The over all goal of this surgical procedure is to enhance the understanding of the cellular and molecular processes of scar and adhesion formation during flexor tendon healing. This method can help answer key questions about the flexor tendon healing process, such as which cell types contribute to scar formation and repair. The main advantage of this technique is that transgenic and knock out mice can be used to better understand the healing process.
This technique may identify potential biological targets for improving the outcomes of flexor tendon healing as it facilitates the investigation of these poorly designed cell populations that contribute to scarring. After confirming a lack of response to toe pinch, apply a ophthalmic ointment to the animals eyes, and clip the fur from the entire hind limb. Sterilize the surgical site with sequential povedone iodine and 70%ethanol scrubs, and locate the flexor digitorum longus, or FDL tendon within the medial aspect of the calf.
Next, make a 0.5 to once centimeter incision in the skin to expose the tendon. Use the forceps to separate the FDL tendon from the surrounding tissue and trace the tendon up to the miotendinous junction. Using spring scissors, release the tendon at the junction, taking care to avoid the posterior tibial artery.
Then close the skin with 5-0 nylon sutures. Now transfer the mouse under a stereo microscope and use spring micro scissors to make a three millimeter incision over the posterolateral aspect of the hind paw. Use the forceps to gently retract the surrounding soft tissue and muscle, taking care to minimize the tissue damage, and identify the FDL tendon.
Then gently raise the tendon and use micro scissors to completely transect the tissue. Using 8.0 sutures in a modified Kessler pattern, ligate the ends of the FDL tendon. Periodically hydrating the tendon with saline throughout the procedure if necessary.
When the ends have been sutures, return the soft tissue and muscle over the tendon and use 5.0 nylon sutures to close the incision. Then place the mouse on a 37 degrees Celsius slide warmer with monitoring until fully recovered. 24 hours before the experimental end point, inject the mice with 100 micro liters of EDU.
The next day, locate the repaired tendon as just demonstrated, and harvest the transected tissue. Isolating the granulated tissue surrounding the healing tendon is a critical step for allowing the eventual aminocytochemical analysis. And care must be taken to minimize the inclusion of peripheral tissues.
Place the tendon in a petri dish containing one milliliter of collagenase. Then, mince the tissues with the scalpel followed by trituration of the fragments with several passes through an 18 gauge needle. Transfer the slurry into 1.5 milliliter centrifuge tubes and digest the tendon pieces for one hour and 37 degrees Celsius with shaking.
At the end of the digestion, centrifuge the tissue suspensions and resuspend the pellets in 500 micro liters of three percent BSA in PBS. Next, filter the cells through a 70 micron strainer to remove the debris and large pieces of undigested tendon. And place the cells in a 37 degrees Celsius incubator.
Transfer the undigested tendon pieces to new, 1.5 milliliter micro centrifuge tubes containing one milliliter of fresh collagenase, and mix the pieces by pipe heading. After another hour of digestion with shaking, spin down the tissue pieces again, and resuspend the pellets in 500 microliters of BSA supplemented PBS. Then filter the suspension through a new 70 micron cell strainer into the tube of cells collected after the first digestion.
Count the cells and spin them down by centrifugation. Then resuspend the pellet at a three to six times 10 to the fourth cells per 100 microliters of PBS supplemented with BSA concentration. To detect EDU incorporation by immunocytochemistry, first use a hydrophobic barrier pen to circle the cells on the slides and immediately fix the samples with 150 microliters of three percent paraformaldehyde and PBS at room temperature.
After 15 minutes, rinse the cells with two five minute washes in 150 microliters of BSA supplemented PBS. Then permeableize the samples with 0.5%triton X 100 in PBS for 20m minutes. After washing as just demonstrated, add 150 microliters of freshly prepared EDU reaction cocktail to the cells for a 30 minute incubation at room temperature.
Then wash the cells again and counter stain the samples with 150 microliters of Hoechst. After 30 minutes at room temperature, add one to two drops of antifade mounting medium to the cells, and carefully cover each sample with the cover slip, taking care to avoid bubbles. Cure the mounting medium over night in the dark at room temperature.
Then seal the cover slips with clear nail polish. Finally, image the slides on a fluorescent microscope using 40X magnification. In a successful repair, the granulation tissue displays a greatly increased cellularity compared to the uninjured tendon.
In this image, a histological example of a ruptured flexor tendon repair is shown. EDU labeling highlights only those cells that actively proliferate during the 24 hours following the EDU pulse in life mice. Facilitating a quantitative assessment of the proliferation within the granulation tissue of the healing tendon.
After watching this video you should have a good understanding of how to perform the flexor tendon injury and repair surgery in mice, to better understand the cell populations that contribute to scar formation in the flexor tendon during healing.
