Time to Redefine Endometriosis Including Its Pro-fibrotic Nature

P. Vigano; M. Candiani; A. Monno; E. Giacomini; P. Vercellini; E. Somigliana


Hum Reprod. 2018;33(3):347-352. 

In This Article

Fibrosis and Myofibroblasts in Endometriotic Lesions

Peritoneal Lesions

The first study on peritoneal endometriosis with a monoclonal antibody against α-SMA was published back in 1996 by Khare et al. (1996) who used immunoperoxidase and Masson's trichome stains to determine, respectively, the presence of myofibroblasts and collagen in 10 pelvic wall samples. Well-formed smooth muscle bundles and dense type I collagen were found in these lesions. In 2000, Anaf and coworkers (2000) demonstrated by immunohistochemistry that all the 21 peritoneal lesions considered were consistently positive for α-SMA staining, although variable in intensity, whereas unaffected peritoneum and eutopic endometrial biopsies were negative. Leyendecker et al. (2002) analyzed 35 endometriotic lesions with a specific α-SMA antibody by immunohistochemistry and all of them stained positively for the marker. Although the authors did not formally discriminate between various disease forms, they clearly showed representative sections of peritoneal endometriotic lesions stained for α-SMA. The group of Sylvia Mechsner similarly evaluated peritoneal endometriosis specimens in two different studies. In the first one, 76% of 120 lesions showed α-SMA expression (Mechsner et al., 2005) while in the second one, all 60 lesions showed positivity (Barcena de Arellano et al., 2011). Therefore, smooth muscle content seems to represent an important and consistent feature of peritoneal endometriosis lesions.

Interestingly, TGF-β1 levels were found to be significantly increased in the peritoneal fluid of women with peritoneal lesions compared to women without the disease. Exposure of mesothelial cells to TGF-β1 increased the production of lactate, with reduction in the local pH. This increase in the amount of lactate resulted in acid activation of the TGF-β ligand with secondary induction of myofibroblast differentiation (Young et al., 2014).

Ovarian Cysts

It is well known that fibrosis is present in the ovarian cyst wall. Indeed, the cyst's pseudocapsule is mostly constituted of fibrotic tissue. Of note, the inner surface of the cyst is usually not entirely covered by an endometrial lining and where the endometrial lining is missing, only fibrotic tissue is identifiable. Positive immunostaining for α-SMA antibody was demonstrated in all of 10 and 13 ovarian cysts by Khare et al. (1996) and Anaf et al. (2000), respectively. According to Mechsner et al. (2005) smooth muscle content was present in 87% of the 40 ovarian lesions they evaluated. Liu et al. (2017) investigated the histologic features of deep and ovarian endometriotic lesions and observed a higher fibrotic content in the former compared with the latter lesion type. Nevertheless, the 25 ovarian samples consistently showed markers of fibroblast-to-myofibroblast transdifferentiation and stained positively for fibrosis at Masson's trichrome technique. Fibrosis was also identified in ovarian cortex surrounding the endometrioma. Indeed, follicular density was found to be lower in the ovarian cortex adjacent to the endometriotic cyst and this phenomenon is thought to be associated with tissue alterations, such as formation of fibrosis and vascular deficiency, and does not seem to be related to mere mechanical stretching. Kitajima et al. (2014) compared the histologic features in apparently normal ovarian cortical tissue from ovaries with small endometriomas and from the contralateral healthy ovaries. Fibrosis, as determined by Masson's trichrome staining with methyl green, was significantly more frequent in cortex from ovaries with endometriomas (80%) than in those without (27%) and the presence of fibrosis with concomitant loss of cortex-specific stroma was observed in 55% of cortical samples from ovaries with endometriomas but in none of those from contralateral healthy ovaries.

