Approval from the University of Arizona Institutional Animal Care and Use Committee (IACUC) was obtained for all experiments (control number: 2021-0828). This protocol uses 15-week-old C57BL/6J male mice although it could be applied to other ages and strains9,26.
1. Creating the HTS biomechanical loading device
NOTE: Modifying the palatal expanders into the HTS device can occur at any point before the experiment.
- Take the unmodified palatal expander and present to makerspace core or fabrication core with specifications according to Figure 1A,B.
NOTE: If no makerspace or fabrication core is available, continue to the following steps.
- Use a Mini Universal Bender (MUB) wire bending tool to alter the palatal expander to conform to the specifications of Figure 1A,B.
- Place the device flat on a surface as shown in Figure 1A1.
- Bend the arms 90° up for the upper arms and 90° down for the lower arms when the device lays flat with the MUB as seen in Figure 1A2. See Figure 1A3 for the appearance of the device when resting flat on a surface, where the arms now are perfectly parallel with the body of the device.
- Bend each arm back into the page 90° along the axis of the dotted line as seen in Figure 1A4. See Figure 1A5 for the device after bending.
2. Hair removal and initial incision on postoperative day 0 (POD 0)
NOTE: Clean and autoclave several sets of surgical instruments before surgery (e.g., dissection scissors, scalpel, Adson forceps, Needle driver). Prepare sterilized 5-0 sutures for use and have a surgical marker on hand.
- Clean the operating space with 70% ethanol spray.
- Place a heating pad set at 35 °C on the presurgical prep table. Tape down an absorbent pad over the heating pad. Place another absorbent pad over the previous pad (to collect shaved hair). Tape the anesthesia cone (or cones) on the absorbent pad.
NOTE: [Optional] If a benchtop fume extractor is available, place the cone near the workspace and turn on the suction.
- Place the mouse in the anesthesia induction chamber with 1-3% isoflurane and 2 L/min oxygen flow until the mice breathe calmly (2-3 min).
- Insert each mouse's nose into the nose cone opening, allowing for inhalation of anesthesia composed of 1-3% isoflurane with 2 L/min of oxygen. Confirm adequate anesthesia by the lack of reaction to a toe pinch. Apply ophthalmic lubricating ointment to the eyes of the mouse.
- Use an electric razor to shave the hair on the dorsum in the area as seen in Figure 1C.
- Apply the depilatory paste with a gloved finger or cotton swab onto the skin, covering the shaved area. After 45 s, wipe away the paste with gauze and then an alcohol swab. Wipe the dorsum with another alcohol swab to remove the depilatory paste, leaving behind a hairless patch (Figure 1C).
NOTE: It is critical to use a moderate amount of paste, remove the paste promptly, and wipe the mouse after removal of the paste to prevent dermal burns by the depilatory paste. Murine skin is thin, making it susceptible to rapid chemical burns that can have detrimental effects both on mouse health and on experimental quality.
- Ear tag the mouse for identification.
- Upon removing the hair, place the mouse back in the cage to prevent excessive isoflurane exposure. Monitor the mouse.
- Remove the top absorbent pad covered with hair to have a clean presurgical pep work surface.
- Set up a separate surgical work area. Place a heating pad set at 35 °C on the surgical table. Tape down an absorbent pad over the heating pad. Tape the anesthesia cone (or cones) on the absorbent pad.
- Place a mouse in the anesthesia induction chamber with 2-4% isoflurane and 2 L/min oxygen flow until the mouse breathes calmly (2-3 min).
- Take the mouse out of the induction chamber and place it on the operating surface. Insert each mouse's nose into the nose cone opening, allowing for inhalation of anesthesia composed of 1-3% isoflurane with 2 L/min of oxygen. Confirm adequate anesthesia by the lack of reaction to a toe pinch. Apply ophthalmic lubricating ointment to the eyes of the mouse.
- Inject 0.05 mg/kg sustained-release buprenorphine into the shoulder subcutaneously using a 21 G needle for postsurgical pain treatment.
- Disinfect the mouse's dorsum with three alternating rounds of iodine/chlorhexidine based scrub and alcohol swab. With the surgical marking pen and a ruler, mark a 2 cm line on the dorsal midline where the full-thickness incision will be made, as shown in Figure 1D.
- Use a scalpel or dissection scissors (surgical preference) to make the full-thickness dorsal midline incision through the marked area.
