Casts

Casts:
Casts are structures formed in the distal tubules and collecting ducts. They are reminiscent of a snake skin.

Tamm-Horsfall protein:
Tamm-Horsfall protein (aka uromodulin) is secreted by cells of the distal tubule and collecting duct. Under certain conditions, Tamm-Horsfall protein forms fibrils which become attached to the tubule walls. If urinary flow is decreased, more protein accumulates around the fibrils forming casts.

Nerdy Note
Tamm-Horsfall protein was named after two prominent virologists, Igor Tamm (1922-1995) and Frank Horsfall (1906-1971). Tamm and Horsfall were looking for a natural inhibitor of viral replication. In the early 1950’s they isolated and purified a protein from urine. The protein demonstrated viral inhibition properties and their studies contributed to basic virology knowledge. The protein today is known as Tamm-Horsfall protein.

Hyaline casts:
Hyaline casts are composed of simple Tamm-Horsfall protein with no inclusions. They have a low refractive index and can be difficult to visualize.

Granular casts:
Granular casts can arise from the degeneration of cellular casts or the aggregation of serum protein granules in the Tamm-Horsfall protein matrix. A yellow color is indicative of the presence of bilirubin. These casts can be observed in the urine after exercise amongst other conditions.

Waxy casts:
Waxy casts represent the last stage in the degeneration of hyaline, granular, or cellular casts. They have smooth blunt ends and are more easily seen under the scope than hyaline casts. Urine protein will always be positive in the presence of waxy casts. Waxy casts are indicative of end stage renal disease.

Fatty casts:
Fatty casts may contain lipid droplets or oval fat bodies. They may be seen in conjunction with free fat droplets in the urine.

Renal casts:
Renal casts are hyaline casts with renal cells incorporated in them. They are occasionally seen normally due to sloughing of tubular cells. If there are too many or they’re seen too frequently it’s indicative of tubular injury.

RBC casts:
The presence of RBC casts is always pathological and associated with an acute glomerulus disorder.

WBC casts:
WBC casts are associated with inflammation or infection of the nephrons.