Cylindruria (casts in the urine)
Cylinders are cylindrical formations of protein or cellular origin, of different sizes. Protein casts (casts of the renal tubules) are formed in the lumen of the convoluted, narrow part of the distal tubule in an acidic medium (pH 4.0 - 5.0) in the presence of plasma protein and Tamm-Horsfall mucoprotein produced by the renal epithelium.
In normal urine, it is contained in a dissolved form. It is deposited in the tubules with the formation of hyaline casts with an excess of mucoproteins (nephrotic syndrome) and changes in the physicochemical properties of urine (pH, viscosity). The formation of cylinders is facilitated by a decrease in renal blood flow, intoxication, dehydration, hypothermia, and the presence of bile acids in the urine.
In alkaline urine, cylinders are not formed; at a high concentration of uropepsin, they dissolve. The size of the cylinders in cylindruria depends on the size of the tubules, the types - on the adhesion of various elements (erythrocytic, epithelial) to the protein cast.
Due to the influence of various factors on the formation of cylinders in cylindruria and the variability of proteinuria, the correlation between the number of cylinders in the urine sediment and the concentration of protein in urine is not always observed.