Tubal Origin of 'Ovarian' Low-grade Serous Carcinoma

Jie Li; Nisreen Abushahin; Shujie Pang; Li Xiang; Setsuko K Chambers; Oluwole Fadare; Beihua Kong; Wenxin Zheng

Disclosures

Mod Pathol. 2011;24(11):1488-1499. 

In This Article

Results

The patients in this study ranged in age from 30 to 72 years (mean 49.5 years). All were without a known family history of breast cancer, ovarian cancers, or BRCA mutations.

Biomarker Expression and Cell Composition in Tube Fimbria

Sections of the fimbriated ends of a total of 50 fallopian tubes were studied immunohistochemically using PAX8, tubulin, calretinin, and MIB1 antibodies. All tubal secretory cells were positive for PAX8, but negative for tubulin, whereas the tubal ciliated cells showed the opposite (Figure 1). All tubal cells were negative for calretinin. The number of tubulin-positive ciliated cells ranged from 34 to 77%, with an average of 65%. MIB1 immunohistochemical stain was done in 42 tubal fimbriated sections, and an average of 3% of the epithelial cells was positive in nuclear pattern. The comparison of cellular proliferative activity between tubal epithelium and ovarian epithelial inclusions is described below.

Figure 1.

Immunophenotype of tubal fimbria and ovarian epithelial inclusions. Top panel shows morphologic appearance of tubal fimbria epithelia, mesothelium-derived ovarian epithelial inclusions, and fallopian tube-derived ovarian epithelial inclusions, followed by immunohistochemical stainings of PAX8, calretinin, and tubulin, respectively. PAX8 (nuclear) and tubulin (apical border) staining was seen in tube and fallopian tube-derived ovarian epithelial inclusions, but not in mesothelium-derived ovarian epithelial inclusions. Conversely, calretinin was positive in mesothelium-derived ovarian epithelial inclusions and negative in tube and fallopian tube-derived ovarian epithelial inclusions (original magnification: left panels × 200).

Morphologic and Immunophenotype of Ovarian Surface Epithelia

As previously noted, 48 ovaries showing ovarian surface epithelium covering at least 10% of the ovarian surface were studied. Microscopically, the ovarian surface epithelium predominantly comprised uniform attenuated (flattened), low cuboidal cells. However, 2 (4%) of 48 ovarian sections contained a columnar type of ovarian surface epithelium. Admixed within these columnar-appearing epithelial cells was a minority population of ciliated cells. Immunohistochemical studies for calretinin, PAX8, and tubulin were performed on all ovarian sections, and the results largely mirrored the aforementioned morphologic findings. The cases could be classified into two distinct groups by immunophenotype. The great majority (46 (96%) of 48 cases) were lined only by typical calretinin-positive PAX8-negative (calretinin+/PAX8−) ovarian surface epithelium, whereas the remaining 2 cases displayed ovarian surface epithelium with calretinin+/PAX8− as well as areas of calretinin−/PAX8+ immunophenotype. For descriptive purposes, ovarian surface epithelium with mesothelial phenotype (calretinin+/PAX8−) was designated as mesothelium-derived ovarian surface epithelium, whereas with fallopian tubal phenotype (calretinin−/PAX8+) as fallopian tube-derived ovarian surface epithelium (Figure 2). Mesothelium-derived ovarian surface epithelium was present in all ovarian sections examined. Ciliated cells were not found in mesothelium-derived ovarian surface epithelium, whereas they were identified in the fallopian tube-derived ovarian surface epithelium. The cell proliferative index (MIB1) was 2.5% in fallopian tube-derived ovarian surface epithelial cells, which was significantly higher than the proliferative index in mesothelium-derived ovarian surface epithelial cells (0% (12 per 1000); Table 1). Compared with the immunophenotype of tubal fimbria, fallopian tube-derived ovarian surface epithelium was largely similar, whereas mesothelium-derived ovarian surface epithelium was entirely different.

Figure 2.

Ovarian surface epithelia with two different immunophenotypes. One comprised flattened, calretinin-positive, PAX8-negative cells with no significant proliferative activity (negative MIB1), consistent with a mesothelium derivation (right side of the panel, mesothelium-derived ovarian surface epithelium), whereas the other comprised columnar, calretinin-negative, PAX8-positive secretory cells with a high number of MIB1-positive cells, consistent with tubal phenotype (left side of the panel, fallopian tube-derived ovarian surface epithelium). These two types of ovarian surface epithelia showed no gradual transitions or junctional zone in between (original magnification: left panels × 200).

