A Syngeneic Murine Model of Endometriosis using Naturally Cycling Mice

Our mirroring model of endometriosis is a simplified and efficient system that integrates the best features of existing models. And perhaps most importantly, relies on microscopic quantification in lieu of subjective grading. Modeling endometriosis can be difficult because there can be many confounding factors and sources of bias.

In our approach, endometriosis is induced without ovariectomy or survival surgery. And the endometrial lesions are objectively quantified. These Innovations provide a highly reliable and reproducible animal model of endometriosis that can be used to further endometriosis disease research.

To facilitate ongoing estrous cycling, place urine soaked bedding from a male mouse cage into the recipient female mouse cage periodically throughout development, and again at 72 hours before endometrial tissue transplant. When the donor female mice are between 22 to 24 days old, use a one milliliter syringe equipped with a 25 to 27 gauge needle to subcutaneously inject two international units of pregnant mare serum gonadotropin in 200 microliters of sterile saline into the lower mouse abdomen. 38 to 42 hours after injection, use dissecting scissors to make a shallow transverse incision through the skin and subcutaneous tissue of the euthanized mouse at the midline and use blunt traction to open the skin on either side of the incision.

Before removing the uterus, trim the adjacent connective tissue and transact each uterine horn just below its respective fallopian tubes, then transect the cervix to allow removal of the entire uterus and block. Inspect the uterus carefully, removing any additional peripheral fat or connective tissue and place the uterus in a droplet of cold PBS on a Petri dish for weighing. Transect each horn across the uterine fundus as close to the fundus as possible and place the dish under a dissecting microscope.

Placing one blade of the dissecting scissors inside the lumen of the first horn, cut along the major axis of the tube keeping in mind, which side is this serosa and which side is the epithelium. Then use forceps to grasp and peel away the endometrial layer and mince the tissue into 10 to 12, one by one millimeter fragments. Alternatively, mince the tissue into one by one millimeter pieces without separating off the myometrium.

Then place the tissues into 500 microliters of saline in a new Petri dish and harvest the endometrium from the second uterine horn as demonstrated. For peritoneal delivery of the tissue fragments into the recipient animal, use the blunt end of a one milliliter syringe to collect the tissue pieces. A total volume of one milliliter of tissue and saline should be collected.

Attach an 18 gauge needle to the syringe and gently depress the plunger to load the fluid into the needle. A mock injection into the Petri dish can be performed to ensure that all of the tissue will pass through the needle tip. Either before or after the injection, use a bulb syringe to administer 10 microliters of saline to obtain a vaginal smear of the recipient animal for estrous cycle documentation.

Holding the syringe at a 45 degree angle, intraperitoneally inject the fragments. If fragments remain after the injection, draw an additional 200 microliters of saline into the syringe to allow the rest of the pragmatists to be injected. Once you're assured of no bleeding or complications, place the mouse back in the home cage with a normal diet.

Approximately 21 days after fragment injection, spray the recipient mouse abdomen was 70%ethanol, intent the skin to allow creation of a superficial subcutaneous incision to bluntly open the abdomen. Perform a complete survey of the abdominal wall and peritoneum, pancreas and mesenteric fat and para uterine connective fat and tissue for gross lesions. Harvest each region of tissue entirely and place the dissected tissue samples into an appropriate labeled cassette for formalin fixation and processing according to standard protocols.

Then section the formalin blocks onto two slides per tissue area at two uniform depths. To score the endometriosis lesions, scan the tissue sections at a 40 times magnification and archive the slides. After scanning, use an appropriate digital slide reading software program to define and mark the longest distance between the edges of each endometriosis lesion.

A continuous lesion is defined by glands surrounded by stroma. The line does not necessarily traverse only the endometriosis tissue, but the two end points must be connected by continuous stroma and or glands. Make a second line, 90 degrees across the first line as demonstrated.

If multiple non-contiguous lesions are encountered, give each their own X and Y measurements. Calculate the final score for each slide as the summation of the areas of each lesion and use the larger of the scores from the two sections on each slide as the final score for that tissue region. Then total the scores from each region to give the final microscopic score for that animal.

In this representative sample, histopathologic analysis after donor endometrium injection revealed a classic architecture of endometriosis lesion. Fluorescent microscopy of the tissue sample confirmed that the lesion originated from the donor. It is important to note that the macroscopic examination for lesions alone is not sufficient for the quantification of the endometriosis disease burden as the gross lesions observed in these representative samples were ultimately determined not to be endometriosis upon histological examination.

Collection of all the tissue within the designated regions reduces bias by allowing for a systematic, objective approach to lesion quantification. Once this reliable and reproducible model has been achieved, researchers can use it to investigate a multitude of endometriosis pathways and therapies.