Urinary bladder

The urinary bladder is a hollow organ designed to store and excrete urine. The layers of the bladder consist of the mucosa lined by the urothelium, muscularis propria and adventitia.

The urothelium is approximately three to seven layers of relatively flat cells, the exact number depends on bladder distension. The superficial layer that faces the lumen is made up of a single row of large, elliptical and multinucleated cells that are called umbrella cells, which have abundant eosinophilic cytoplasm that store small amounts of mucin. These cells help provide the permeability barrier that keeps the urine in place.

The intermediate cells have a cuboidal to low columnar shape, oval nuclei with stippled chromatin, moderate amounts of cytoplasm and well-defined cell membranes. The basal layer is made up of cuboidal cells that rest on the basal lamina. The lamina propria is composed of loose connective tissue containing a vascular and lymphatic network. Smooth muscle cells are present in association with blood vessels in the mid-portion of the lamina propria.

The bladder consists of three muscle layers called detrusor muscles that are solely smooth muscle; the inner and outer layer has a longitudinal orientation and a middle layer with a circular orientation. What makes it possible for voluntarily emptying of urine is that the urethra is made up of both smooth and striated muscle.