There are cavities inside the tissue of the central nervous system that are filled with a fluid called cerebrospinal fluid. The lining of these spaces is called the ependyma, named from a Greek word for covering. The ependyma is composed of glia called ependymal cells.
Ependymal cells form a simple (single layer) cuboidal (cube shaped) epithelium (covering). The side of the cells facing the cerebrospinal fluid has microvilli (tiny short processes) to increase surface area, and cilia (larger mobile processes) that help move the fluid. The main function of ependymal cells is to form a barrier between the cerebrospinal and interstitial fluids. This barrier is relatively leaky compared to the blood-brain barrier.
This is medically useful because analysis of the cerebrospinal fluid can provide information about conditions in the tissue of the central nervous system. Specialized ependymal cells and capillaries form tufts called choroid plexus that secrete the cerebrospinal fluid, as well.