Angiogenesis, neurogenesis and lymphangiogenesis in the early development of endometriosis using a menstruating mouse model

Endometriosis is an estrogen-dependent gynecological disease, characterized by the growth of endometrial-like tissue outside the uterus. The most common symptoms are dyspareunia (pain during sexual intercourse), dysmenorrhea (pain during menstruation), chronic pelvic pain, infertility, and chronic fatigue.

Sampson’s theory of retrograde menstruation is the most accepted theory to explain the pathogenesis of endometriosis, and states that it originates from menstrual endometrium flowing backwards through the fallopian tubes into the pelvic cavity. Here, the endometrial cells attach onto the peritoneal wall and pelvic organs where they form endometriotic lesions.

Mouse models for endometriosis can be used to investigate the pathophysiology of the disease. However, the most important drawback of the currently available mouse models for endometriosis is the limited preclinical translational value due to the lack of standardization, validation and replication. Therefore, aim of this dissertation was to optimize a novel mouse model for endometriosis and to investigate lesion ontogenesis and endometriosis-related pain using this model.

First, we optimized and standardized the protocol to obtain menstrual endometrium in donor mice by adjusting the decidualization stimulus, a crucial step in order to create menstruation in mice. We showed that the amount of endometrial decidualization can be maximized by administering an intra-uterine stimulus containing 100 µl oil, combined with a mechanical stimulus.

Secondly, we investigated safety, efficacy and reliability of different slow-release estrogen capsules and we indicated that the in-house manufactured silastic capsules are best suited for long-term slow release of estradiol in mice, when compared to polyethylene capsules. Moreover, we developed a new technique to introduce endometriosis and the 100% incidence and 60% peritoneal implant take rate indicated that laparoscopic surgery can indeed successfully be used to introduce endometriosis in mice.

Thirdly, we investigated lesion ontogenesis and endometriosis-related pain after induction of menstrual endometrium from donor mice in syngeneic recipient mice in relation to normal estrogenic and hypo-estrogenic conditions. We demonstrated that a higher number of menstrual endometrium pieces attached in the peritoneal cavity when using laparoscopic induction, compared to adipose tissue. Next, we showed that estrogen did not have an influence on tissue attachment, proliferation or appearance, while time between surgery and sacrifice (1, 2, 3, 4, 6, or 8 weeks) showed to decrease tissue proliferation. Additionally, we observed that the attached peritoneal implants derived from menstrual endometrium did not show the typical endometriotic features, existing of endometrial glands and stroma. Finally, we indicated that 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.

In conclusion, we successfully optimized the different steps of the laparoscopic mouse model for endometriosis induced with menstrual endometrium. Therefore, we had expected that this would lead to a novel mouse model for endometriosis in which lesion ontogenesis and endometriosis-related pain could be investigated. Nevertheless, we showed that the current methodology used to induce endometriosis was not sufficient to develop endometriotic lesions and to detect endometriosis-related pain and that further optimization is needed in order to create a relevant model to study endometriosis.