Morphology and Immunophenotype of Ovarian Epithelial Inclusions

Ovarian epithelial inclusions have traditionally been thought to be formed via an invagination of ovarian surface epithelium into ovarian cortex, with malignant transformation of the ensuing cystic lining, possibly after Mullerian metaplasia, forming the histogenetic basis for ovarian serous cancers.[29] We sought to determine whether the morphologic and immunophenotypic attributes of ovarian epithelial inclusions are consistent with an ovarian surface epithelium derivation or a tubal derivation.

A total of 856 ovarian epithelial inclusions were identified in sections of 45 ovaries. The number of ovarian epithelial inclusions ranged from 1 to 103 with an average of 19 per ovarian section. Cytologically, there were mainly two types of ovarian epithelial inclusions identified. The majority of ovarian epithelial inclusions contained secretory and ciliated cells, as is characteristic of tubal epithelium, whereas the others were lined by flattened indifferent type of cells without cilia, which are identical to mesothelium-derived ovarian surface epithelium. The majority of ovarian epithelial inclusions were <2 mm. When ovarian epithelial inclusions were >2 mm in diameter, they were lined predominantly by flattened cells. Occasionally, ovarian epithelial inclusions with ciliated and secretory cells formed papillae. No cytologic atypia was identified in all ovarian epithelial inclusions epithelial cells. Stromal cells adjacent to ovarian epithelial inclusions were mainly ovarian stromal cells.

Similar to ovarian surface epithelium, there were two groups of immunophenotypically distinct ovarian epithelial inclusions: one with tubal phenotype (calretinin−/PAX8+) was defined as fallopian tube-derived ovarian epithelial inclusions and the other with mesothelial phenotype (calretinin+/PAX8−) as mesothelium-derived ovarian epithelial inclusions. Of 856 ovarian epithelial inclusions, 667 (78%) were fallopian tube-derived ovarian epithelial inclusions, whereas only 188 (22%) were mesothelium-derived ovarian epithelial inclusions. Therefore, fallopian tube-derived ovarian epithelial inclusions was 3.54 times more common in the ovarian cortex than mesothelium-derived ovarian epithelial inclusions (P<0.001). Ciliated cells were seen in all fallopian tube-derived ovarian epithelial inclusions, and none were clearly discernable in the mesothelium-derived ovarian epithelial inclusions. A total of 400 randomly selected ovarian epithelial inclusions from 20 ovaries were selected for additional microscopic analysis: the number of the ciliated cells ranged from 1 to 35 (average of 12 per fallopian tube-derived ovarian epithelial inclusions). However, analysis of the corresponding tubulin-stained sections showed a higher number of ciliated cells (average of 18 per fallopian tube-derived ovarian epithelial inclusions), indicative of a greater sensitivity of immunohistochemical stains in identifying ciliated cells when compared with morphologic observation. Overall, the morphologic and immunophenotypic attributes of the fallopian tube-derived ovarian epithelial inclusions were very similar to those of the tubal fimbriated end epithelium, and were notably different from those of the mesothelium-derived ovarian surface epithelium. The converse was true of mesothelium-derived ovarian epithelial inclusions, which was more comparable with mesothelium-derived ovarian surface epithelium than tubal epithelium (Table 2 and Figure 1).

To further confirm that two types of ovarian epithelial inclusions were present in ovarian cortex, we performed dual immunohistochemical staining with PAX8 and calretinin in the aforementioned 20 ovarian sections (Figure 3).

Figure 3.

Ovarian epithelial inclusions with double PAX8 and calretinin staining. Ovarian epithelial inclusions were occasionally morphologically indifferent (left panel). However, double staining (right panel) with both calretinin (red) and PAX8 (brown) showed that these ovarian epithelial inclusions had discrepant immunostaining patterns. Fallopian tube-derived ovarian epithelial inclusions (PAX8+/calretinin−) are on the top right panel and mesothelium-derived ovarian epithelial inclusions (PAX8−/calretinin+) in the middle right, whereas the panel in the low right shows one large fallopian tube-derived ovarian epithelial inclusion (middle) and one small mesothelium-derived ovarian epithelial inclusion (upper right). The ovarian surface epithelia on the top right panel show PAX8−/calretinin+, indicative of mesothelial origin. A few calretinin+/PAX8− cells seen in the low right (low right panel) represent luteinized ovarian stromal cells (original magnification: × 100).

