Pathogenesis of the pancreatic MCNs is unclear. Zamboni et al.  reported that the OLS exhibits a variable degree of luteinization, characterized by the presence of epithelioid cells with round to oval nuclei, and abundant clear or eosinophilic cytoplasm. Expression of ER and PgR in the OLS has also been reported  in most cases of pancreatic MCNs and hepatobiliary MCNs. In our study, the OLS showed strong immunopositivity for a-SMA and vimentin, and weak focal immunopositivity for desmin, findings which suggest a myofibroblastic character. These findings were similar to the descriptions of Thompson et al. . However, with double staining of the OLS in our present study, immunoreactivity for ER and PgR were found in the nuclei of myofibroblasts. In addition to ultrastructural features, the OLS was seen with myofi-broblasts and was composed of mainly spindle-shaped cells which on electron microscopy showed few granular endoplasmic reticula and numerous small pinocytotic vesicles along all membranes. a-SMA in the ovarian stroma has been related to stimulation of the latter by a variety of factors . Cytoskeletal protein expression can also be modulated in some cells by sex hormones. Thus, estrogen and progesterone were found to stimulate smooth muscle cells . Moreover, the stroma of ovarian MCNs was strongly immunopositive for aSMA and vimentin, as pancreatic MCNs, whereas normal ovarian stroma was immunonegative for a-SMA. In addition, double staining of the OLS showed immunopositivity for a-SMA and ER and for a-SMA and PgR . Hence, we emphasize that the OLS represents myofibroblastic proliferation in response to tumor development, but not ovarian stroma, and that OLS might be dependent on hormonal activation, and the response to tumor development mentioned above might be one reason why MCNs develop predominantly in females.
In our series, pancreatic duct communication was confirmed in 7 (31%) of 22 pancreatic MCNs by both examination of resected materials and preoperative pancreatography. Therefore, we propose MCN criteria as follows: a cystic tumor lined by mucin-producing epithelium, usually supported by OLS, with or without pancreatic duct communication.
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