[A case of small cell carcinoma of the urinary bladder].

A case of small cell carcinoma with a high serum alpha-fetoprotein level of the urinary bladder is reported. Histologically, in addition to the small cell carcinoma, there were small areas of atypical carcinoids and larger epithelial cells. Immunohistochemically, the small cell carcinoma and the atypical carcinoids showed positivity for neuron-specific enolase, chromogranin, and vimentin. On the other hand, the larger epithelial cells showed positivity for alpha-fetoprotein, keratin, and carcinoembryonic antigen. Electron microscopically, a few neurosecretory granules were noted in the cytoplasm of the small cell carcinoma. After a total cystectomy, the serum alpha-fetoprotein level decreased to within a normal range. Postmortem examination of recurrent tumorous masses revealed only features of the small cell carcinoma. This case supports a theory that a small cell carcinoma of the urinary bladder develops from multipotential epithelial cells.