Magnetic resonance images of the anal canal show small, circular, low-intensity areas arranged in a row and a high-intensity area surrounding them internally and externally in the longitudinal muscle layer that cannot be explained by current anatomical findings. The purpose of this study was to elucidate the detailed structure of the longitudinal smooth muscle of the anal canal and to interpret the magnetic resonance image of the longitudinal muscle. Specimens for macroscopic anatomy and histology were obtained from six and seven cadavers, respectively. The histological nature of the longitudinal muscle was examined by staining serial transverse and coronal sections of the lateral wall of the anal canal with Masson's trichrome stain and using immunohistochemistry for smooth and skeletal muscle fibers. Dense and sparse areas of smooth muscle fibers coexisted in the longitudinal muscle layer. The dense areas formed columnar muscle bundles approximately 1.0-1.5 mm in diameter, and they continued from the longitudinal muscle bundles of the rectum. The columnar muscle bundles of the longitudinal anal muscle were internally and externally surrounded by sparsely arranged smooth muscle fibers that ran longitudinally. The coexistence of dense and sparse areas of smooth muscle fibers suggests that the structure of the smooth muscle is optimized for its function. This histological nature is probably reflected in the magnetic resonance image of the longitudinal muscle as the coexistence of low-and high-intensity areas. Clin. Anat. 33:619-626, 2020.

An extremely rare case of a lymphoepithelial cyst associated with persistent elevation of serum carbohydrate antigen (CA) 19-9 levels is described. A 72-year-old man was incidentally found to have a cystic tumor in the uncus of the pancreas and to have a high serum CA 19-9 level. At 2-year follow up, the tumor size had gradually increased and the high CA 19-9 level persisted. Because the malignant potential of the tumor could not be ruled out, a laparotomy was performed. The tumor was excised from the pancreas. On pathological examination, the tumor was proven to be a lymphoepithelial cyst. After the operation, the patient's serum CA 19-9 level decreased to the normal range. Lymphoepithelial cysts of the pancreas are a rare, benign entity. They are true pancreatic cysts, characterized by a mature, keratinizing, squamous lining surrounded by lymphoid tissue. Because the treatment options and the prognosis of these entities are quite different from those of other cystic neoplasms of the pancreas, preoperative differential diagnosis is the main issue. Thus, although they are rare, lymphoepithelial cysts should be considered in the differential diagnosis of cystic tumors of the pancreas.

Preoperative diagnosis of asymptomatic paraganglioma is difficult due to the lack of specific symptoms. In this report, we present a rare case of a small and asymptomatic para-aortic paraganglioma. A 34-year-old woman who complained of back pain was admitted for further examination. No abnormal findings were observed on physical or laboratory examinations. An abdominal CT scan and an abdominal MRI incidentally noted a mass about 3 cm in diameter adjacent to the right edge of the inferior vena cava. The following aortic angiography showed the tumor with a feeding artery diverting directly from the aorta. The tumor was completely resected by laparotomy. The resected tumor, 3 × 3 × 3 cm in size, was soft, dark-reddish and encapsulated. Immunohistochemical examinations showed that it was positive for neuron-specific enolase, chromogranin A and adrenocorticotropin. Under these findings, the diagnosis of para-aortic paraganglioma was determined. Seven years after the operation, she remains asymptomatic and free of disease.

These findings show that HPV16 infection is closely involved in the development of anal SCC and suggest that the change in the genome occurs at the stage of microinvasive cancer.

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