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Organ system histology
Let’s first take a look at a cross-section of the ureter at low magnification, which shows the four main concentric layers of the ureter: the transitional epithelium that lines the mucosa, the lamina propria, the muscularis externa, and the adventitia.
The transitional epithelium is also further divided into 3 layers.
At high magnification, we can see that the most superficial layer consists of large ovoid cells that are called dome or umbrella cells because of their curved apical surface and because they often cover other epithelial cells beneath them.
When the ureter is distended, there are fewer folds in the epithelium, but the epithelium is also able to expand further by flattening and decreasing the amount of overlap of the umbrella cells.
The second layer that comprises the transitional epithelium is an intermediate layer of cuboidal and low columnar cells.
When umbrella cells are damaged, cells from the intermediate layer are able to quickly differentiate in order to replace the damaged umbrella cells.
The third layer of the transitional epithelium is the basal layer, which is a single layer of cuboidal basal cells that rest on the basement membrane.
In this cross-section of the ureter, the lumen is on the far left.
The histology of the ureters, bladder, and urethra share some common features, such as the concentric layers of transitional epithelium, lamina propria, muscularis externa, and adventitia. The transitional epithelium of these organs is composed of three layers, including a superficial layer with umbrella cells, an intermediate layer of cuboidal and low columnar cells, and a single layer of basal cells.
The muscularis externa of the ureters has an inner longitudinal layer and a circular outer layer of smooth muscle, while the distal ureters, bladder, and proximal urethra have an additional outer layer of longitudinal smooth muscle. Both males and females have a distal urethra lined with stratified squamous epithelium, but only males have pseudostratified and stratified columnar epithelium, which is found in the membranous and spongy portions of the urethra.
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