ependymal cell

Deep inside the brain there are hollow, fluid-filled chambers called ventricles, and the spinal cord has a thin channel running down its centre. Ependymal cells are the tidy single layer of cells that line the inside walls of all these spaces, like the smooth tiling around the edge of a swimming pool. They are one of the brain's several kinds of glia, the supporting cells that look after neurons rather than firing electrical signals themselves. Each ependymal cell has a roughly cube-like shape, and packed across the surface that faces the fluid are hundreds of tiny hair-like projections called cilia.

Those cilia are not just decoration: they beat back and forth in coordinated waves, like rows of swimmers all rowing in time, and this gentle stroking keeps the cerebrospinal fluid moving steadily through the chambers instead of going stagnant. The same lining also helps make the fluid in the first place. In certain spots the ependymal layer folds into tufts of cells draped over tiny blood vessels, called the choroid plexus, and it is here that fresh cerebrospinal fluid is filtered out of the blood. Across the rest of the lining, ependymal cells act as a soft border between the watery chambers and the solid brain tissue, letting some substances pass while holding others back.

Because they sit right at the meeting point of fluid and tissue, ependymal cells matter for keeping the brain clean and well cushioned, and damage to this lining or to the cilia can disturb how the fluid flows and is renewed.