Urination is a way to secrete water-soluble irritants and other undesirable chemicals from the body. The generation of urine occurs in kidneys, when chemicals and salts are filtered out of the blood in the kidney tubules. Urine then travels to the bladder, which acts as a storage unit until urination. During the passage of urine, the urine then travels down the urethra and exits the body. Epithelial cells in the blood may originate from one of three lineages and enter the urine from different sites within the urinary system.
Squamous Epithelial Cells
One type of epithelial cell that may be present in urine is a squamous cell. These cells appear relatively large and have a flattened, pancake-like morphology under a microscope. The cells also have a large nucleus---a compartment that houses a cell's DNA---that appears as a small dot in the center of the cell. Squamous cells enter the urine when they are sloughed away from the outer urethra or from the skin surrounding the opening of the urethra, according to The University of Utah. A large number of squamous cells in the urine sample may indicate contamination of the sample with skin tissue and interfere with urinalysis.
Transitional Epithelial Cells
Urine may also contain transitional epithelial cells, which originate from the lining of the bladder and the ureters, which are the tubes that connect the kidneys to the urinary bladder. Under normal conditions, the transitional epithelial cells maintain the elasticity of the bladder and can expand and contract to change the size of the bladder and allow for urine storage. Tissue turnover in the bladder can lead to a shedding of transitional epithelial cells into the urine. The University of Utah indicates that transitional epithelial cells have a flattened appearance and can be distinguished from squamous epithelial cells by their large nucleus, which appears as a large dot within the cell.
Cuboidal Epithelial Cells
In some cases, the urine may contain cuboidal epithelial cells from the kidney tubules---the initial site of urine formation. The kidney tubules are lined with cuboidal epithelial cells; each cell has a cube-like shape, with a large round nucleus in the center of the cell, according to Peninsula College. Under normal conditions, these cuboidal cells facilitate the transfer of salts, sugar and water from the bloody into the urine, helping control both urine formation and the levels of nutrients in the bloodstream. Cuboidal renal tubule cells may enter the urine due to normal tissue analysis, and doctors typically check the numbers of cuboidal cells in the urine during urinalysis to help assess the health of the kidneys.
