Final answer:
Casts are the tubular structures in urine made from mucoprotein secreted by kidney cells, indicative of kidney health and can increase with renal stress or disease.
Step-by-step explanation:
The tubular structures found in the urine and composed mainly of mucoprotein secreted by certain cells of the kidney are known as casts. These casts form in the distal convoluted tubule (DCT) or the collecting ducts when certain cells there secrete a type of mucoprotein into the urine. During times of kidney stress or disease, such as inflammation or infection, the number of casts can increase. They are useful in diagnosing the health of the kidney and can indicate various renal diseases when observed under a microscope during urinalysis.