Introduction: Human neonatal PMNs exhibit decreased mobility, adherence, and transendothelial migration in vitro compared to adult PMNs. Neonatal animals also demonstrate decreased extravasation of leukocytes to inflamed foci. Using intravital microscopy we examined directly leukocyte(LEU) margination and adhesion in mesenteric vessels of neonatal and adult rabbits.

Materials and Methods: Term New Zealand white rabbits were studied at 0-7 days (neonate, N), at 8-14 days (infant, I), and adult (1-1.5 Kg, A). Through a midline abdominal incision, a small portion of the mesentery was exteriorized and an intravital microscope with a video camera was used to observe and record the mesentery's microcirculation. We determined venular diameter, number of rolling and adherent leukocytes and leukocyte rolling velocity. Shear rate was calculated from determinations of RBC velocity. WBC was determined.

Results: There were no differences in WBC between the groups, though N had decreased absolute neutrophil counts (2458 ± 1364), compared A (5901± 705,p=0.008). The number of LEU rolling past a fixed point on the vessel wall (LEU flux) was not different between groups. There were fewer LEU rolling along 60μm of vessel in one minute in N and I groups compared to A (LEU ± SEM: N, 13±4; I, 15 ±5; A, 31±15; p=0.033). In addition there were fewer LEU adherent/60μm in N and I groups compared to A (LEU ± SEM: N, 1.7±1; I, 1.2±1; A, 14±7; p=0.004). There were no differences in shear rate and red blood cell velocity between the groups. The velocity of LEU rolling was decreased in adult rabbits (p=0.051).

Conclusion: N and I animals have an equal number of LEU that interact with the vessel wall (LEU flux), but have a decreased number of LEU that roll for a long segment and remain adherent relative to adult. The differences in leukocyte rolling and adhesion between immature and A animals may be due to differences in expression and/or function of adhesion molecules. These differences may result in delayed leukocyte emigration, and are the subject of future investigation.