A schematic diagram of structures and specialized cell types bordering the different parts of the mammalian ventricular system, and in contact with the cerebrospinal fluid (CSF). The complexity of the system suggests that CSF functions are not limited to metabolic support of the brain and the release of waste products. Abbreviations: CO: caudal opening of the central canal of the spinal cord;H: hypothalamic CSF-contacting neurons; HY: Hypophysis; LV: lateral ventricle; ME: median eminence; O: vascular organ of the terminal lamina; PIN: pineal organ; R: raphe nuclei; RET: retina; RF: Reissner's fiber; SE: septal region; SCO: subcommissural organ; SP: medullo-spinal CSF-contacting neurons; TEL: telencephalon; TF: terminal filum; (Fig. 1 was kindly provided by Prof. B. Vigh. For details about specific cell types, the reader is referred to: Vigh and Vigh-Teichmann, , and to Vigh et al, ).