PLATE 78 • URETER The ureters are paired tubular structures that convey urine from the kid-neys to the urinary bladder. They are lined with transitional epithelium (urothelium), an impervious layer that lines the urinary excretory passa-ges from the renal calyces through the urethra. The ability of this epithe-lium to become thinner and flatter allows all of these passages to accommodate to distension by the urine. The epithelium rests on a dense collagenous lamina propria, which in turn, rests on an inner longitudinal and an outer circular layer of smooth muscle. Regular peristaltic contractions of this muscle contribute to the flow of urine from the kidney to the urinary bladder. ORIENTATION MICROGRAPH: As shown in this low-power orienta-tion micrograph, the wall of the ureter consists of a mucosa (Muc), a muscularis (Mus), and an adventitia (Adv). Note that the ureters are lo-cated behind the peritoneum of the abdominal cavity in their course to the bladder. Thus, a serosa (Ser) may be found covering a portion of the circumference of the tube. Also, because of contraction of the smooth muscle of the muscularis, the luminal surface is characteristically folded, thus creating a star-shaped lumen. ATATAT BVBVBV AdvAdvAdv MucMucMuc MusMusMus Ser
Ureter, monkey, H&E ×160.
The wall of the ureter from the rectangular area in the orientation micrograph is examined at higher magnification in this figure. One can immediately recognize the thick epithelial lining, which appears distinct and sharply delineated from the remainder of the wall. This is the transitional epithelium (urothelium), (Ep). The remainder of the wall is made up of connective tissue (CT ) and smooth muscle. The latter can be recognized as the darker-staining layer. The section also shows some adipose tissue (AT ), a component of the adventitia.
The transitional epithelium and its supporting connective tissue constitute the mucosa (Muc). A distinct submucosa is not present, although the term is sometimes applied to the connective tissue that is closest to the muscle.
The muscularis (Mus) is arranged as an inner longitudinal layer (SM(l)), a middle circular layer (SMc), and an outer longitudinal layer (SM(l)). However, the outer longitudinal layer is present only at the lower end of the ureter. In a cross section through the ureter, the inner and outer smooth muscle layers are cut in cross section, whereas the middle circular layer of the muscle cells is cut longitudinally. This is as they appear in this figure.
Transitional epithelium, ureter, monkey, H&E ×400.
This figure shows the inner longitudinal smooth muscle layer (SM(l)) at higher magnification. Note that the nuclei appear as round profiles, indicating that the muscle cells have been cross-sectioned. This figure also shows the transitional epithelium (Ep) to advantage. The surface cells of the transitional epithelium (urothelium) are characteristically the largest, and some are binucleate (arrow). The basal cells are the smallest, and typically, the nuclei appear crowded because of the minimal cytoplasm of each cell. The intermediate cells appear to consist of several layers and are composed of cells larger in size than the basal cells but smaller than the surface cells.
KEY Adv, adventitia AT, adipose tissue BV, blood vessels CT, connective tissue Ep, transitional epithelium Muc, mucosa Mus, muscularis Ser, serosa SM(c), circular layer of smooth muscle SM(l), longitudinal layer of smooth muscle arrow, binucleate surface cell