Cerebrospinal fluid (CSF)-contacting neurons are located periventricularly or inside the brain ventricles; they contact the CSF via their dendrites, perikarya or axons. Most of the CSF-contacting nerve cells send dendritic processes into the ventricular cavity where they form ciliated terminals. These ciliated dendritic endings resemble those of known sensory cells, yet their role is still unknown. There are two types of CSF-contacting dendritic terminals. One bears solitary 9x2+0 cilia; it is present in different hypothalamic regions such as the paraventricular organ and the vascular sac. The magnocellular neurosecretory nuclei also contain CSF-contacting neurons, which probably furnish information about the parameters of the CSF for the regulatory function of the hypothalamo-hypophyseal system. CSF-contacting nerve cells of the parvocellular hypothalamic nuclei are suspected to participate in hypothalamo-adeno-hypophyseal regulation. A second type of CSF-contacting dendritic terminal bears many stereocilia and is found in the central canal of the spinal cord. This type of terminal is also supplied with a 9x2+2 kinocilium that may contact Reissner's fiber, the secretory material of the subcommissural organ. Resembling mainly mechanoreceptors, these spinal CSF-contacting neurons appear to form axon terminals of the neurosecretory type at the external circumference of the spinal cord. Developing and/or regressing photoreceptor cells of the retina and pineal complex may display a similar dendritic structure characteristic of hypothalamic CSF-contacting neurons. Axons penetrating into the ventricles innervate the apical surface of the ependyma and/or the CSF-contacting dendritic terminals. Some bipolar neurons of the retina form so-called Landolt's clubs; these may be considered as the retinal component of the CSF-contacting neuronal system. Since in the lancelet nearly all nerve cells contact the CSF, the CSF-contacting neurons represent a specialized, but phylogenetically old cell type, a “protoneuron” in the vertebrate brain. They may be derived phylogenetically by inversion of the ciliated neurons found in the plate-like nervous system of more primitive deutero-stomians.
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