There are two types of bladder cancers—urothelial and non-urothelial types.
The urothelium is special type of transitional cell epithelium that forms a stretchy, protective barrier in the bladder. The majority of primary bladder tumors are urothelial cell carcinomas also known as transitional cell carcinomas that obviously originate from the urothelium.
Somewhat confusingly, non-urothelial tumors, like squamous cell carcinomas and adenocarcinomas, also often arise from the urothelium layer but are distinguished by the way that their cells differentiate.
For example, the normal bladder surface is not normally lined with squamous epithelium, yet the cells in the urothelium can change shape and take on a flat, pancake-like appearance of squamous cells. This non-cancerous change is called squamous cell metaplasia.
If these cells begin to grow unchecked they can turn into a squamous cell carcinoma. These tumors typically pop up in multiple locations, and show extensive keratinization, which is where the cytoplasm of the cells is filled with keratin, the same tough material in hair and nails.
Squamous cell carcinomas typically arise in response to chronic irritation like from recurrent urinary tract infections, and long-standing kidney stones.
Another cause, common in some parts of the world, is an infection with Schistosoma haematobium - a type of flatworm. In this infection, the infective larvae linger in the water and then burrow into human skin when given the opportunity, and travel to the liver to mature into adult flukes. From there young flukes migrate to the urinary bladder veins to sexually reproduce and lay eggs in the bladder wall. These eggs can get urinated out, but they also cause chronic inflammation in the bladder wall which is how they lead to squamous cell carcinomas.