Resident immune cells populate the human endometrium at menstruation (23
) and the inflammatory nature of menstrual tissue transferred to the peritoneal cavity likely affects the development of endometriosis (3
). Using intraperitoneal endometrial tissue injection to mimic retrograde menstruation, our studies suggest that surgery near the time of menstruation may contribute to peritoneal inflammation, potentially affecting the development of endometriosis and adhesive disease. Specifically, in our model, peritoneal adhesion formation was maximally enhanced by a surgical wound and
the presence of endometrial fragments. Mice receiving endometrial tissue injection at 0 to 16 hrs postovariectomy developed multiple adhesions at the wound site and elsewhere in the peritoneal cavity whereas no adhesions distal to the surgical injury site itself were noted in the model after 36 hrs from the time of surgery. Since sham-operated mice lacked adhesions, the presence of endometrial fragments within the peritoneum did not independently trigger adhesion formation. Similarly, in the absence of human tissue injection, the surgical procedure alone also failed to promote adhesions. Importantly, the study reported herein only examined the effects of ovariectomy since this surgical procedure is a standard component of our previously established experimental endometriosis model (15
). Therefore, the potential impact of other, perhaps more common, peritoneal surgical procedures will need to be examined in future studies to determine whether our results are specific to the surgical removal of ovaries or to pelvic surgery in general.
Compared to infection-related adhesions, endometriosis-associated adhesions exhibit greater numbers of inflammatory cells (25
), likely reflecting the proinflammatory influence of ectopic endometrial growth on the entire peritoneal microenvironment. Importantly, a recent surgical injury can equally affect the manner in which experimental endometriosis develops. Specifically, endometrial tissue preferentially attached at the injury site when injected 0 to 16 hours after surgery while tracking to the wound site is limited at later times. Certainly, proinflammatory cytokines affect both wound healing and angiogenesis during menstruation (26
) and surgery (27
) and thus the presence of these cytokines likely contributed to our findings.
In previous studies, we have demonstrated that, establishing experimental endometriosis requires that human tissue fragments acquire a peritoneal vascular supply (22
). Interestingly, in the current study, the extent of angiogenesis in the ectopic lesions reflected the timing of human tissue injection relative to the ovarietomy. Microvessel density (MVD) was highest in ectopic lesions established near the time of surgery and decreased thereafter. Although the MVD of adhesions appeared less dependent upon the timing of human tissue injection postsurgery, the risk of adhesion development diminished quickly after 16 hrs suggesting an “inflammatory threshold” may be required for adhesion formation.
In the last series of studies in our endometriosis-adhesion model we examined a potential therapeutic agent for adhesion prevention. PIO, a thiazolidinedione with PPAR-γ agonist activity, is currently in use as an anti-diabetic medication. PPAR-γ receptors are expressed in numerous cell types including immune and endothelial cells (28
); thus, treatment with thiazolidinediones can have both anti-inflammatory and anti-angiogenic effects. Rosiglitazone, another thiazolidinedione, has previously been found to prevent/reduce adhesions in a rat uterine horn adhesion model (14
). In our study, we found that treatment of mice with PIO both prior to surgery and for 5 days postsurgery reduced the extent of lesion formation as well as the development of postsurgical adhesions.
In summary, we report that the proinflammatory microenvironment of a recent, peritoneal surgical procedure combined with the presence of endometrial tissue fragments is associated with the development of adhesive disease, even at sites distal to the surgical injury. Current models of experimental adhesion formation require direct trauma to the mesothelium (30
) and thus future studies with our model may provide useful information on the mechanisms of adhesion formation not directly related to local tissue trauma. Although translation of our chimeric experimental model findings to women is premature at this juncture, it would be difficult to conduct a prospective human study to determine whether avoiding surgery near the time of menstruation is prudent. Therefore, the use of our chimeric model represents a useful experimental approach to understanding the association of peritoneal inflammation to the related disease processes of endometriosis and post surgical adhesion formation. Certainly, our therapeutic intervention with PIO provides preliminary evidence that similar approaches may aid in developing new medical strategies for adhesion prevention.
In this study, we describe a novel model for examining the role of a recent surgical injury on the development of endometriosis-related adhesions.