Casts are cylindrical bodies formed either in the distal convoluted tubules or the collecting ducts of the kidney. Since the walls of the tubule act as a mold for cast formation, the width of the tubule determines the width of the cast. Thus, narrow casts are formed in the distal tubules, while broad casts are formed in the collecting ducts.
The matrix of all casts is uromodulin (formerly known as Tamm-Horsfall protein), a glycoprotein secreted by the distal loop of Henle and the distal tubule. This protein is soluble under normal conditions. When conditions change, entrapment of cells and granular material of tubular origin can happen.
A significant number of urinary casts may indicate the presence of renal disease.