Interestingly, Sun-Wei Guo's group has recently shown that, in cells derived from ovarian endometriosis, activated platelets promoted epithelial to mesenchymal transition, fibroblast-to-myofibroblast transdifferentiation and differentiation to smooth muscle cells, resulting in increased cell contractility, collagen production and ultimately to fibrosis, via the release of TGF-β1 and the induction of TGF-β/Smad signaling pathway. TGF-β1 blockade could reverse these phenomena (Zhang et al., 2016).

Deep Infiltrating Endometriosis

Donnez and coworkers (1996) demonstrated for the first time that deep endometriotic nodules were histologically composed of scanty stroma and glandular epithelium disseminated in extensive fibromuscular tissue. Gömöritrichrome stain was used to detect muscle tissue. They speculated that this smooth muscle content pre-existed in the correspondent normal area and was invaded by the ectopic endometrium. Subsequently, Anaf et al. (2000) evaluated 12 rectovaginal nodules and eight uterosacral lesions and found them to be consistently positive for an anti-α-SMA antibody; they disputed the pre-existence of smooth muscle tissue in the rectovaginal nodules and conversely supported a transdifferentiation of endometrial stromal cells. Itoga et al. (2003) examined 90 rectovaginal nodules for the presence of fibrosis by elastic-van Gieson staining of collagen and for positivity to anti-α-SMA and anti-desmin antibodies. Fibrosis was observed in all but one of the samples, and immunoreactivity for smooth muscle actin and desmin was observed in 89% of the specimens. In deep nodules (n = 20), staining levels for α-SMA, desmin, collagen I and extent of fibrosis were shown to be higher than those of ovarian disease (Liu et al., 2017). van Kaam et al. (2008) not only showed that all the 20 deep infiltrating endometriotic lesions studied comprised fibromuscular tissue containing α-SMA-, desmin- and myosin-positive myofibroblastic cells, but again raised reasonable doubts on the origin of this muscle content. Indeed, they demonstrated that the inoculation of human endometrium into a nude mouse could induce α-SMA expression in the surrounding murine tissue. This would suggest that a reaction of the local environment to the presence of ectopic endometrium, rather than the stromal differentiation toward smooth muscle cells, could be at the basis of fibrosis development.

Despite the identification of a fibrotic component in deep infiltrating disease, Matsuzaki et al. (2017) showed that the TGF-β1 signaling may be absent when culturing endometriotic cells taken from this type of lesions. They suggested that endometrial stromal cells from patients affected might differentiate into myofibroblasts without TGF-β1 treatment and produce collagen type I. Increased stiffness through increased myofibroblast collagen production may then further increase matrix stiffness resulting in a fibrotic environment in deep disease over time.

Summary of the Literature Overview

Regardless of the different hypotheses provided to explain the origin of myofibroblasts and fibrosis in endometriotic lesions (summarized in Figure 1) (Young et al., 2014; Zhang et al., 2016; Matsuzaki et al., 2017; Albertsen and Ward, 2017), all investigators agree on the importance of this component. One may argue that fibrosis represents a secondary event triggered by an insult (the presence of ectopic cells) in an affected tissue (Walton et al., 2017). However, fibrosis appears as the phenomenon underpinning endometriosis-associated morbidity and some manifestations of the disease (i.e. adhesions). Thus, in line with what is recognized for other conditions of unknown etiology such as scleroderma (Tsou and Sawalha, 2017), fibrosis seems to represent a self-amplifying event of endometriosis.

Figure 1.

Main pathogenetic models proposed to explain the presence of myofibroblasts and the development of fibrosis in endometriosis. Epithelial to mesenchymal transition, fibroblast-to-myofibroblast transdifferentiation, increased collagen production and ultimately fibrosis have been suggested to be triggered in endometriotic cells by the presence of stimulating factors (e.g. Tranforming Growth Factor (TGF) β1 [B and C], platelets [B] or a stiff tissue matrix [D]). Similar phenomena in other tissues (A, surrounding connective tissue or C, mesothelial barrier) have been also proposed.