General Plan of Wall Structure
The wall of the urinary tract throughout its length (except for most of the urethra) has a similar structure. It consists of four layers:
- Mucous membrane (tunica mucosa).
- Submucosa (tela submucosa).
- Muscular layer (tunica muscularis).
- Outer layer (represented by the adventitia or, in certain areas of the urinary bladder, the serosa).
The presence of a well-developed loose submucosa allows the mucosa to form folds, ensuring the organs' ability to significantly distend. In the ureters, these folds are longitudinal, whereas in an empty bladder, they run in various directions. An exception is the trigone of the bladder, where the submucosa is absent, and the mucosa is firmly fused with the muscular layer, remaining smooth.
Histological Features of the Mucosa
The mucosa of the urinary tract is adapted to continuous contact with urine and changes in organ volume.
- Epithelium: The lining is represented by transitional epithelium (urothelium), consisting of basal, intermediate, and superficial layers. Surface (umbrella) cells change their shape depending on the degree of wall stretch: in a collapsed state, they are dome-shaped, whereas upon stretching, they flatten. Meanwhile, their nuclei always remain round, serving as an important diagnostic feature.
- Lamina propria: Formed by loose fibrous connective tissue. Glands are generally absent here (except for small tubuloalveolar glands in the area of the bladder trigone).
- Muscularis mucosae: Virtually absent everywhere. Only in the urinary bladder can isolated bundles of smooth myocytes be found, not forming a continuous layer.
Muscular Layer and Cystoid Principle
The muscular layer is formed by bundles of smooth myocytes separated by connective tissue layers. The number of layers varies: in the upper sections (up to the middle of the ureter), there are two, while in the lower half of the ureter and the urinary bladder, there are three. The fibers run in a spiral fashion, with the turns in adjacent layers being mutually perpendicular.
Urine transport occurs not in a continuous stream, but in portions, thanks to the cystoid principle of organization. The urinary tract is divided into functional segments (cystoids) separated by functional sphincters—cavernous-like structures (CLS) in the submucosa and muscular layers. At rest, CLS are filled with blood and occlude the lumen. When a segment is overfilled with urine, the CLS reflexively collapse, the lumen opens, and smooth muscles contract, pushing urine into the next cystoid. This prevents retrograde urine flow.
Differential Diagnosis on Microscopic Slides
On histological sections, the organs of the urinary system can be confused with other organs having a similar tubular structure or stratified epithelium.
- Ureter vs. Esophagus: Both have a folded lumen and stratified epithelium. However, in the ureter, the epithelium is transitional (round nuclei at the surface), there is no muscularis mucosae and no proper glands in the submucosa. In the esophagus, the epithelium is stratified squamous, there is a prominent muscularis mucosae, and numerous glands are present.
- Urinary Bladder vs. Tongue: Both preparations are characterized by massive muscular layers and stratified epithelium. The main differences are the relief pattern (smooth folds in the bladder versus sharp papillae in the tongue) and the shape of the surface cell nuclei (round in the transitional epithelium of the bladder, flattened in the stratified squamous epithelium of the tongue).
Briefly: answers to topical questions
What are the names and orientation of the muscular layer fibers in the upper and lower halves of the ureter?
The myocytes of the ureteral muscular layer have a spiral arrangement, with the spiral turns in adjacent layers running mutually perpendicularly. In literature, these layers may also be referred to as longitudinal and circular. The number of layers varies depending on the region:
- Upper half (from the calyces to the middle) — represented by 2 layers (inner and outer).
- Lower half — includes 3 layers (inner, middle, and outer).
In which specific areas is the urinary bladder covered by a serous membrane, and where by an adventitial membrane?
The urinary bladder has an outer layer, which, depending on the region, is either serous or adventitial.
- Serosa (tunica serosa) — is the peritoneum; it covers the dome and partially the lateral surfaces of the urinary bladder.
- Adventitia (tunica adventitia) — the main type of outer layer, formed by loose fibrous connective tissue; it pertains to the remaining areas not covered by the serosa.
Class and exam questions
How to distinguish a cross-section of the ureter from the esophagus?
In the ureter, the nuclei of the surface transitional epithelial cells are round, there are no glands in the submucosa, and there is no muscularis mucosae. In the esophagus, the epithelium is stratified squamous, with a muscularis mucosae and glands present.
What is the cystoid principle of ureter function?
This is the portioned transport of urine. The tract is divided into segments (cystoids). Urine accumulates in one segment, then vascular cavernous structures at the outlet collapse, the wall contracts, and urine is propelled into the next segment, preventing its backflow.
How do surface cells of the transitional epithelium change upon organ stretching?
In the unstretched state, surface (umbrella) cells are dome-shaped. Upon organ filling and wall stretching, they flatten, but their nuclei retain a rounded shape.
What else to study on this topic
- Structural features of the intrarenal urinary tract (calyces and pelvis).
- Structure and functions of cavernous-like structures in the ureteral wall.
- Detailed structure of the muscular layer of the urinary bladder.
- Features of urinary tract innervation and the role of intramural ganglia.
- Histology of the male and female urethra.
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