Endometriosis Induces Colonic Inflammation and Reactive Gliosis in an Animal Model

Endometriosis is a complex gynecological disorder characterized by the ectopic growth of endometrial tissue. Symptoms of endometriosis are known to impair the quality of life of patients, among these symptoms are dysmenorrhea, chronic pelvic pain, and gastrointestinal (GI) issues. These GI issues are one of the common reasons for misdiagnosis with irritable bowel syndrome (IBS), a pain syndrome characterized by abdominal pain, painful bowel movements, diarrhea, and constipation, all of which are often present in endometriosis and persist even after treatment. Enteric glial cells (EGC) are known to play a role in pain associated with IBS, and reactive gliosis has been reported in patients with IBS, but the role of EGC in endometriosis has yet to be elucidated.We hypothesize that endometriosis will induce reactive gliosis, with increased expression of the glial fibrillary acidic protein (GFAP) and S100B, in the myenteric plexus of colonic sections in an animal model of endometriosis.We used 15 female Sprague Dawley rats and divided them into three groups (n=5 per group): Naïve, Sham, and Endo. Endometriosis was induced in the Endo group by suturing 4 pieces of autologous uterine tissue in the intestinal mesentery, while the Sham group received only sutures without tissue. Von Frey and Hot Plate tests were done the day before surgery, two weeks after surgery, and before sacrifice to assess pain behavior. 21 days after surgery, animals were sacrificed, and endometriosis development and colonic macroscopic inflammation were assessed. Colonic tissue was collected to assess GFAP and S100B immunoreactivity, microscopic damage, and mast cell whole-tissue cell count.We found Endo animals showed a decrease in allodynia threshold, but no hyperalgesia. All Endo animals developed endometriosis with an average area per vesicle of 13.89±1.43mm2 , and we found Endo animals had a significant increase in the colonic macroscopic score (3.01±0.22;p<0.001) compared to Sham (0.35±0.12) and Naïve (0.47±0.20). Paraffin-embedded colon sections stained with H&E didn't show statistical differences in microscopic damage between groups, yet toluidine blue staining for mast cells showed a 36% increase in the mean number of cells in Endo compared to Naïve but didn't reach statistical difference. GFAP whole-mount immunostaining showed significantly increased integrated density in Endo (25655.00±2395.00) when compared to Sham (13558.00±719.80;p<0.001) or Naïve (9035.00±1362.00;p<0.001), while no statistical difference was found in S100B integrated density between groups.Endometriosis can alter GI homeostasis by inducing macroscopic inflammation, reactive gliosis, and some mast cell infiltration. Taken together these suggest endometriosis can mimic IBS histopathology beyond the symptomatology, reinforcing this disease's complexity and the need to treat it beyond the gynecological setting.