The distal convoluted tubule is situated between the straight part of the proximal convoluted tubule and the collecting tubule, and is composed of a straight part and a distal convoluted part. The epithelial cells lining the distal convoluted tubule are cuboidal, with a round nucleus and basal striations at the base of the cells. The distal convoluted tubule is responsible for the resorption of sodium, an activity that is promoted by the hormone aldosterone. At the portion of the distal convoluted tubule contacting the parent renal corpuscle between afferent and efferent arterioles, the epithelial cells become densely packed, with the nuclei close together. Thus the region appears darker and is known as macula densa. The macula cells are immediately adjacent to the afferent and efferent arterioles, to the juxtaglomerular cells, and to the extraglomerular mesangial cells. These three cell groups together are called the juxtaglomerular apparatus.