Background: The growth of mature allografts is a critical issue in pediatric lung transplantation. This study explores the architectural changes of mature sheep lung when submitted to two different compensatory growth forces: either transplantation into a neonatal host or expansion in an otherwise empty adult hemithorax.
Methods: Right upper lobes (RUL) (mean+/-SEM, 66.7+/-1.9 kg) from 4- to 5-year old (adult sheep) were transplanted into newborn (n=6) lambs (5.4+/-0.3 kg, 5+/-2 days old) that were then allowed to survive for 45 days. Changes in pulmonary volume and architecture were measured before and after transplantation. Allografts were compared with both normal adult RUL (n=10) and adult (65.8+/-2.2 kg and 4 to 5 year old) RUL that remained in situ for 45 days after resection of the corresponding middle and lower lobes (n=6). Statistical differences were analyzed using two-sample and paired t tests.
Results: In adult animals, RUL remaining in the otherwise empty hemithorax compensated by an 85% increase in volume (251.5+/-18.7 ml vs. 466+/-32.8 ml) (P<0.0001). Concomitant increases in total internal alveolar surface area (48%) and alveolar size were prominent. The number of alveoli per volume decreased proportionately to the increases in volume (P<0.0001). There was no significant change in the calculated number of alveoli (345.6+/-40.5 x 10(6)) compared with the normal adult RUL (402.4+/-40.7x10(6)) (P=0.37). Transplant recipients received a reduced-size normal adult RUL (49%) in volume (125.3+/-21.5 ml). Allografts 45 days after transplantation showed a 73% increase in volume (216.4+/-21.3 ml) (P<0.0001) with a parallel (83%) increase in total internal alveolar surface area (P=0.008). The number of alveoli per volume remained constant (P=0.21) despite the increase in volume. There was therefore a significant increase in the calculated number of alveoli from before transplantation (172.5+/-35.9x 106) compared with that observed 45 days after transplantation (389.7+/-77.7x10(6)) (P=0.012).
Conclusions: We conclude that mature sheep RUL parenchyma compensates with dilation of the respiratory structures in the adult animal, whereas there is alveolar multiplication when transplanted into newborn recipients.