Epithelial (true) splenic cysts: pathogenesis of the mesothelial and so-called epidermoid cyst of the spleen

Nonparasitic cysts of the spleen are classified as primary or epithelial cysts if their inner surface has a cellular lining. Depending upon the pattern of the inner surface cell layer, the primary splenic cysts are divided into mesothelial or epidermoid subtypes. In order to study their pathogenesis, five epithelial splenic cysts (two with mesothelial lining, and three with foci of stratified squamous epithelium) were investigated by scanning and transmission electron microscopy and by immunohistochemistry. In places, the squamous epithelium was continuous with a flattened mesothelial layer. Furthermore, within the stratified epithelium, a continuous spectrum of cytodifferentiation from mesothelial to squamous cells was found. Thus, the epidermoid splenic cyst seems to be a variant of the mesothelial cyst with focal squamous metaplasia. Rupture of the splenic capsule with hemosiderin deposits and infoliation of the mesothelium was seen in one case, implying a traumatic genesis. In addition, stoma-like channels providing connections between the lumen of the cyst and the sinus of the adjacent splenic tissue were detected by scanning electron microscopy. As with other cystic lesions, the influx of blood cells and proteinaceous fluid from these stomata may contribute to the growth of the cysts, which are usually of considerable size when surgically removed.