Urine is produced and excreted by the kidneys, two bean-shaped retroperitoneal organs. In the cortical part of the kidney the blood is filtered in the glomeruli. The filtrate passes through the proximal convoluted tubule, the Loop of Henle and the distal convoluted tubule. In these sections of the nephron the filtrate changes its chemical composition and becomes concentrated urine. Finally, the urine passes through the straight collecting tubule to the renal pelvis, into the ureters and thence into the urinary bladder; here it is stored until voided via the urethra, which, in males, traverses the prostate gland.
The mucosa of the urinary tract is made by a multilayered epithelium with an average of 7-8 layers of specialized cells connected to a basement membrane.
The thickness of bladder mucosa varies depending on the state of the bladder. When distended, there are only few layers of urothelial cells, while an empty bladder appears to have a thick multi-layered epithelium. The cells of the deeper layers are small with a single nucleus. The superficial cells are larger and often multinucleate with between 2 and 55 nuclei. Each superficial cell overlies two or more deeper cells. For this reason they are called ‘umbrella’ cells.
The urothelium of normal bladder can show several epithelial invaginations called von Brunn’s nests. They can be lined by mucus secreting columnar cells rather than urothelium. When they become distended with mucus the histological picture is called cystitis cystica or cystitis glandularis.