This protocol describes a procedure for inducing uterine injury in a mouse model, followed by downstream analyses during a subsequent pregnancy.
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Method Article
This protocol describes a procedure for inducing uterine injury in a mouse model, followed by downstream analyses during a subsequent pregnancy.
Uterine injury from Cesarean sections (C-sections) and other procedures (dilation and curettage, myomectomies, etc.) can lead to several morbidities in subsequent pregnancies, including placenta previa, placenta accreta spectrum (PAS), Cesarean scar pregnancy, and uterine rupture. C-sections account for approximately 30% of deliveries in the US, with rates projected to rise. Therefore, a deeper understanding of the mechanisms by which injury-associated pregnancy disorders arise and a platform for testing possible interventions are essential. To achieve these objectives, there is a critical need for animal models of uterine injury, particularly those that address the impact of injury on subsequent in utero outcomes. This protocol describes a novel surgical procedure for a model of mechanically induced uterine injury in the laboratory mouse (Mus musculus). Steps taken in preparation for surgery, induction of the uterine injury itself, and post-surgical recovery procedures are demonstrated. Additional information is provided regarding the downstream dissection of both non-pregnant and pregnant mice. Overall, by detailing a set of procedures for inducing uterine injury in an in vivo model, this protocol presents a tractable method for elucidating the molecular and cellular events of uterine wound healing and its effects on subsequent pregnancy.
Although 1 in 3 deliveries in the US are conducted via Cesarean section (C-section), little is known about how the uterus heals and how an injured uterus leads to complications in subsequent pregnancies1,2,3,4,5,6,7,8,9,10,11,12,13. These complications include uterine rupture, placenta previa, PAS, and Cesarean scar pregnancy, which together affect ~2.5% of patients who have had one or more C-sections1,2,6,8,12,14,15,16,17. Placenta previa is characterized by the abnormal placement of the placenta in the lower portion of the uterus4,16. This not only causes physical obstruction of the cervix and an elevated risk of bleeding during pregnancy, but is also a significant risk factor for the more severe, invasive condition of PAS8,12. PAS is defined by over-invasion of the placenta too deeply into maternal tissues, even as far as the bladder or rectum, and often requires a C-section hysterectomy to preclude life-threatening maternal hemorrhage12,17. Cesarean scar pregnancies, though rare, carry a high rate of related morbidities, including miscarriage, fetal death, and preterm birth18,19. In addition to complications associated with a history of prior C-sections, other uterine procedures such as myomectomy or dilation and curettage (D&C) also lead to patient morbidities. For instance, ~30% of patients undergoing a D&C after a late spontaneous abortion and up to 46% of patients undergoing a second hysteroscopic resection for leiomyoma develop intrauterine adhesions characteristic of Asherman syndrome (AS)20,21. AS impacts ~2% of females, causing amenorrhea, dysmenorrhea, embryo implantation failure, recurrent pregnancy loss, preterm labor, and/or PAS21,22. Therefore, given the substantial major morbidities and mortalities associated with prior uterine trauma, an experimental animal model is critical not only for elucidating the mechanisms by which uterine wounds heal and how uterine injury impacts subsequent pregnancy outcomes, but also as a platform for the development of novel diagnostics and therapeutics23,24.
Several studies have thus developed mouse models of uterine injury. However, early studies either did not examine the consequences of uterine injury on subsequent pregnancies25 or limited the damage to the uterine luminal surface to investigate embryo implantation outside the window of endometrial receptivity in ovariectomized mice26. To address these knowledge gaps, the model described in this protocol, along with other recent studies, presents a novel surgical methodology for introducing uterine injury into mice27,28. This protocol uses the common laboratory mouse as the model organism due to its wide accessibility, genetic tractability, short gestation, rapid time from birth to puberty, large litter sizes, and cost-effectiveness. In addition, the anatomical feature of two uterine horns in mice allows for an internal control with an uninjured horn in each experimental animal, as well as the ability to separate local defects directly at the scar vs. global defects throughout the entire uterine environment. The approach described in this Protocol entails a simple dorsal laparotomy, followed by the use of a burred needle to create an incision through both the endometrial and myometrial layers of a single uterine horn. This methodology most closely mimics injuries seen during C-sections and myomectomies, though it can easily be modified to investigate non-incisional injuries such as D&Cs, which precede AS29,30. Following injury, animals are allowed to recover and then mated with stud males for timed pregnancies to examine the consequences of injury on subsequent in utero outcomes27,28.
