Deep pulmonary lymphatics in immature lungs.

Recently, we found that the translocation of inhaled nanoparticles from the air space to secondary organs is age dependent and substantially greater in neonates than in adults (J Respir Crit Care Med 177: A48, 2008). One reason for this difference might be age-dependent differences in alveolar barrier integrity. Because the neonate lung is undergoing morphogenetic and fluid balance changes, we hypothesize that the alveolar barrier of developing lungs is more easily compromised and susceptible to foreign material influx than that of adult lungs. On the basis of these hypotheses, we predict that the postnatally developing lung is also more likely to allow the translocation of some materials from the air space to the lymphatic lumens. To test this idea, we intratracheally instilled methyl methacrylate into immature and adult lungs and compared lymphatic filling between these two age groups. Scanning electron microscopy of the resultant corrosion casts revealed peribronchial saccular and conduit lymphatic architecture. Deep pulmonary lymphatic casts were present on the majority (58.5%) of airways in immature lungs, but lymphatic casting in adult lungs, as anticipated, was much more infrequent (21.6%). Thus the neonate lung appears to be more susceptible than the adult lung to the passage of instilled methyl methacrylate from the air space into the lymphatics. We speculate that this could imply greater probability of translocation of other materials, such as nanoparticles, from the immature lung as well.