To begin, place the dehydrated heart and lungs isolated from a mouse into a dissection dish containing 40 to 50 milliliters of PBS. Position it under a bright-field microscope with 10x magnification and set up the lighting. Then place the heart's dorsal surface on the dissection dish.
Using surgical scissors, carefully separate the ventricles from the atria after identifying the transition between the ventricular and atrial wall. Place the separated atria on the dissection dish with the heart base facing up. Orient the atria with the lung lobes positioned posteriorly, the left atrium and its appendage situated on the right and the right atrium and its appendage situated on the left.
Pin the remaining left ventricular tissue to the dissection dish to secure the preparation. Spread the lung lobes gently without exerting excessive force and pin them to the dissection dish. Next, find the aorta with the aortic arch in the center of the heart base.
Remove connective and fat tissue around the heart base while preserving all identified landmarks. Also, remove the aortic arch to clear the view of the heart base. Detect the pulmonary trunk directly to the right of the aorta and trace its path toward the back to identify the pulmonary arteries.
To confirm their position, track their route to the left lobe and right superior or middle lobes to pinpoint the lung hilum. Determine the location of the left and right main bronchi behind the aorta. Confirm their position by tracing their path towards the lung hilum.
Then identify the superior vena cava to the left of the aorta and trace its path into the right atrium near the right atrial appendage. Using blunt dissection with fine forceps, carefully detach the pulmonary arteries from the atrium's top surface, then sever them in the lung hilum and move them aside to unveil the left atrial free wall and the pulmonary veins in full view. Remove the main bronchi from the lung hilum and discard the bronchial tissue.
Examine the lung hilum to identify the ostium of the pulmonary artery and the main bronchus. To separate the atria and ventricles, make a cut from the front part of the remaining right ventricle and move upward through the tricuspid valve to the front side of the vena cava superior, opening the right atrium from the frontal side. Then cut the posterior right atrial free wall adjacent to the interatrial septum to create a division between the right and left atrium.
Cut the posterior right ventricular wall next to the interventricular septum to completely disconnect the left and right ventricles. Remove some of the basal lung tissue to trim the size of the lung lobes, leaving three to four millimeters of the lung tissue.
