What is the bladder?
The urinary bladder, or the bladder, is a hollow organ in the pelvis. Most of it lies behind the pubic bone of the pelvis, but when full of urine, it can extend up into the lower part of the abdomen. Its primary function is to store urine that drains into it from the kidney through tube-like structures called the ureters. The ureters from both the kidneys open into the urinary bladder. The bladder forms a low-pressure reservoir that gradually stretches out as urine fills into it. In males, the prostate gland is located adjacent to the base of the bladder where urethra joins the bladder. From time to time, the muscular wall of the bladder contracts to expel urine through the urinary passage (urethra) into the outside world. The normal volume of the full bladder is about 400 ml-600 ml, or about 2 cups.
What are the layers of the bladder?
The bladder consists of three layers of tissue. The innermost layer of the bladder, which comes into contact with the urine stored inside the bladder, is called the "mucosa" and consists of several layers of specialized cells called "transitional cells," which are almost exclusively found in the urinary system of the body. These same cells also form the inner lining of the ureters, kidneys, and a part of the urethra. These cells form a waterproof lining within these organs to prevent the urine from going into the deeper tissue layers.
The middle layer is a thin lining known as the "lamina propria" and forms the boundary between the inner "mucosa" and the outer muscular layer. This layer has a network of blood vessels and nerves and is an important landmark in terms of the staging of bladder cancer (described in detail below in the bladder cancer staging section).
The outer layer of the bladder comprises of the "detrusor" muscle and is called the "muscularis." This is the thickest layer of the bladder wall. Its main function is to relax slowly as the bladder fills up to provide low-pressure urine storage and then to contract to compress the bladder and expel the urine out during the act of passing urine. Outside these three layers is a variable amount of fat that lines and protects the bladder like a soft cushion and separates it from the surrounding organs such as the rectum and the muscles and bones of the pelvis.
Picture of the urinary system: kidneys, ureters, and bladder
What is bladder cancer?
Bladder cancer is an uncontrolled abnormal growth and multiplication of cells in the urinary bladder, which have broken free from the normal mechanisms that keep uncontrolled cell growth in check. Bladder cancer (like cancers of other organs) has the ability to spread (metastasize) to other body parts, including the lungs, bones, and liver.
Bladder cancer invariably starts from the innermost layer of the bladder (for example, the mucosa) and may invade into the deeper layers as it grows. Alternately, it may remain confined to the mucosa for a prolonged period of time. Visually, it may appear in various forms. Most common is a shrub-like appearance (papillary), but it may also appear as a nodule, an irregular solid growth or a flat, barely perceptible thickening of the inner bladder wall (see details in subsequent sections).
What is the burden of bladder cancer in the U.S.?
Bladder cancer has the dubious distinction of inclusion on the top 10 list of cancers, with an estimated 74,690 new cases occurring in 2013-2014 within the U.S. Bladder cancer is three to four times more likely to be diagnosed in men than in women and about two times higher in white men than in African-American men. Bladder cancer killed an estimated 15,210 people in 2013. In the U.S., the lifetime bladder cancer risk for men is about one in 26 and for women about one in 90.
What are the types of bladder cancer?
Bladder cancer is classified on the basis of the appearance of its cells under the microscope (histological type). The type of bladder cancer has implications in selecting the appropriate treatment for the disease. For example, certain types may not respond to radiation and chemotherapy as well as others. The extent of surgery required for maximizing the chances of cure may also be impacted by the histological type of the cancer. In addition, bladder cancers are often described based on their position in the wall of the bladder. Noninvasive bladder cancers are located in the inner layer of cells (transitional cell epithelium) but have not penetrated into deeper layers. Invasive cancers penetrate into the deeper layers such as the muscle layer. Invasive cancers are more difficult to treat.
The more common types of bladder cancer and their relative incidence are given below:
- Urothelial carcinoma (previously known as "transitional cell carcinoma") is the most common type and comprises 90%-95% of all bladder cancers. This type of cancer has two subtypes, papillary carcinoma (growing finger-like projections into the bladder lumen) and flat carcinomas that do not produce fingerlike projections. Urothelial carcinoma (transitional cell carcinoma) is strongly associated with cigarette smoking.
- Adenocarcinoma of the bladder comprises about 1%-2% of all bladder cancers and is associated with prolonged inflammation and irritation. Most adenocarcinomas of the bladder are invasive.
- Squamous cell carcinoma comprises 1%-2% of bladder cancers and is also associated with prolonged infection, inflammation, and irritation such as that associated with longstanding stones in the bladder. In certain parts of the Middle East and Africa (for example, Egypt), this is the predominant form of bladder cancer and is associated with chronic infection caused by Schistosoma worm (a blood fluke, that causes schistosomiasis, also termed bilharzia or snail fever).
- Other rare forms of bladder cancer include small cell cancer (in bladder nerves), pheochromocytoma (rare), and sarcoma (in muscle tissue).