Although similar in overall strategy to other recent studies, there are aspects of this protocol that provide distinct advantages. For instance, both Li et al. and Burke et al. accessed the uterus abdominally as opposed to dorsally as described here31,32. A dorsal approach is both simpler to perform and more conducive to rapid post-operative recovery of the animal. For the non-pregnant mice in the Burke et al. study, bent needles or curettes were used to scrape the endometrium, thus more closely mimicking D&Cs, as opposed to the full-thickness incisional injuries in this Protocol that more closely model C-sections and myomectomies32. In peripartum mice, Burke et al. introduced incisions on the mesometrial as opposed to the anti-mesometrial surface where embryos implant, as described here. By using a burred needle as opposed to scissors used by Li et al.31, this approach can more easily be modified from full-thickness to endometrium-restricted injuries to model both D&Cs as well as C-sections and myomectomies. Both Li et al. and Burke et al. closed uterine wounds with sutures, which, though more faithful to the events during a C-section, introduces additional variability into the healing process as well as the potential for foreign body effects31,32,33,34,35,36. Similar studies in rats have used abdominal routes to generate full-thickness incisional37,38 or excisional39 uterine wounds that are closed using sutures37,38,39 in both non-pregnant38,39 and pregnant37 animals. Therefore, the protocol presented here is particularly suitable for researchers interested in a facile and reproducible approach that requires minimal surgical experience, addresses embryo implantation- and placentation-related events following a full-thickness uterine injury, and is easily adaptable to a wide variety of questions, including D&C-like injuries to the endometrium27,28.
Step 1 of this protocol outlines pre-operative steps. Step 2 describes the surgical procedure for inducing injury to a uterine horn. Finally, Step 3 details the dissection of a pregnant dam to investigate the effects of uterine surgery on a subsequent pregnancy. Altogether, this study presents a unique rodent surgical model of uterine wound healing and associated pregnancy outcomes that is quick to perform and easy to adopt.
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All procedures demonstrated in this protocol have been approved by the Institutional Animal Care and Use Committee (IACUC) of the University of California, Davis (IACUC Protocol #23985). The procedures described in this protocol are typically performed on C3H females between 7-10 weeks of age and 19-25 g in weight, though this protocol is suitable for female mice of all strains, ages, and weights, depending on the researcher's question. Monitor animals prior to surgery to ensure that only healthy animals with no signs of injury or distress are used for studies. The details of the reagents and the equipment used are listed in the Table of Materials.
1. Preparation of the surgical area
2. Surgical induction of uterine injury
NOTE: This section describes the surgery and procedure for introducing injury into a single uterine horn of a non-pregnant female mouse. First, a dorsal laparotomy is performed to expose the top of the uterine horn. Then, a burred 25 G needle is inserted into the uterine lumen to damage the uterine endometrium and, if desired, to cut through the myometrium. This creates a longitudinal wound in the injured uterine horn, enabling further experimentation to address questions such as how the uterus heals and how injury impacts a subsequent pregnancy. It is important that the animal is properly sedated before performing the surgery. As described in Step 1, several parameters indicate that an animal is adequately anesthetized. These include the cessation of movement, a reduction in respiratory rate, and the absence of a forelimb reflex. In addition, the hindlimb reflex upon a light touch should be absent, though this reflex sometimes remains present despite adequate anesthesia if sufficient pressure is applied40,42.
3. Mating and dissection of pregnant mice to assess consequences of uterine injury
NOTE: Depending on the specific research question, various endpoint analyses may be performed post-uterine injury. For example, non-pregnant animals may be dissected at different timepoints post-injury to examine the kinetics of wound healing27. In addition, injured animals may be impregnated via natural mating or embryo transfer to examine the impact of injury on subsequent pregnancies27. This Step describes the assessment of embryonic and placental development in a damaged uterine environment. In particular, this Step provides details on setting up timed matings and dissection of animals at embryonic day 12.5 of gestation (E12.5, with the morning of vaginal plug defined as E0.5; more information on identifying vaginal plugs in step 3.2 below). Altogether, Step 3 provides an example of downstream analyses that can be conducted post-injury to examine a specific set of experimental questions.
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Following surgery with the use of ketamine/xylazine as the anesthetic, animals are typically partially mobile within ~1 h. and fully mobile within ~4-6 h. Recovery from isoflurane anesthesia typically occurs within minutes. Any complications that may arise, such as wound dehiscence or pain, are typically evident within 1-3 days following surgery. Perform daily welfare checks to monitor for pain or complications until the skin incision has healed and the skin clips are ready to be removed.
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C-sections account for ~33% of deliveries in the US, and up to 55%-65% of deliveries in countries such as Brazil and Egypt12,52, and yet little is known about how the uterus heals following injury or how these injuries impact subsequent pregnancies. While prior C-section is a major risk factor for pregnancy disorders like placenta accreta spectrum (PAS) and placenta previa, the mechanisms by which these arise are unclear3,
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The authors have no conflicts of interest to declare.
