General Plan of the Wall Structure
The wall of the urinary tract throughout its entire length (except for most of the urethra) has a similar structure. It consists of four layers:
- Mucosa (tunica mucosa).
- Submucosa (tela submucosa).
- Muscular layer (tunica muscularis).
- Outer layer (represented by adventitia or, in certain parts of the urinary bladder, serosa).
The presence of a prominent loose submucosa allows the mucosa to form folds, ensuring the organs' ability to undergo significant distension. In the ureters, these folds are longitudinal, while in an empty bladder, they run in various directions. The exception is the trigone of the urinary bladder, where the submucosa is absent, and the mucosa is firmly fused with the muscle 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 transitional epithelium (urothelium), consisting of basal, intermediate, and superficial layers. The superficial (umbrella) cells change their shape depending on the degree of wall distension: in a collapsed state, they are dome-shaped, whereas upon stretching, they flatten out. Notably, their nuclei always remain round, which is an important diagnostic feature.
- Lamina propria: Formed by loose fibrous connective tissue. Glands are typically absent here (except for small alveolar-tubular glands in the region of the urinary bladder trigone).
- Muscularis mucosae: Virtually absent everywhere. Only in the urinary bladder can individual bundles of smooth myocytes be found, without forming a continuous layer.
Muscular Layer and the 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 segments (up to the middle of the ureter), there are two layers, while in the lower half of the ureter and the urinary bladder, there are three. The fibers are arranged in a spiral fashion, with turns in adjacent layers running perpendicular to each other.
Urine transport does not occur in a continuous stream, but rather in portions, thanks to the cystoid principle of motility organization. The urinary tract is divided into functional segments (cystoids) separated by functional sphincters—cavernous-like bodies (CBs) located in the submucosa and muscular layers. At rest, the CBs are filled with blood and occlude the lumen. When a segment is distended with urine, the CBs reflexively collapse, the lumen opens, and smooth muscles contract, expelling urine into the next cystoid. This prevents retrograde urine flow.
Differential Diagnosis in Microscopic Specimens
On histological sections, organs of the urinary system can be confused with other tubular structures or organs with stratified epithelium.
- Ureter vs. Esophagus: Both have a folded lumen and stratified epithelium. However, the ureter features transitional epithelium (round surface nuclei), lacks a muscularis mucosae, and has no glands in the submucosa. The esophagus has stratified squamous epithelium, a prominent muscularis mucosae, and numerous glands.
- Urinary Bladder vs. Tongue: Both specimens feature massive muscle bundles and stratified epithelium. Key differences include 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 bladder's transitional epithelium, flat in the tongue's stratified squamous epithelium).