Ureter, bladder and urethra histology

The ureters are fibromuscular tubes that transport urine from the renal pelvis to the urinary bladder, where it’s stored until it’s emptied through the urethra during urination.The ureters, bladder, and the initial portion of the urethra are all lined with transitional epithelium, which is also called urothelium.This specialized epithelium is only found in the urinary system and it allows the conducting passageways and bladder to expand a lot while staying impermeable to water and ions.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.Although the basement membrane is often hard to identify when using light microscopy.In this cross-section of the ureter, the lumen is on the far left.So the first layer starting from the left is the transitional epithelium, which is purple when stained with hematoxylin and eosin.The next main layer beneath the transitional epithelium is a supporting layer called the lamina propria, which is a thick layer of dense collagenous and elastic connective tissue.Surrounding the lamina propria, is the muscularis externa, which is made of 2-3 layers of smooth muscle.The proximal portion of the ureter has an inner longitudinal layer and an outer circular layer.The direction of these muscle layers are actually the opposite of how they’re arranged in the GI tract.The distal portion of the ureter has an additional outer longitudinal layer of smooth muscle.Keep in mind that this is a cross section of the ureter, so the individual muscle fibers that run longitudinally will look round in this image, and the middle circular layer of smooth muscle will appear to have long fibers, since they are arranged parallel or along the same plane as this section of tissue.Finally, the outermost layer of the ureter is the adventitia, which is composed of loose connective tissue, blood vessels, adipocytes, and nerves.Alright, let’s move onto the bladder, which has a similar overall structure as the distal ureter.So it also has the same four main concentric layers, which includes the transitional epithelium with umbrella cells, the lamina propria, the muscularis externa, and the outer adventitia.The muscularis externa is also called the detrusor muscle.And similar to the distal ureter, it also has an inner longitudinal layer, middle circular layer, and outer longitudinal layer of smooth muscle.Also similar to the ureters, the transitional epithelium is able to expand and stretch to accommodate a large volume of urine.These two images show the transitional epithelium of the bladder wall in a relaxed state on the left and an extended or stretched state on the right.When comparing the images, we can see that the stretched epithelium is thinner and it also has umbrella cells that are elongated and look flat when compared to the umbrella cells found in the relaxed transitional epithelium.When urinating, the urethra is the fibromuscular tube that carries the urine from the bladder to the exterior of the body.In females, there will be transitional epithelium proximally, similar to the bladder, and stratified squamous epithelium distally.In this low power image from the distal portion of a female’s urethra, the stratified squamous epithelium seen at the top of the image.Just below the epithelium is a thick layer of lamina propria that is very vascular, with many veins throughout the layer.And at the bottom of the image, we can also see a portion of the smooth muscle that surrounds the lamina propria.The distal portion of a male’s urethra would also look very similar to this image as well.The male urethra has three main parts.The first portion is the prostatic urethra, then the membranous urethra, which is a short section of the urethra before leading to the final section called the spongy or penile urethra.Each segment of the urethra is lined by a different type of epithelium.The prostatic urethra is lined by urothelium or transitional epithelium similar to the bladder.The membranous and spongy urethra are both lined by stratified columnar and pseudostratified columnar epithelium, although the spongy urethra will also have stratified squamous epithelium distally, which is similar to the distal portion of the urethra in females as well.##SummaryAlright, as a quick recap.The ureters, bladder, and proximal urethra have the same main concentric layers: the transitional epithelium that lines the mucosa, lamina propria, muscularis externa, and an outer adventitia.The transitional epithelium is further divided into 3 layers: a superficial layer with umbrella cells, an intermediate layer of cuboidal and low columnar cells, and a single layer of basal cells that rest on the basement membrane.The muscularis externa of the ureters have an inner longitudinal layer and a circular outer layer of smooth muscle, but the distal portion of the ureters, bladder, and proximal portion of the urethra have an additional outer layer of longitudinal smooth muscle.Both males and females have distal urethras 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.