library_construction_protocol: Dissected mitral valves, including anterior and posterior leaflets, were digested using digestion buffer containing 2 mg/ml Collagenase II, 2 mg/ml Collagenase IV, 0.65 mg/ml Dispase II, 0.1 mg/ml Elastase, and 1 mg/ml HAase I in HBSS. The valves were incubated in digestion buffer at 37°C for 15 minutes × 2. The digestion was then stopped by DMEM+10% FBS on ice. The cell suspensions were filtered using Falcon Test Tube with 35 μm Cell Strainer and centrifuged. The cell pellet was resuspended in 50 μl DMEM+10% FBS for subsequent single-cell RNA sequencing. Cells were processed using the Chromium Single Cell 3' v3.1 Kit from 10× Genomics.