2.5. Preparation of Percoll solution with different concentrations (densities)

1× HBSS/10 mM HEPES [4-(2-hydroxyerhyl)piperazine-1-erhaesulfonic acid] solution preparation: 5 mL of 1 M HEPES (Invitrogen, USA) was added into 495 mL of 1× HBSS and mixed upside-down at room temperature. Stock isotonic Percoll (SIP) solution preparation: 18 mL of stock Percoll solution (GE Healthcare, USA) was added into 2 mL of 10× HBSS and mixed upside-down at room temperature, and the SIP solution was defined as 100% Percoll solution. Seventy percent (70%) Percoll solution preparation: 7 mL of SIP solution was added into 3 mL of 1× HBSS/10 mM HEPES and mixed upside-down at room temperature. Thirty-seven percent (37%) Percoll solution preparation: 3.7 mL of SIP solution was added into 6.3 mL of 1× HBSS/10 mM HEPES and mixed upside-down at room temperature. Thirty percent (30%) Percoll solution preparation: 3 mL of SIP solution was added into 7 mL of 1× HBSS/10 mM HEPES and mixed upside-down at room temperature [30,31].