The medulla is arranged into 10-18 pyramidal structures, known as renal pyramids. The base and sides of the renal pyramids are within the cortex, and their vertices, the renal papilla, protrude into the minor calyx. The renal pyramids are separated by a layer of connective tissue and the renal column, which possesses cortical structure. One renal pyramid is also called a lobe. A renal lobule is composed of a single medullary ray and the cortical tissue surrounding it.