Endometriosis remains a serious problem for reproductive-age women. The etiology and pathogenesis of endometriosis have been the subject of numerous investigations, although answers remain unclear [4, 9, 12, 15]. Starting in 2005, our group described the earliest morphologic changes of endometriosis in the ovary, which we named “initial endometriosis” [4]. That study, which was largely based on light microscopic observations, provided morphologic evidence that retrograde menstruation may not explain how the initial endometriosis forms either on the ovarian surface or within the ovarian cortex. That study indirectly provided some supportive evidence that ovarian endometriosis may be derived from the fallopian tube. Morphologically, foci of initial endometriosis show gradual transitions from a typical example of endosalpingiosis (tubal type epithelia with ovarian stroma but without appreciable vascular changes) to areas with a classic appearance of endometriosis (endometrioid epithelia associated with endometrioid stromal cells and increased density of microcapillary vessels) [4]. Such morphologic transitions suggest that the foci of ovarian endometriosis started from the site within the ovary, rather than being deposited from retrograde menstrual endometrial tissue.
From what being presented, we questioned the hypothesis of retrograde menstruation and proposed that tubal cells are a plausible tissue source for ovarian endometriosis [16]. Recent studies of the tubal origin of ovarian serous carcinomas [5, 17, 18] have highlighted many previously under-recognized biologic and physiologic properties of the fallopian tube. In vivo, the fallopian tube is in close spatial proximity to the ovary [5, 7, 17, 19, 20], tubal epithelia are easily sloughed from the tubal mucosae [17, 21], and the majority of the endosalpingiosis or OEIs are originated from the fallopian tube. Moreover, endosalpingiosis or OEIs are readily observed in ovarian cortex in many ovarian endometriosis [5]. It is likely therefore that the relationship between endosalpingiosis and initial endometriosis represents a trans-differentiation process from the former to the latter, although detailed underlying mechanisms remain to be clarified.
Given the well-known limitations of morphologic assessment to support the novel concept of tubal origin of ovarian endometriosis, we have shifted morphologic observations to molecular studies addressing the issue since 2014. In one of our previous studies, we identified a set of novel genes that are either highly expressed in the normal tubal or endometrial tissue through a gene differential array study [9]. In that study, FMO3 and DMBT1 were examined as the biomarkers to test the hypothesis if the fallopian tube contributes the formation of ovarian endometriosis. These biomarkers were then validated in those paired samples of ovarian endometriosis, normal tubal and endometrial tissue similar to the current study. We found that FMO3 was highly expressed in the fallopian tube while was low in the paired endometrial samples, whereas DMBT1 had the reverse findings. Based on those observations, we concluded that tubal epithelia contribute at least partially to the development of ovarian endometriosis.
The current study used another biomarker, FRA (folate receptor alpha), which is highly differentially expressed in the tubal and endometrial tissues. FRA, the product of the FOLR1 gene, is a glycosylphosphatidylinositol (GPI)-anchored protein that binds plasma folate (5-methyltetrahydrofolate) and transports it into the cell via endocytosis [22]. Folate is essential for 1-carbon metabolism, transferring single carbon units in reactions involving purine and pyrimidine synthesis, DNA repair, and methylation of various biomolecules including DNA, proteins, phospholipids, and neurotransmitters [23, 24]. Folate deficiency has been linked with dysregulation of these processes and, in some cases, is associated with an increased risk of developing cancer, including serous type epithelial ovarian tumors [14, 25–27]. However, no specific data have been reported on the expression of FRA in normal fallopian tube relative to ovarian endometriosis or to evaluate what role, if any, FRA may play in the development of ovarian endometriosis.
Since FOLR2 and its protein product FRA were highly differentially expressed in the tube and the eutopic endometrium, we assessed the tubal and endometrial samples with the FRA antibody on 50 patients with ovarian endometriosis (32 paired and 18 non-paired). We found that all fallopian tubal epithelia expressed FRA with strong intensity. In contrast, FRA expression in the endometrium was either negative or weakly and focally positive. Therefore, FRA may be considered a tubal-specific biomarker when compared to that of the endometrium. This result was quite consistent with whole-genome expression microarray analysis [9]. Among 18 non-matched ovarian endometriosis, 15 cases had strong or moderately membrane and intracellular staining. Only 3 (18%) samples showed a weak staining on apical luminal borders. Furthermore, Pax-8 positive epithelial cells on the ovarian surface and OEIs showed moderate to strong FRA expression, while Pax-8 negative epithelial cells on the ovarian surface and OEIs were negative for FRA, which is supportive of tubal origin of ovarian endometriosis. Morphologically ovarian surface like epithelia and OEIs have two origins with one originated from classic ovarian surface epithelia (OSE) (Pax-8 negative) and the other from fallopian tube (Pax-8 positive) [5]. Original OSE negative for FRA expression suggests that ovarian endometriosis is unlikely derived from OSE through a metaplastic process. In short, our findings from this study further support the hypothesis that ovarian endometriosis is at least partially derived from the fallopian tube. In our previous study [9], we estimated that approximately 60% of ovarian endometriosis may be originated from the tubal epithelia based on the FMO3 and DMBT1 expression study. This is correlated to the number of OEIs from fallopian tube [5]. The design of the current study does not allow such quantification; however, the findings bolster the argument that the majority of ovarian endometriosis comes from the fallopian tube rather from retrograde menstrual endometrium.
Two essential conditions must be met to consider the fallopian tube to be the origin of endometriosis when present in the ovary. First, tubal cells must enter the ovary. Frequent detachment of tubal epithelia from fimbriated ends makes this possible. Tubal cells are easily retrieved by flushing the fallopian tube [17, 21]. The process is further facilitated by juxtaposed spatial relationship between tubal fimbria and ovarian surface [19, 28]. The rupture of ovarian surface caused by ovulation [29, 30] provides a favorable condition for tubal epithelia to implant onto the ovary then get into ovarian cortex. The latter process, from a morphologic perspective, has long been described as endosalpingiosis [31–33]. Second, endosalpingiosis or OEI must transform itself into endometriosis. The latter probably occurs through metaplasia or trans-differentiation, a process that is commonly seen in the Müllerian system [34], although detailed molecular mechanism remains elucidated. Initial endometriosis within the ovary describes the morphologic transition of OEIs, with some glands of OEIs displaying the earliest morphologic changes of endometriosis in only half of the gland [4]. Metaplasia from tubal epithelia is the most likely explanation for this morphologic observation, especially since transitional areas from normal-appearing tubal epithelia to endometrial like tissue are commonly present [17, 35]. In summary, a graphic abstract is illustrated in Fig. 5. We may conclude, therefore, endometriotic or endometrioid epithelial cells are likely originated from tubal epithelia. However, by all means, so far we don’t have solid scientific evidence that ovarian endometriosis are either derived from or not coming from the endometrial cells from retrograde menstruation. Further studies in this regard are needed.
The novel findings of this study have provided further evidence supporting our previously proffered theory of the tubal contributions for ovarian endometriosis. Although our findings remain preliminary, they might provide an alternative way of thinking about the etiology and pathogenesis of endometriosis that might aid the prevention and early treatment of ovarian endometriosis.