Normal Urothelial Histology and Cytology

The majority of the collecting system is lined by urothelium (transitional epithelium). Variable areas of the bladder and urethra may be lined by glandular epithelium (simple columnar), especially in the trigone and the dome of the bladder (the vestigial urachus); pa-raurethral glands, which provide lubrication for the urethra, might also be a source of glandular epithelium from that area. Cystitis cystica or glandularis, arising in Brunn's nests in the bladder mucosa, may shed groups of atypical glandular cells not to be confused with those cells of an adenocarcinoma of the bladder or prostate. In addition, the prostate and accessory sex glands are lined by columnar epithelium. Therefore, if glandular cells are seen within a urine sample, these sources should be considered.

The urothelium is a unique mucosa, specialized for the urinary tract for its ability to expand and contract, and as a barrier against the toxic urine. This stratified epithelium is morphologically intermediate between cuboidal and squamous, hence its old name, "transitional". When contracted, the bladder is lined by a layer 4-5 cells thick with the basal cells assuming a cuboidal shape; the intermediate cells, polygonal; and the surface cells round and large, and often binucleate. When the bladder is distended, the mucosa may be only 2-3 layers thick and the intermediate and surface cells may appear flattened.

The surface cells, the largest ones found in cytologic samples, have abundant cytoplasm, the luminal surface of which may appear thickened (Fig. 1.1). The nuclei of these superficial cells, often called umbrella cells, because of their position over more than one intermediate or basal cell (Fig. 1.2), may have prominent nucleoli, and may be multinucleated (Fig. 1.3).

The physiologic role of the urothelium is fascinating, and as unique as its cytologic appearance. The purpose of the urinary epithelium is to provide a barrier between the blood and the usually hypertonic toxic urine, which contains the majority of wastes from the body. The plasma membranes of the surface of umbrella cells are thicker than most other cell membranes. This rigid trilaminar membrane, the so-called "asymmetric unit membrane" is composed of a unique family of proteins, uroplakins. Interdigitating cell junctions permit great distension of the epithelium without damage to the integrity of the mucosal surface. The epithelium is connected to a basement membrane that appears invisible by light microscopy. The basal layer may be deeply indented by strands of underlying connective tissue which contain capillaries.

The histology of the other parts of the urinary tract, the ureters, pelves and calyces, and urethra, is essentially identical to the bladder, except that the size of the cells is smaller. Cross section of a contracted ureter reveals large mucosal folds that flatten if the ureter distends.

Columnar cells are infrequently present, but their identity is readily recognized as the cellular features are the same as any other benign columnar cell (Figs. 1.4, 1.5). Their origin may be in glandular remnants in the dome or trigone of the bladder. Urothelial cells on the surface of an hyperplasia may also appear to be columnar (Figs. 1.6, 1.7). Any atypia needs to be assessed in the context of accompanying inflammation, as from cystitis cystica/glandularis or suspicion of glandular neoplasia, based on history and cytologic features.

Squamous epithelium (Figs. 1.8, 1.9) can occur as a result of metaplasia or as a congenital area, especially within the trigone of women. The distal portion of the penile urethra is lined by squamous epithelium. In females, vaginal contamination during a voided urine collection (Fig. 1.10) can be a source of benign and neoplastic squamous and glandular epithelium (see Chapter 5).

Urine Cytology Umbrella Cell

Figure 1.1. Normal Umbrella Cells—bladder washing: The thickened unilateral aspect of the cytoplasmic boundary is a manifestation of the asymmetric unit membrane whose purpose is to prevent toxic urine from entering the blood stream. In addition to the thickened asymmetric membrane, the frothy perinuclear cytoplasm is also characteristic of benign urothelial cells. Chromatin is fine and uniform in texture and distribution. (600x)

Figure 1.1. Normal Umbrella Cells—bladder washing: The thickened unilateral aspect of the cytoplasmic boundary is a manifestation of the asymmetric unit membrane whose purpose is to prevent toxic urine from entering the blood stream. In addition to the thickened asymmetric membrane, the frothy perinuclear cytoplasm is also characteristic of benign urothelial cells. Chromatin is fine and uniform in texture and distribution. (600x)

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Responses

  • caden
    Are urothelial cells in urine cytology normal?
    8 years ago
  • Charles Richardson
    Are urothelial cells normal in a urine sample?
    7 years ago
  • Jarno Katajisto
    Are urothelial cells epithelial cells?
    7 years ago
  • tabitha
    What is the umbrella cells in histology of urothelium?
    4 years ago

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