. The synaptic junctions of the pinealocytes differ from those in the CSF contacting neurons. Many synapses occur on the latter, but they appear only rarely on pinealocytes. The axons of the CSF contacting neurons form synaptic connections with other cells, or terminate as neurohormonal synaptic hemidesmosomes on the basal lamina of the brain surface. The pinealocyte axons give rise to terminals containing synaptic ribbons. Such ribbons do not occur in CSF contacting neurons. In Lacertilians, we found pinealocytic terminals without ribbons on dendrite-like profiles. On the basis of the ultrastructural comparisons, we consider the CSF contacting neurons and pinealocytes to be very similar, but not to represent precisely the same cell type.