ULTRASTRUCTURAL QUANTIFICATION OF PULMONARY INTRAVASCULAR MACROPHAGESIN NEWBORN AND 2-WEEK-OLD LAMBS

Background: Pulmonary intravascular macrophages are resident cells in the pulmonary circulation of sheep. Sheep, unlike species without pulm onary intravascular macrophages, exhibit pulmonary hypertension in res ponse to intravenously injected particles. We reported in lambs that p ulmonary vascular reactivity to intravenous particles increases with a ge as the population of intravascular macrophages develops. Preliminar y quantitative histologic data showed that newborn lambs are born with few intravascular macrophages, but a large population develops over 2 weeks after birth. In this study, we present a complete quantitative analysis at the ultrastructural level. Methods: We fixed five newborn and five 2-week-old lamb lungs by vascular perfusion and examined the tissue by electron microscopy. Results: The fraction of capillary lume n taken up by intravascular macrophages/monocytes is about three times greater in the lungs of 2-week-old lambs than that in newborn lambs ( 16% vs. 5%; P < 0.05). The fraction of capillary surface density assoc iated with intravascular macrophages/monocytes is about three times gr eater in a-week-old lambs than that in newborn lambs (8% vs, 3%; P < 0 .05). The number of macrophages more than doubles with age (16 +/- 4 v s, 7 +/- 2; P < 0.05) and the estimated size (volume-weighted mean vol ume) increases by more than 1.5 times (294 +/- 46 mu m(3) vs. 184 +/- 29 mu(3); P < 0.05). Conclusions: These data agree closely with Monast ral blue retention by the lung (reported previously); there are more t han twice as many mature pulmonary intravascular macrophages at 2 week s than at 1 day after birth, and the cells are 1.5 times larger. (C) 1 996 Wiley-Liss, Inc.