Overgangsepitheel

Overgangsepitheel, also known as transitional epithelium or urothelium, is a specialized type of stratified epithelial tissue found lining the urinary tract of mammals. It is characterized by its unique ability to stretch and recoil, accommodating the fluctuating volume of urine within organs like the bladder, ureters, and renal pelvis. The appearance of overgangsepitheel changes depending on the degree of distension. When the organ is empty or contracted, the cells are typically rounded and stratified, forming multiple layers. As the organ fills and stretches, the apical cells flatten and the overall thickness of the epithelium decreases, allowing for significant expansion.

The cellular structure of overgangsepitheel is complex. The basal layer consists of cuboidal or columnar cells, which are mitotically active and responsible for cell renewal. The intermediate layers are composed of polygonal cells. The most distinctive cells are those of the superficial layer, often referred to as umbrella cells or dome cells. These large, binucleated cells possess a unique apical membrane reinforced by plaques, which contributes to their resistance to urine's osmotic pressure and potential toxicity. This protective barrier is crucial for preventing the reabsorption of water and solutes and for protecting the underlying tissues from damage. The smooth muscle in the walls of the urinary tract contracts to expel urine, and the specialized nature of overgangsepitheel is essential for its functioning during these processes.