NOTE: Be careful not to cut the underlying tissue (e.g., muscle), as seen in Figure 1D.
- Using 5-0 sutures in a simple interrupted pattern, close the incision by bisecting the wound as shown in Figure 1D. Use at least 5 evenly spaced sutures.
- Cut a Telfa gauze into a 3 cm x 1 cm piece. Place it in the center of a foam adhesive dressing and place the adhesive dressing on the mouse dorsum such that the gauze covers the incision as shown in Figure 1D. Place a halved dressing on the abdomen and wrap circumferentially until it meets the dorsal dressing.
- After the dressing is complete, place the mouse in a separate sterile cage and monitor it until it has fully recovered from the anesthetic.
- Repeat the procedure with all mice, regardless of the experimental group. Let the incision heal over the next 4 days before the next step.
3. Placement of HTS biomechanical loading device (POD 4)
NOTE: Clean and autoclave the HTS devices and several sets of surgical instruments before surgery (e.g., dissection scissors, scalpel, Adson forceps, Needle driver, skin stapler, skin staples). Prepare sterilized 5-0 sutures for use. [Optional] If a benchtop fume extractor is available, place the cone near workspace and turn on the suction.
- Prepare the area of surgery as described in steps 2.1 and 2.2.
- Anesthetize the mouse and provide analgesia by following steps 2.11-2.13.
- Use the forceps or needle driver to separate the dressing from the mouse's abdomen by working the tool side to side against the ventral side. Leaving the tool between the wrap and skin to elevate the dressing off the skin, use scissors to cut the dressing off.
NOTE: Be careful not to cut the mouse skin, rip the dressing off, or disturb the healing incision.
- Clean the dorsum with an alcohol swab. Examine the incision for wound dehiscence or signs of infection.
- Ensure the HTS device is not extended/expanded and is in its most reduced form, as shown in Figure 1B. Lightly coat the arms of the device with medical glue.
- With one hand, render the skin on the mouse dorsum slightly taut in the transverse direction. With the other hand, place the HTS device on the mouse dorsum such that the incision is equidistant from each arm of the HTS device, thereby centering the device over the incision. Ensure the skin between the arms of the HTS device is uniformly taut. Hold the device in place until the glue has dried (~30 s), as seen in Figure 2A.
NOTE: Keeping the skin uniformly taut is important to ensure equal amounts of tension are placed on the incision with the device. Leave the sutures intact during this process because the process of placing the device adds mechanical tension to the skin, which can reopen the healing incision. The sutures remain in place until the first day of stretching to ensure that the wound remains closed.
- Secure four sutures around each arm and through the skin as shown in Figure 2B. While suturing, make sure that the needle exits the skin towards the incision.
NOTE: Inserting the needle from the incision side of the device can sometimes tear the incision open due to the force necessary to puncture the skin.
- Now place three skin staples around the arms and through the skin, securing the HTS device to the skin as shown in Figure 2B.
- Bandage the mouse by following steps 2.17 and 2.18.
- Repeat the procedure with all mice, regardless of the experimental group.
NOTE: All mice receive the same preparation; however, they are randomly assigned in each experimental group (e.g., control, stretch) to ensure an unbiased distribution of mice in the experimental design. Specifically, both control and stretch mice will have the device attached to their dorsum. The control mice's devices will remain untouched, while the stretch mice will undergo the next steps.
4. Initial stretch of HTS biomechanical loading device (POD 5)
NOTE: Clean and autoclave several sets of surgical instruments (e.g., dissection scissors, scalpel, Adson forceps, Needle driver) prior to surgery. [Optional] If a benchtop fume extractor is available, place the cone near the workspace and turn on the suction.
- Randomly assign each mouse via ear tag to the desired group. For control mice, simply remove the sutures on the incision and change the dressings of the wounds (steps 1-6, and then 8-9). If desired, remove the device at the time of stretching and take a photo with a measuring implement for scale to track the scar size.
- Prepare the surgical area, anesthetize the mouse, and remove the dressing by performing steps 3.1-3.4.
NOTE: Be careful not to tear the dressing off or disturb the healing incision. If an arm of the HTS device has detached from the skin, lightly glue back into place and add a skin staple or suture depending on the size of the detachment.
- Remove the sutures from the wound with dissection scissors or other method of choice.
- Insert the HTS device key into the device and turn to expand the device until the skin is taut but not at risk of tearing the skin, as seen in Figure 2C.
