The distribution of smooth muscle actin, collagen fibres and immune cells within superficial peritoneal endometriotic lesions is distinctly different to their surrounding microenvironment

Research questionDo different subtypes of superficial peritoneal endometriotic lesions exist, based on the presence and morphology of smooth muscle, collagen fibres and immune cell populations?DesignThis was a retrospective cohort study of 24 patients, from across the menstrual cycle, with surgically and histologically confirmed endometriosis. Immunofluorescence was utilised to delineate the CD10 stromal area of lesions (n = 271 lesions from 67 endometriotic biopsies), and then smooth muscle actin (SMA) positive tissue and immune cell populations (CD45+ and CD68+) were quantified within and adjacent to these lesions. Second harmonic generation microscopy was employed to evaluate the presence and morphology of type-1 collagen fibres within and surrounding lesions.ResultsOverall, immune cell numbers and the area of SMA and collagen within endometriotic lesions tended to be low, but there was still a spectrum of presentations with significant variation, particularly in the adjacent tissue microenvironment, based on lesion locations and/or the morphology of endometriotic gland profiles. We identified lesions where collagen fibres formed well aligned capsules around the CD10+ stromal border, versus lesions where collagen fibre distribution was random. We also observed considerable inter- and intra-patient variability in the morphology of SMA and collagen within and surrounding lesions.ConclusionThese data demonstrate there is considerable diversity in the presence of immune cells and morphology of SMA and collagen within, but even more so, surrounding endometriotic lesions, even within individual patients. This heterogeneity, especially within individual patients, presents a challenge to incorporating these cell and tissue types into any new endometriosis classification systems or prognostic approaches.