Adenomyosis is used to be called endometriosis interna, and deep endometriosis is now called adenomyosis externa. Thus, there is a question as to whether adenomyosis is simply endometriosis of the uterus, either from the perspective of pathogenesis or pathophysiology. In this manuscript, a comprehensive review was performed with a literature search using PubMed for all publications in English, related to adenomyosis and endometriosis, from inception to June 20, 2019. In addition, two prevailing theories, i.e., invagination—based on tissue injury and repair (TIAR) hypothesis—and metaplasia, on adenomyosis pathogenesis, are briefly overviewed and then critically scrutinized. Both theories have apparent limitations, i.e., difficulty in falsification, explaining existing data, and making useful predictions. Based on the current understanding of wound healing, a new hypothesis, called endometrial-myometrial interface disruption (EMID), is proposed to account for adenomyosis resulting from iatrogenic trauma to EMI. The EMID hypothesis not only highlights the more salient feature, i.e., hypoxia, at the wounding site, but also incorporates epithelial mesenchymal transition, recruitment of bone-marrow-derived stem cells, and enhanced survival and dissemination of endometrial cells dispersed and displaced due to iatrogenic procedures. More importantly, the EMID hypothesis predicts that the risk of adenomyosis can be reduced if certain perioperative interventions are performed. Consequently, from a pathogenic standpoint, adenomyosis is not simply endometriosis of the uterus, and, as such, may call for interventional procedures that are somewhat different from those for endometriosis to achieve the best results.

Do platelets play any role in the development of adenomyosis? As in endometriosis, adenomyotic lesions show significantly increased platelet aggregation, increased expression of transforming growth factor (TGF)-β1, phosphorylated Smad3, markers of epithelial-mesenchymal transition (EMT) and fibroblast-to-myofibroblast transdifferentiation (FMT), and smooth muscle metaplasia (SMM), in conjunction with increased fibrosis as compared with normal endometrium. Both EMT and FMT are known to play vital roles in fibrogenesis in general and in endometriosis in particular. EMT has been implicated in the development of adenomyosis. SMM is universally seen in endometriosis and also in adenomyosis, and is correlated positively with the extent of fibrosis. However, there has been no published study on the role of platelets in fibrogenesis in adenomyosis, even though adenomyotic lesions undergo repeated cycles of tissue injury and repair, which suggests the involvement of platelets and their possible roles in fibrogenesis. Cross-sectional studies of ectopic endometrial and control endometrial tissue samples from three sets of women with and without adenomyosis (n= 34 and 20, 12 and 10, and 8 and 8, respectively) were carried out from 2014 to 2015. Immunohistochemistry analysis of ectopic endometrial tissues from women with (n= 34) and without (n= 20) adenomyosis with respect to biomarkers of EMT, FMT and highly differentiated smooth muscle cells as well as TGF-β1, phosphorylated Smad3, markers of proliferation, angiogenesis and extracellular matrix (ECM) deposits. Masson trichrome staining, Van Gieson staining and Pico-Sirius staining were performed to evaluate and quantify the extent of fibrosis in lesions. Progesterone receptor isoform B (PR-B) staining also was performed. In addition, CD42b-positive platelets in ectopic (n= 12) and control (n= 10) endometrium were counted by confocal microscopy and compared. The protein expression levels of TGF-β1 and phosphorylated Smad3 in both ectopic (n= 8) and control (n= 8) endometrium were measured by western blot analysis. Immunofluorescent staining of both platelets and hepatocyte growth factor (HGF) was also performed for adenomyotic tissue samples (n= 10). Adenomyotic lesions had a significantly higher extent of platelet aggregation and increased staining for TGF-β1 and phosphorylated Smad3 (both P-values <0.001 versus control). In addition, E-cadherin staining was decreased while vimentin staining in adenomyotic epithelial cells was increased, along with increased staining of proliferating cell nuclear antigen, vascular endothelial growth factor and CD31 (all P-values <0.001), markers of proliferation and angiogenesis. Staining for α-SMA, a marker for myofibroblast, desmin, smooth muscle myosin heavy chain and oxytocin receptor was significantly increased in adenomyotic lesions versus control, concomitant with increased staining of collagen I and lysyl oxidase (all P-values <0.001). Histochemistry analysis indicates that the extent of fibrosis is high in adenomyotic lesions (P < 0.001), and the extent appeared to correlate negatively with the microvessel density (P < 0.05). PR-B staining was significantly decreased in adenomyotic lesion as compared with control endometrium (P < 0.001). Platelets and HGF were co-localized mostly in the stromal component of adenomyotic lesions, near the glandular epithelium. The results are limited by the cross-sectional nature of the study and the use of histochemistry and immunohistochemistry analyses only, but nonetheless is a validation of our previous finding in mouse experiments. The data presented are consistent with the notion that platelet-induced activation of the TGF-β/Smad signaling pathway may be a driving force in EMT, FMT and SMM in the development of adenomyosis, leading to fibrosis. This study provides the first piece of evidence that adenomyotic lesions are wounds that undergo repeated injury and healing, and, as such, platelets play critical roles in the development of adenomyosis by promoting proliferation, angiogenesis, increasing ECM deposits, and SMM, resulting in fibrosis. Platelets may also be involved in uterine hyperactivity and myometrial hyperinnervation. Our results provide one explanation as to why adenomyosis is a challenge for medical treatment, and shed new light onto the pathophysiology of adenomyosis. Support for data collection and analysis was provided by grants from the National Science Foundation of China. None of the authors has anything to disclose.

Do platelets play any role in the development of endometriosis? Activated platelets aggregate in endometriotic lesions and play important roles in the development of endometriosis. Platelets play important roles in cancer development and metastasis but there is no published study on their role in the development of endometriosis, even though endometriotic lesions undergo repeated cycles of tissue injury and repair, which characteristically involve platelets. Cross-sectional clinical studies of women with and without endometriosis, in vitro experimentation, and animal studies using platelet and/or macrophage depletion. Immunohistochemistry analysis of ectopic/eutopic endometrial tissues from 58 women with and 47 without endometriosis. Proliferation assay, cell cycle analysis by flow cytometry, gene expression and protein analysis for COX-2, VEGF, MMP-9, and Bcl-2 using primary cell culture, and evaluation of the rate of platelet activation induced by peritoneal fluid from women with and without endometriosis. Two mouse experiments, one that evaluated the effect of platelet depletion on lesion development and its associated phenotypic changes, and the other, the effect of platelet and/or macrophage depletion. We found that platelets aggregated in endometriotic lesions, concomitant with elevated VEGF expression and microvessel density. Co-culture of endometriotic stromal cells with platelets enhanced cellular proliferation, and increased the expression of COX-2, MMP-9, VEGF, and Bcl-2. IL-1β-induced COX-2 up-regulation and increased production of the coagulant TXA² in endometriotic stromal cells. Tissue factor (TF) expression was elevated in endometriosis and TF concentrations were significantly elevated both in the supernatant of cultured primary endometriotic stromal cells and in peritoneal fluid from women with endometriosis. Platelet depletion resulted in significantly reduced lesion size and improved hyperalgesia in mice with induced endometriosis. This study is limited by its cross-sectional design and by its focus on ovarian endometriomas. The demonstration that platelets are involved in the development of endometriosis provides a rationale for the use of anti-coagulants to treat endometriosis, and opens prospects for developing novel biomarkers for diagnostic or prognostic purposes. Financial support for this study was provided by grants from the National Science Foundation of China, a grant from the Shanghai Science and Technology Commission, support from the Key Specialty Project of the Ministry of Health, People's Republic of China. None of the authors has any conflict of interest to disclose.