Small cell carcinoma of urinary bladder. Ultrastructural study.

Two cases of small cell carcinoma of the urinary bladder are presented. On gross examination they were found to be polypoid and shared light microscopic features of oat cell carcinoma of the lung. Ultrastructurally, however, they did not reveal features of neuroendocrine cells containing neurosecretory granules. In regard to histogenesis, they appear to arise from undifferentiated cells present in the transitional epithelium.