Ultrastructural quantification of pulmonary intravascular macrophages in newborn and 2-week-old lambs

Pulmonary intravascular macrophages are resident cells in the pulmonary circulation of sheep. Sheep, unlike species without pulmonary intravascular macrophages, exhibit pulmonary hypertension in response to intravenously injected particles. We reported in lambs that pulmonary vascular reactivity to intravenous particles increases with age as the population of intravascular macrophages develops. Preliminary 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.

We fixed five newborn and five 2-week-old lamb lungs by vascular perfusion and examined the tissue by electron microscopy.

The fraction of capillary lumen 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 associated with intravascular macrophages/monocytes is about three times greater in 2-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 vs. 7 +/- 2; P < 0.05) and the estimated size (volume-weighted mean volume) increases by more than 1.5 times (294 +/- 46 microns 3 vs. 184 +/- 29 microns 3; P < 0.05).

These data agree closely with Monastral blue retention by the lung (reported previously); there are more than twice as many mature pulmonary intravascular macrophages at 2 weeks than at 1 day after birth, and the cells are 1.5 times larger.