Histologically, the cyst walls and septa in cystic tumors are composed of three layers: mucinous epithelial cells, the OLS, and a thin or thick fibrous capsule. The lining epithelia of MCNs are composed mostly of tall columnar, mucin-secreting cells, and they exhibit a range of atypia from benign-looking to atypical cells. The epithelial cells show a variable degree of papillary growth. Adding a supplementary explanation, Albores-Saavedra et al.  has reported that many of the tall columnar cells are similar to those of the major pancreatic ducts, while others resemble those of the colon or of the superficial gastric epithelium. Goblet cells, which may form invaginations, are seen in nearly all tumors, and Paneth cells occur in 10% of tumors.
The OLS is composed mostly of oval to spindle-shaped cells with round to elongated nuclei without atypia and with a small amount of cytoplasm, resembling ovarian stroma (fig. 2). Immunohistologically, the OLS shows strong immunopositivity for vimentin and a-SMA and focal immunopositivity for a
Fig. 3. Immunohistochemical study of OLS of pancreatic mucinous cystic neoplasms. a The OLS shows immunopositivity for estrogen receptor. b The OLS also shows immunopositivity for progesterone receptor.
b a desmin. Both ER and PgR are expressed in the nuclei of OLS cells (fig. 3). Sometimes, MCNs contain pseudosarcomatous mural nodules . Garcia et al.  reported a case of pancreatic mucinous cystic neoplasm with pseudosarcomatous mural nodules in which both the epithelial component and the pseudosarcomatous mural nodules were believed to be malignant. They considered the mucinous cystic component to represent a cystadenocarcinoma and the pseu-dosarcomatous mural nodules an anaplastic carcinoma. The epithelial origin of the mural nodules was confirmed on the basis of cytokeratin and EMA reactivity. The stroma component was mesenchymal in origin and the sarcomatous stroma portended an adverse clinical course. The presence of a sarcomatous element is related to the aggressiveness of MCNs .
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