HISTOLOGICAL EVIDENCE FOR CELL-PROLIFERATION ACTIVITY IN CYSTIC TUMOR(ENDODERMAL HETEROTOPIA) OF THE ATRIOVENTRICULAR NODE

Cystic tumor (endodermal heterotopia) of the atrioventricular (AV) nod e in a 95-year-old female is described. Electrocardiograms showed comp lete AV blockage with a narrow QRS morphology resulting from the long- standing first degree of AV blockage since age 61. A cardiac pacemaker was implanted at age 83. The patient died of pneumonia at age 95. Aut opsy demonstrated a normal heart weight (320 g), normal coronary arter y and normal myocardium. Histological examination of the conduction sy stem revealed a marked fibrotic sinoatrial (SA) node and a cystic lesi on in the lower right atrium, including the approaches to the AV node. The bundle of His and its branches were completely intact. Cellular n ests and cystically dilated tubules were scattered within a loose tiss ue stroma, which measured 18 x 12 x 5 mm. There were several mitotic f igures among the tubules, as well as nuclear fragments similar to apop totic bodies and exfoliation of the tumor cells. Immunohistochemical s tudies demonstrated a positive reaction for epithelial membrane antige n, carcinoembryonic antigen, CA19-9, cytokeratin and secretory compone nt in the tubular cells, and a negative reaction for thrombomodulin. E ndocrine cells among the tubules were positive only for calcitonin and serotonin. Ultrastructurally, sparse microvilli and desmosome-like st ructures between the tumor cells were observed. These findings support that this lesion originates from the endoderm. Moreover, it is likely that progression in the degree of AV block may be explained by the ce ll proliferation activity of the tumor.