The role of fibrosis in endometriosis: a systematic review

Abstract

Background: Fibrosis is an important pathological feature of endometriotic lesions of all subtypes. Fibrosis is present in and around endometriotic lesions, and a central role in its development is played by myofibroblasts, which are cells derived mainly after epithelial-to-mesenchymal transition (EMT) and fibroblast-to-myofibroblast transdifferentiation (FMT). Transforming growth factor-β (TGF-β) has a key role in this myofibroblastic differentiation. Myofibroblasts deposit extracellular matrix (ECM) and have contracting abilities, leading to a stiff micro-environment. These aspects are hypothesized to be involved in the origin of endometriosis-associated pain. Additionally, similarities between endometriosis-related fibrosis and other fibrotic diseases, such as systemic sclerosis or lung fibrosis, indicate that targeting fibrosis could be a potential therapeutic strategy for non-hormonal therapy for endometriosis.

Objective and rationale: This review aims to summarize the current knowledge and to highlight the knowledge gaps about the role of fibrosis in endometriosis. A comprehensive literature overview about the role of fibrosis in endometriosis can improve the efficiency of fibrosis-oriented research in endometriosis.

Search methods: A systematic literature search was performed in three biomedical databases using search terms for 'endometriosis', 'fibrosis', 'myofibroblasts', 'collagen', and 'α-smooth muscle actin'. Original studies were included if they reported about fibrosis and endometriosis. Both preclinical in vitro and animal studies, as well as research concerning human subjects were included.

Outcomes: Our search yielded 3441 results, of which 142 studies were included in this review. Most studies scored a high to moderate risk of bias according to the bias assessment tools. The studies were divided in three categories: human observational studies, experimental studies with human-derived material, and animal studies. The observational studies showed details about the histologic appearance of fibrosis in endometriosis and the co-occurrence of nerves and immune cells in lesions. The in vitro studies identified several pro-fibrotic pathways in relation to endometriosis. The animal studies mainly assessed the effect of potential therapeutic strategies to halt or regress fibrosis, for example targeting platelets or mast cells.

Wider implications: This review shows the central role of fibrosis and its main cellular driver, the myofibroblast, in endometriosis. Platelets and TGF-β have a pivotal role in pro-fibrotic signaling. The presence of nerves and neuropeptides is closely associated with fibrosis in endometriotic lesions, and is likely a cause of endometriosis-associated pain. The process of fibrotic development after EMT and FMT shares characteristics with other fibrotic diseases, so exploring similarities in endometriosis with known processes in diseases like systemic sclerosis, idiopathic pulmonary fibrosis or liver cirrhosis is relevant and a promising direction to explore new treatment strategies. The close relationship with nerves appears rather unique for endometriosis-related fibrosis and is not observed in other fibrotic diseases.

Registration number: N/A.

Keywords: deep endometriosis; endometriosis; fibrosis; myofibroblasts; transforming growth factor-β.

Pathways involved in fibrosis development in endometriosis.

Figure 1.

PRISMA flow diagram: schematic representation of the study selection process.

Figure 2.

Schematic representation of cellular transitions contributing to myofibroblasts in endometriotic lesions. Fibroblast-to-myofibroblast transdifferentiation (FMT), epithelial-to-mesenchymal transition (EMT), and, to a lesser extent, endothelial-to-mesenchymal transition (EndoMT) are sources of myofibroblasts in endometriosis, marked by expression of α smooth muscle actin (α-SMA). In FMT, expression of vimentin increases, and fibroblasts thin and elongate. In EMT, the expression of E-cadherin decreases, and expression of vimentin increases. In EndoMT, expression of CD-31 decreases, and expression of fibroblast-specific protein (FSP1) increases. Smooth-muscle-metaplasia (SMM) can lead to smooth muscle-like cells in endometriosis, expressing desmin and smooth muscle myosin heavy chain (SM-MHC).