We thank the UC Davis Teaching and Research Animal Care Services (TRACS) for their ongoing husbandry, procedural, and logistical support. We thank the UC Davis Cardiovascular Research Institute (CVRI) Animal Model Core and core director Dr. Yi-Je (Jay) Chen, D.V.M., Ph.D. for technical guidance and support as well as surgical suite use for filming. We thank all members of the Zhang lab for their support of this work. Figure 5A was created with BioRender.com (https://BioRender.com/ig9sfon).
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| Name | Company | Catalog Number | Comments |
|---|---|---|---|
| 0.9% NaCl for injection, sterile | Fresenius Kabi | 63323-186-10 | |
| 25 G x 5/8 in. BD PrecisionGlide™ Needle | BD | 305122 | |
| 70% ethanol | Fisher Scientific | BP82031GAL | |
| Absorbable 4-0 vicryl suture | Ethicon | J214 | |
| Animal cages | VWR | 10712-110 and 10715-323 | Cages provided by the researcher's animal facility are acceptable |
| Animal ear-punch | Fisher Scientific | 13-812-201 | |
| AutoClip wound closing system | Fine Science Tools | 12020-00 | |
| U-100 insulin syringe | Fisher Scientific | 14-826-79 | BD brand |
| Betadine swabsticks | Medline | MDS093901 | |
| Buprenorphine Extended Release, 1.3 mg/ml (Ethiqa XR) | Fidelis Pharmaceuticals | 099114 | Alternative extended release Buprenorphine or Buprenorphine HCl are acceptable |
| Carprofen (RIMADYL) 50 mg/ml | Zoetis | RIM-00284R1 | |
| Curved iris scissors | World Precision Instruments | 501759G | |
| Dry glass bead surgical tool sterilizer | VWR INTERNATIONAL | 75999-324 | |
| Dry glass bead refill | VWR INTERNATIONAL | 75999-332 | |
| Female mice, typically 8 to 12 weeks of age, e.g., strain C3H | Charles River | 025 | |
| Fine scissors - curved 10.5cm | Fine Science Tools | 14370-23 | |
| Fur trimmer | Fisher Scientific | 50-195-4544 | |
| Glad Press ‘n Seal | Fisher Scientific | NC1089054 | |
| Gloves, nitrile | Fisher Scientific | 19-130-1597C | |
| H2O, sterile distilled | Fine Science Tools | 15230170 | |
| Illuminator for surgeries | AmScope | HL150-AY | |
| Ketamine, 100 mg/mL (Zetamine) | Dechra | 501072 | |
| Lab coat | Uline | S-15376W-50 | |
| Light microscope | Fisher Scientific | S13039 | |
| Lint roller | Fisher Scientific | 17-018-329 | |
| Nair Hair Removal Cream | Target | 049-06-3304 | |
| Needle holder | Fine Science Tools | 12500-12 | |
| Paper towels | Scott | 1804 | |
| Petri dish | Fisher Scientific | 08-757-100 | |
| Phosphate-buffered saline (PBS) | Thermo Fisher Scientific | 10010023 | |
| Plastic beaker | Thermo Fisher Scientific | 12010250 | |
| Probe seeker with bent end | Fisher Scientific | 08-995 | |
| Fisherbrand™ Instant Sealing Sterilization Pouches 9 cm (W) x 13.3 cm (H) | Fisher Scientific | 181250 | |
| Fisherbrand™ Instant Sealing Sterilization Pouches 9 cm (W) x 23 cm (H) | Fisher Scientific | 181251 | |
| Forceps, Fine, #5 | Fine Science Tools | 11254-20 | |
| Forceps, Fine, #55 | Fine Science Tools | 11295-51 | |
| Scale | Fisher Scientific | S93805 | |
| Scissors, surgical | Fine Science Tools | 14060-09 | |
| Spray bottle | Fisher Scientific | S413505P | |
| Sterile alcohol prep pads | Fisher Scientific | 22-363-750 | |
| Sterile ophthalmic veterinary ointment | Puralube | PH-PURALUBE-VET | |
| Sterile swabs for eye ointment application | Fisher Scientific | 18-366-472 | |
| Sterilization pouches | Fisher Scientific | 19-910-673 | |
| Surgical drape | Fisher Scientific | 50-209-1792 | |
| Surgical hair cap | Uline | S-10480BLU | |
| Surgical masks | Fisher Scientific | 18-048-010 | |
| Thomas Scientific Absorbent Bench Underpad, Bench pads, 16.5 x 23 in./42 x 58cm | Fisher Scientific | NC1588452 | |
| Warming/induction chamber with full floor warming | Fisher Scientific | 14-370-302 | |
| Wound clip refills | Fine Science Tools | 12022-09 | |
| 100mg/ml xylazine | Dechra | Rompun 100 mg/mL Xylazine |
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