Immunophenotypes of Ovarian Serous Tumors

All serous tumors, including cystadenomas (n=48), borderline tumors (n=42), and low-grade serous carcinomas (n=28), showed a positive PAX8 and negative calretinin immunophenotype (Figure 4). Tubulin-positive ciliated cells were present in all benign and borderline tumors but were essentially absent in low-grade serous carcinoma cases (Figure 4). There was a trend toward progressive loss of tubulin expression from serous cystadenomas to serous borderline tumors. As expected, the reverse trend was seen regarding MIB1 proliferative indices, which was significantly increased from cystadenomas (5±2%) to borderline tumors (15±5%), and to low-grade serous carcinomas (32±12%).

Figure 4.

Secretory cell expansion in the process of low-grade serous carcinoma development. The left panels (a, c, e and g) show PAX8+ secretory cells in fallopian tube-derived ovarian epithelial inclusions, serous cystadenoma, serous borderline tumor, and low-grade serous carcinoma. The right panels (b, d, f and h) show tubulin stain that highlights cilia on cell apical border. Tubulin-positive cilia were present in ~30% of the fallopian tube-derived ovarian epithelial inclusions cells, 10% of benign cystadenoma cells (arrow), <5% of borderline tumor cells (arrow), and none in low-grade serous carcinoma cells.

Cellular Proliferative Activity in Ovarian Surface Epithelium, Ovarian Epithelial Inclusions, and Tubal Epithelia

To quantitate the cell proliferation index in cells of ovarian epithelial inclusions, ovarian surface epithelium, and tubal fimbriated epithelia, we examined a total of 20 ovarian cases containing 210 fallopian tube-derived ovarian epithelial inclusions, 48 mesothelium-derived ovarian surface epithelium samples, and 42 tubal fimbria epithelia by using MIB1 staining. Cell proliferation was significantly higher in fallopian tube-derived ovarian epithelial inclusions compared with mesothelium-derived ovarian surface epithelium samples but was significantly less than that in tubal fimbria epithelia. The cell proliferative index was 87.5-fold higher in fallopian tube-derived ovarian epithelial inclusions cells compared with mesothelium-derived ovarian surface epithelial cells (P<0.001). However, compared with tubal fimbria, the proliferative index in fallopian tube-derived ovarian epithelial inclusions showed an approximately threefold reduction (P<0.05; Table 3). As there were only a few cases of fallopian tube-derived ovarian surface epithelium, and proliferative indices of mesothelium-derived ovarian epithelial inclusions were similar to those of mesothelium-derived ovarian surface epithelium, these data were not included.

Secretory/Ciliated Cell Ratio in F-ovarian Epithelial Inclusions, Tubal Epithelia, and Ovarian Serous Tumors

Two methods were used to evaluate the secretory-to-ciliated cell ratios of fallopian tube-derived ovarian epithelial inclusions, tubal epithelia, and ovarian serous tumors: light microscopy by identifying ciliated cells directly, and tubulin immunohistochemical stain to identify ciliated cells. This evaluation was not applied to mesothelium-derived ovarian surface epithelium and ovarian epithelial inclusions, as these did not have either secretory or ciliated cells. Morphologically, the secretory-to-ciliated cell ratio was lowest in fallopian tube, and was significantly increased in fallopian tube-derived ovarian epithelial inclusions and serous cystadenomas, which had similar secretory-to-ciliated cell ratios. The secretory-to-ciliated cell ratio in serous borderline tumors (8.7±2.5) was only slightly higher, but all lesions showed significantly lower secretory-to-ciliated cell ratios than low-grade serous carcinomas (98±1.2). By tubulin immunohistochemical stain, a significant population of ciliated cells was present in fallopian tube-derived ovarian epithelial inclusions, cystadenomas, and borderline tumors with secretory-to-ciliated cell ratios of 5.5, 4.6, and 10.9, respectively (Table 4 and Figure 4). However, ciliated cells were basically all lost in low-grade serous carcinoma cases.

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