Passive Recruitment of Circulating Leukocytes into Capillary Sprouts from Existing Capillaries

Recent evidence implicating leukocytes in angiogenesis raised the question of whether leukocytes and other circulating cells can home to capillary sprouts intraluminally. The objective of this study was to investigate this possibility by studying the traffic of leukocytes in artificial capillaries (8–12 μm in diameter) with sprouts of different length and sprouting angle fabricated using soft lithography. Leukocytes were indeed able to enter capillary sprouts, in spite of the complete absence of blood flow through the blind‐ended sprouts and any chemoattractants. The red blood cells (RBCs) of the “comet tails” (the densely packed cellular trains forming behind leukocytes as they move through narrow capillaries) effectively pushed leukocytes into the sprouts while bypassing them at the sprouting bifurcation. Individual sprouts filled with several leukocytes, as wells as RBCs and platelets were observed. The results of this study suggest that (i) blood cells are likely present in capillary sprouts throughout their development, (ii) leukocytes and other circulating cells may use this mechanism to home to capillary sprouts intraluminally for direct engraftment, and (iii) tissues may use this phenomenon as another mechanism for local recruitment of leukocytes from the blood stream.