Investigating endometriosis development and endometriosis-related pain over time and in relation to estrogen in a laparoscopic mouse model
Authors: Peterse Daniëlle , Verhassel Alejandra , Fassbender Amelie , Dorien F. O , Vanhie Arne , Rompuy Anne-Sophie Van , Saunders Philippa , Vriens Joris , D’Hooghe Thomas M. 
Published: March 2024

Background

Endometriosis is a complex disease, and its pathophysiology is still unclear. Therefore, endometriosis animal models need to be carefully selected and examined to be useful for identification of novel therapies for women with endometriosis. In this study, we evaluated endometriosis-associated pain, and time- and estrogen-related development of endometriotic lesions after laparoscopic implantation of menstrual endometrium in a homologous mouse model for endometriosis.

Methods

Endometriosis was induced by laparoscopic introduction of 10 menstrual endometrial tissue pieces into the peritoneum of ovariectomized recipient mice (59 estrogen-substituted; 59 estrogen-depleted). Sham animals (57 estrogen-substituted; 60 estrogen-depleted) received 10 pieces of perigonadal adipose tissue. The animals were sacrificed at 1, 2, 3, 4, 6 or 8 weeks after induction, the attached peritoneal implants localized and excised and immunohistochemically analyzed. Additionally, endometriosis-related pain was evaluated by measuring mechanical allodynia, thermal hyperalgesia, locomotor activity and anxiety-like behavior before and after tissue implantation.

Results

At least one implant per mouse could be retrieved in 94% (111/118) of the endometrial tissue animals and in 78% (91/117) of the adipose tissue animals (p<0.001). Peritoneal implant take rate was significantly higher in endometrial tissue animals (2.5±1.4) compared to adipose tissue animals (1.6±1.5) (p<0.0001), regardless of estrogen supplementation and time of sacrifice. Hemosiderin could be observed more often (p<0.0001) in attached peritoneal implants of the endometrial tissue animals (67%, 68/101), compared to the adipose tissue animals (37%, 31/83). Ki67 staining showed a higher proliferation index in the attached peritoneal implants retrieved after one week, compared to the other time points of both endometrial tissue and adipose tissue animals. The behavioral test showed no significant difference in mechanical and thermal sensitivity, locomotor activity and anxiety-behavior between the menstrual endometrial tissue and adipose tissue implanted animals. Nevertheless, the estrogen-substituted animals showed decreased activity in the tests featuring thermal nociception and anxiety-like behavior, compared to the estrogen-depleted animals. Additionally, time after implantation showed to have a positive effect on thermal sensitivity, locomotor activity and anxiety-related behavior in all animals, as the mice became less sensitive to thermal stimuli, more active in the open field test and buried less marbles in the marble burying test.

Conclusion

This study showed an increased attachment of menstrual endometrium compared to adipose tissue in the peritoneum when using laparoscopic induction. There was no apparent influence of estrogen on tissue attachment, proliferation or appearance. A decrease in cell proliferation in peritoneal implants occurred over time. Locomotor activity, anxiety-like behavior, and mechanical and thermal sensitivity of the animals was not affected after induction of endometriosis, regardless of the type of implanted tissue. Altogether, we showed that the current methodology used to induce endometriosis was not sufficient to develop endometriotic lesions that contained both stromal and epithelial cells. Moreover, the current methodology was not able to detect specific endometriosis-related pain.

GRAPHICAL ABSTRACT