Lineage tracing suggests that ovarian endosalpingiosis does not result from escape of oviductal epithelium

Abstract

Most high-grade serous carcinomas are thought to arise from Fallopian tube epithelium (FTE), but some likely arise outside of the tube, perhaps from ectopic tubal-type epithelium known as endosalpingiosis. Importantly, the origin of endosalpingiosis is poorly understood. The proximity of the tubal fimbriae to the ovaries has led to the proposal that disruptions in the ovarian surface that occur during ovulation may allow detached FTE to implant in the ovary and form tubal-type glands and cysts. An alternative model suggests that cells present in ectopic locations outside the Müllerian tract retain the capacity for multi-lineage differentiation and can form glands with tubal-type epithelium. We used double transgenic Ovgp1-iCreER^T2 ;R26R^LSL-eYFP mice, which express an eYFP reporter protein in OVGP1-positive tissues following transient tamoxifen (TAM) treatment, to track the fate of oviductal epithelial cells. Cohorts of adult mice were given TAM to activate eYFP expression in oviductal epithelium, and ovaries were examined at time points ranging from 2 days to 12 months post-TAM. To test whether superovulation might increase acquisition of endosalpingiosis, additional cohorts of TAM-treated mice underwent up to five cycles of superovulation and ovaries were examined at 1, 6, and 12 months post-TAM. Ovaries were sectioned in their entirety to identify endosalpingiosis. Immunohistochemical staining for PAX8, tubulin, OVGP1, and eYFP was employed to study endosalpingiosis lesions. Ovarian endosalpingiosis was identified in 14.2% of TAM-treated adult mice. The endosalpingiotic inclusion glands and cysts were lined by secretory and ciliated cells and expressed PAX8, tubulin, OVGP1, and eYFP. Neither age nor superovulation was associated with a significant increase in endosalpingiosis. Endosalpingiosis was also occasionally present in the ovaries of pre-pubertal mice. The findings imply that ovarian endosalpingiosis in the mouse does not likely arise as a consequence of detachment and implantation of tubal epithelium and other mechanisms may be relevant. © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Keywords: Fallopian tube; endosalpingiosis; high-grade serous carcinoma; lineage tracing; mouse model.

Figure 1.. Tamoxifen induces eYFP expression in the oviductal epithelium of Ovgp1-iCreER^T2;R26R^LSL-eYFP mice.

Oviductal expression of eYFP (seen as green fluorescence) is induced by transient treatment with TAM (A) but not vehicle (B) in Ovgp1-iCreER^T2;R26R^LSL-eYFP double transgenic mice. Mouse oviducts are indicated by white arrows, ovaries by white arrow heads and uterine horns by white stars. Representative photomicrographs showing eYFP expression in the oviductal epithelium of TAM-treated mouse based on immunohistochemical staining (C), but not in the vehicle treated mouse (D).

Figure 2.. Representative examples of ovarian endosalpingiosis in Ovgp1-iCreER^T2;R26R^LSL-eYFP mice.

Ovarian endosalpingiosis lesions (indicated by black stars), are usually located in the medulla near ovarian hilum (A), but are occasionally found in the ovarian cortex (B). Some ovaries harbor more than one lesion (C). The epithelium lining endosalpingiotic glands and cysts is OVGP1 positive (inset) and includes both secretory and ciliated cells (D), with ciliated cells expressing tubulin (E) and secretory cells preferentially expressing PAX8 (F, inset shows high-magnification). Note the internal positive control for PAX8 (normal oviduct) in lower left corner of panel F at black arrow. Black arrowheads indicate ciliated cells in panels D and E. Scale bars: 200 μm (panels A, B, F); 100 μm (panel C); 50 μm (panels D, E).

Figure 3.. Endosalpingiosis lesions in Ovgp1-iCreER^T2;R26R^LSL-eYFP mice are different from rete ovarii.

Although extra-ovarian rete also forms glands lined by columnar cells (A) that express PAX8 (C), the rete does not express OVGP1 (E). High magnifications of boxed areas in A, C, and E are shown in B, D, F, respectively. Note the internal positive control for detection of PAX8 and OVGP1 expression (normal oviduct at black arrows) in lower left corners of panels C and E. Scale bars: 200 μm (panels A, C, E); 50 μm (panels B, D, F).

Figure 4.. Ovarian endosalpingiosis lesions express both OVGP1 and eYFP in TAM-treated Ovgp1-iCreER^T2;R26R^LSL-eYFP mice.

A representative example of endosalpingiosis in a TAM-treated double transgenic mouse is shown: H&E stained section (A), and adjacent sections immunohistochemically stained for OVGP1 (C) and eYFP (E). High magnifications of boxed areas in panels A, C, and E are shown in panels B, D, and F, respectively. Note the internal positive control for detection of OVGP1 and eYFP expression (normal oviduct at black arrows) in lower left corners of panels C and E. Scale bars: 200 μm (panels A, C, E); 50 μm (panels B, D, F).