The medullary rays are finely striated extensions of the medullary tissue that project up into the cortex. They run parallel to the axis of the renal pyramid and represent the central axis of a renal lobule. A medullary ray consists of the straight portion of proximal tubules, the thick ascending limb of Henle's loops (distal tubules), and straight collecting tubules.

The collecting tubule is lined by low cuboidal to tall columnar epithelial cells. The cells are generally pale-staining with a clear, regular cell border. Their nuclei are round or ovoid and situated at the center of the cell. The collecting tubules conduct urine from the nephron to the minor calyx, with some resorption of water.