M01.10.09 Bladder

The urinary bladder is a hollow, muscular organ that stores urine until micturition. Its histology is adapted to withstand the stress of stretching and recoiling as the bladder fills and empties. Here’s a breakdown of its layers and their functions.

1. Layers of the Urinary Bladder

The bladder wall is composed of four primary layers:

• Mucosa
• Submucosa
• Muscularis
• Adventitia/Serosa

Mucosa

• Epithelium: The mucosa is lined with transitional epithelium (urothelium), a unique stratified epithelium that can stretch as the bladder fills. Transitional cells are rounded or umbrella-like when the bladder is empty but flatten as the bladder distends.
• Lamina Propria: Beneath the urothelium lies the lamina propria, a connective tissue layer that provides structural support. It contains blood vessels, nerves, and immune cells.

Submucosa

• A thin layer of connective tissue that separates the mucosa from the muscularis. It also contains blood vessels and nerves to support the bladder wall.

Muscularis (Detrusor Muscle)

• Composed of three layers of smooth muscle fibers arranged in an inner longitudinal, middle circular, and outer longitudinal pattern.
• This muscle layer is critical for the contraction and emptying of the bladder.
• The internal urethral sphincter (a thickened middle circular layer) controls urine outflow and is under involuntary control.

Adventitia and Serosa

• Adventitia: Connective tissue covering most of the bladder, anchoring it to surrounding structures.
• Serosa: A peritoneal covering on the superior surface of the bladder in the pelvic cavity.

Normal bladder with transitional epithelium

2. Key Histological Features

• Transitional Epithelium: Can change shape to accommodate bladder filling; umbrella cells form a protective barrier against urine.
• Detrusor Muscle: Contracts under autonomic control to facilitate urine expulsion.
• Rich Vascular and Nerve Supply: The lamina propria and submucosa contain a high density of blood vessels and nerves, essential for bladder control and sensation.

Points to Remember

• Transitional Epithelium: Allows the bladder to expand and provides a barrier.
• Lamina Propria: Supports the urothelium and has immune functions.
• Detrusor Muscle: Essential for urine expulsion; forms the involuntary internal urethral sphincter.
• Adventitia and Serosa: Anchor and protect the bladder.

Clinical Correlation

The bladder’s histological structure makes it resilient, but certain conditions can affect its function:

• Cystitis: Inflammation of the bladder, commonly caused by infection, disrupts the urothelial barrier.
• Overactive Bladder Syndrome: Excessive contractions of the detrusor muscle, often associated with frequency and urgency.

Bibliography

1. Ross, M. H., & Pawlina, W. (2015). Histology: A Text and Atlas (7th ed.). Wolters Kluwer Health.
2. Gartner, L. P., & Hiatt, J. L. (2013). Color Atlas and Text of Histology (6th ed.). Lippincott Williams & Wilkins.