Simple cuboidal epithelium consists of a monolayer of epithelial cells that appear to be square-shaped in cross section. The function of these cells are specialized to perform selective secretion and reabsorption in the kidneys, they also line the thyroid follicles and are called follicular cells. This tissue actively takes up iodine and creates thyroid hormone precursors, which are then processed and secreted into the blood. The epithelium lining the ventricles of the brain and the central canal of the spinal cord is called ependyma and is made of a monolayer of cuboidal epithelial cells. These cells generate cerebrospinal fluid.