A cast is a mold of the tubule that made it

A cast forms inside a renal tubule and takes the tubule's shape. Uromodulin, formerly known as Tamm-Horsfall protein, is secreted into the tubular lumen by cells of the thick ascending limb and early distal convoluted tubule. It is the principal matrix protein of a hyaline cast, the simplest and most common cast type. When urine is concentrated and flow through that tubular segment slows, conditions favor uromodulin aggregating into a solid protein gel that fills the tubule cross-section.

Because the tubule wall acts as the mold, the resulting matrix comes out with broadly parallel sides and rounded or blunt ends rather than sharp, ragged edges. This shape is the single most useful visual fact: a true cast looks like a cylinder because it was cast inside a cylinder. Tubular concentration, low flow or stasis, and a physicochemical environment that favors protein aggregation all favor formation, but these are facilitators rather than fixed independent requirements that must all be present in a fixed combination.

The matrix can pick up cargo as it forms or afterward: cells swept into the lumen, cellular debris from degeneration, lipid droplets, pigment, or crystals. That cargo is what separates a granular cast from a cellular cast from a fatty cast, but the underlying matrix and its cylindrical shape is what makes all of them casts in the first place. A structure should not be called a cast solely because it contains cells or granules; the matrix boundary has to be there too.

Look for the mold before you look at the cargo, because matrix shape identifies a cast and cargo only tells you which cast category it is.