Urothelial Cells: Definition

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Urothelial cells are the cells that line much of the urinary tract. They cover the inside of the renal pelvis, the ureters, the bladder, and the upper urethra. Doctors used to call them transitional cells. Together, they form a lining called the urothelium. The urothelium acts as a barrier that keeps urine from reaching the tissue underneath.

Urothelial cells are normal cells. A pathologist often sees them in urine samples and bladder biopsies. This article explains what urothelial cells are, what they look like, what it means to find them in urine, and which cancers start from them.

What are urothelial cells?

Urothelial cells are a type of epithelial cell that lines the urinary tract. The lining changes with the organ’s size. In a full bladder, the lining stretches and thins to two or three cell layers. In an empty bladder, it contracts and thickens to five to seven layers. The surface cells form a tight barrier, so urine cannot leak into the organ wall.

You can learn more about the organs that urothelial cells line in the bladder, ureters, urethra, and kidneys articles.

What do urothelial cells look like under the microscope?

Urothelial cells form several layers. Each layer has a different type of cell:

Umbrella cells. These cells sit on the surface. They are large and dome-shaped. Many have two nuclei. They have abundant pink cytoplasm.
Intermediate cells. These cells sit in the middle layers. The number of layers depends on how far the organ stretches.
Basal cells. These cells form a single layer at the bottom. They can divide to replace damaged cells.

The nucleus of a urothelial cell is relatively large. It is round to oval. The chromatin looks fine and evenly spread. A nucleolus may be visible.

Is it normal to find urothelial cells in urine?

Finding urothelial cells in urine is normal. The lining sheds some cells, and they pass out in the urine. In a voided urine sample, the cells usually appear one at a time. Pathologists examine them under the microscope. Doctors call this test urine cytology. The cytology report article explains how these reports work.

Why do pathologists look at urothelial cells in urine?

Pathologists look at urothelial cells in urine to search for bladder cancer or cancer in the upper urinary tract. Cancer cells can shed into the urine, just like normal cells. The pathologist looks at the size, shape, and color of the cells and the look of their nuclei. Many reports use the Paris System, which describes how likely high-grade cancer cells are present.

What does atypical mean for urothelial cells?

Atypical means the cells look abnormal under the microscope. The change may involve their shape, size, or color. Atypical cells do not always mean cancer. Many benign conditions cause cytologic atypia. Causes include:

Infection. Bacterial and fungal infections can make urothelial cells look reactive.
Viral infection. BK polyomavirus can infect urothelial cells. The infected cells mimic cancer cells, so pathologists call them decoy cells. See viral cytopathic effects.
Urinary stones. Stones can change the lining and its cells.
Treatment. Radiation, chemotherapy placed in the bladder, and BCG immunotherapy can all change how the cells look.

Atypical cells can also occur in cancer. For this reason, doctors usually follow up an atypical result.

What types of cancer start from urothelial cells?

Doctors call cancers that start from urothelial cells urothelial cancers. The main types are:

Urothelial carcinoma. Urothelial carcinoma is a cancer of the urinary tract lining.
Papillary urothelial carcinoma. Papillary urothelial carcinoma grows as finger-like projections. Pathologists divide it into low grade and high grade types.
Urothelial carcinoma in situ. Urothelial carcinoma in situ is a flat, high-grade cancer. It stays within the lining. See also carcinoma in situ.

Not every growth from urothelial cells is cancer. A urothelial papilloma is a noncancerous growth.

What might my report say about urothelial cells?

Urine reports often use the Paris System categories. Each laboratory phrases them a little differently:

Negative for high-grade urothelial carcinoma. The pathologist did not see cells of high-grade cancer. This result is reassuring, but it does not fully rule out cancer if your doctor remains concerned.
Atypical urothelial cells. Some cells look abnormal, but not enough to call them suspicious or cancer.
Suspicious for high-grade urothelial carcinoma. The cells strongly raise concern for cancer, but the pathologist is not certain.
High-grade urothelial carcinoma. The pathologist saw cancer cells.
Nondiagnostic. The sample had too few cells, or the cells were too poorly preserved, to judge. This does not mean the result is normal.

Urine cytology is less reliable for low-grade tumors. For this reason, the Paris System removed its separate low-grade category in its second edition. Your doctor may use other tests, such as a tissue biopsy, to look for these tumors.

What happens after my report?

The next steps depend on the result. Doctors usually follow up an atypical result. After a suspicious or positive result, they almost always recommend a tissue biopsy. A biopsy or tumor removal gets its own report. The TURBT pathology report article explains that report.

Questions to ask your doctor

What does my report say about the urothelial cells?
Was the urine sample good enough to judge?
If my result is atypical, what could explain it?
Could an infection, a stone, or a recent treatment explain the changes?
Do I need a repeat urine test?
Do I need a biopsy or an exam of my bladder?
How often should I have follow-up testing?
Which symptoms should lead me to call you?