The blood-cerebrospinal fluid (CSF) is produced and secreted to the brain ventricles and spinal cord during early mammalian embryonic development and postnatally, by specialized choroid plexus epithelial cells. The CSF is not only important for the physical protection of the growing brain, but also maintains a specific pressure essential for normal development of the central nervous system. A mechanism unique to the developing brain, transfers specific proteins from blood to the CSF, via the tubulo-cisternal endoplasmic reticulum of the plexus epithelial cells, resulting in a high concentrations of proteins in early CSF. An important role of the CSF is the transport of nutrients, ions, and hormones from the blood via the choroid plexus into the brain, in addition to clearing toxic agents from the brain, back to the blood. In the adult, the CSF is continuously secreted into the ventricular system and drained by bulk flow back into the venous side of the cerebral circulation, via the arachnoid villi of the superior sagittal sinus.