NOTE: This initial distraction may take around 4-8 full turns of the key since the skin may be loose between the arms of the device.
- Bandage the mouse by performing steps 2.17 and 2.18.
5. Subsequent stretch of HTS biomechanical loading device (POD 7, 9, 11, 13, 15, 17)
NOTE: Clean and autoclave several sets of surgical instruments (e.g., dissection scissors, scalpel, Adson forceps, Needle driver) prior to surgery. [Optional] If benchtop fume extractor is available, place the cone near the workspace and turn on the suction.
- For control mice, simply change the dressings of the wounds (steps 1-5, and then steps 13-14). If desired, remove the device at the time of stretching and take a photo of the scar with a measuring implement for scale to track the scar size.
- Prepare the surgical area, anesthetize the mouse, and remove the dressing by following steps 3.1-3.4.
NOTE: If an arm of an HTS device has detached from the skin, glue it in place and add a skin staple or suture depending on the size of the detachment.
- Insert the HTS device key into the device and turn to expand the device until the skin is taut but not at risk of tearing the skin.
NOTE: This may take approximately 4 turns or ~2 mm of total distraction. If the device has reached maximum extension and cannot be extended further, perform steps 5.4-5.7 to take off and re-attach a new device. Otherwise, bandage the mouse by performing steps 2.17 and 2.18.
- If the device has reached maximum extension, remove the device from the mouse's dorsum by removing the staples with a staple remover or scissors by prying the prongs of the staples open. Then, remove the sutures and carefully remove the device.
- Clean the device with a scalpel, alcohol swabs, and paper towels. Soak the device in 70% ethanol for ~20 min to aid in cleaning. Then, use the key to contract the device to its thinnest form.
- Reattach the HTS device following steps 3.5-3.8.
- Bandage the mouse by following steps 2.17 and 2.18.
6. Harvesting the HTS tissue (POD 19)
NOTE: Harvesting tissue can take place at any point in the process. We have harvested tissue after only 4 days of stretch to examine early time points; however, tissue is most consistently harvested at POD 19 (2 weeks after strain was initiated). Clean and autoclave several sets of surgical instruments (e.g., dissection scissors, scalpel, Adson forceps) prior to surgery. To get photos of the scar over time, the device can be removed before each stretching step to take a photo of the scar before re-applying the device and re-initiating mechanical strain. [Optional] If a benchtop fume extractor is available, place the cone near the workspace and turn on the suction. The benchtop fume extractor may be turned off when the isoflurane gas is no longer being used.
- Prepare the surgical area, anesthetize the mouse, and remove the dressing by following steps 3.1-3.4.
- Sacrifice the mouse via cervical dislocation.
NOTE: Take care not to pull the skin and tear the mouse's dorsum.
- Use dissecting scissors or a skin staple remover to remove the skin staples. Cut off the sutures. Gently remove the HTS device, taking care not to tear the skin.
- Using either a scalpel or scissors, cut the skin surrounding the HTS scar. Preserve the skin for histological analysis in the desired fashion.
NOTE: If the skin is to be used for transcriptomic or protein analysis (e.g., qPCR, western blot, single-cell analysis), be sure to excise just the HTS scar tissue to minimize the amount of surrounding healthy tissue. This will ensure that the analysis will only capture the scar tissue.
- Scrape the HTS devices clean with a scalpel. Place devices in a beaker with 70% ethanol to soften any remaining adhesive or tissue.
NOTE: After soaking in 70% ethanol, the devices may require further wiping, scraping, and cleaning before autoclaving.
7. Measuring average scar width
NOTE: This was accomplished with image analysis software ImageJ, and the information was recorded on a spreadsheet.
- Open images in ImageJ. Trace the edges of the scar with the polygon tool. Click analyze | measure to measure this area.
NOTE: The scar can be identified by its discoloration and lack of hair follicles.
- Measure the length of the scar from end to end using the segment tool. Click analyze | measure to measure this length.
- Measure the length of 1 cm or any other standardized unit of length in the picture, using the segment tool. Click analyze | measure to measure this length.
- Using the spreadsheet, take the Area of the scar (in pixels; px) and divide that by the Length of the scar (px length). That result gives the average scar width in pixels.
- Divide that result by the measured standard unit of length (px length). The result will be the average scar width in the unit of the standard unit of length (e.g., cm) used for the experiment.