We have shown that dimensions of alveoli determined by 3HeMR increase with age and lung size during childhood and adolescence at a rate much less than would be expected if lung growth occurred only by expansion of the preexisting airspaces. This is best explained by postulating that lung grows largely by neoalveolarization through childhood and adolescence. This contradicts the prevailing hypothesis that alveolarization is restricted to fetal life and early childhood. If confirmed by future studies, our findings have important clinical implications for the prognosis of lung disease and for the impact of drugs and environmental exposures during childhood.
The reliability of the 3
HeMR technique is critical to this interpretation. Our measurements were highly repeatable, with a within-subject coefficient of variation of ADC of 3.1%. The validity of ADC as a measure of pulmonary alveolar dimensions has been demonstrated by morphometry in animal models (27
) and in human lungs (29
). These studies demonstrate the use of ADC in noninvasively assessing peripheral airspace dimensions. Parameters derived from the second technique of MR (R
) have also been validated against human lung morphometry (30
We considered the possibility that our results could be explained by changes in geometry of lung acinus with growth rather than neoalveolarization. However, our q-space data, analyzed using the acinar model of Yablonskiy and coworkers (26
), show that both alveolar sleeve diameter and alveolar duct diameter increased less than expected with lung growth. In addition, because ADC and the parameters derived from q-space MR are measured with diffusion times of 14 and 5 milliseconds, respectively, they measure different geometric aspects of the peripheral airspaces and changes in relative geometry should be reflected in different relationships of these parameters with growth. However, both increase with FRC at a rate considerably slower than expected in the absence of neoalveolarization. Therefore, it is unlikely that changes in geometry occur with growth.
In the only other report of ADC in childhood, Altes and coworkers (31
) measured ADC in 29 healthy subjects ranging from 4–30 years. They also report an increase of ADC with age. However, they did not measure lung size by independent means and there was no attempt to determine an expected line for increase of ADC with age. Shanbhag and coworkers (17
in five children aged 6 years and in adults and found that
was lower in children than adults. There was no assessment of lung volume in this study. The mean age of adult subjects was 49 years, when effects of alveolar enlargement caused by senescence (25
) may have begun. In support of this,
reported for the five children was very close to the values in the two youngest adults (both 28 yr) in their study.
The belief that alveolarization in humans was complete by 3 years was based on studies using older methods of morphometry, the drawbacks of which have been discussed previously (33
). New methods of measuring alveolar number and size have since been developed (34
). However, human alveolarization has not yet been studied using these new techniques, largely because of ethical constraints that preclude the acquisition of suitable postmortem lung tissue.
Recent studies in mammals using new morphometric techniques support continued alveolarization to adulthood in rabbits (14
), rhesus monkeys (12
), and after pneumonectomy in mature dogs (13
). Massaro and coworkers (36
) showed evidence of calorie intake-related alveolar gain in adult mice refed after starvation. Schittny and coworkers (37
) observed an increase in alveolar number after completion of microvascular maturation in rats using design-based stereology. Using three-dimensional synchrotron radiation X-ray tomography, they showed local duplication of single capillary layers in areas of postmaturity septal growth, which indicated a potential mechanism for postmature alveolarization. This dispelled the notion that the immature double capillary layer in alveolar walls is a prerequisite for new septation. Thus, there is emerging evidence of alveolarization in maturing lungs in other mammals. There is indirect evidence that this also happens in humans. Brown and coworkers (38
) used electrical impedance tomography to determine an average alveolar number of 90 million at age 2–3 years compared with 300 million in adults, implying that alveolarization continues to take place after 3 years of age. Using a new unbiased morphometric technique, Ochs and coworkers (35
) showed that alveolar number was closely related to adult lung volume and that mean alveolar size was almost constant between subjects. If alveolarization were completed by 2–3 years, the final number of alveoli, and by extrapolation the final size of the lung, would have to be set by then, which is implausible. This provides more indirect evidence in favor of human alveolarization beyond early childhood.
The strengths of the study include the large number of volunteers from a wide age range spanning most of the period of lung growth. Prospectively collected data on early life exposures and preexisting lung disease were available (20
), which allowed selection of volunteers known to be healthy. Repeated MR measurements at varying inflation volumes were performed in some subjects, which facilitated the statistical test of the hypothesis of no new alveolarization. Independent measurement of lung size by plethysmography enabled the calculation of expected slope in the models. Finally, the application of two different techniques of MR permitted evaluation of lung geometry.
Studies of alveolar size and number in subjects at different ages, including our own, share the common assumption that the cross-sectional data are representative of longitudinal changes. We found wide interindividual variability in ADC, but our large number of volunteers and the repeatability of ADC measurements (within subjects) minimize the effect of this problem. Also, preliminary longitudinal data (not shown) corroborate the evidence from this study. Further longitudinal measurements of ADC will help refine the results of this study and begin to explain the wide population scatter of ADC values. The assumptions underpinning the multilevel regression model are transparent and the model fits the data well, suggesting that the assumptions are realistic (see online supplement).
There are important implications to our observation that alveolarization is not confined to early life. Children who die after extremely preterm birth have been shown to have larger, simpler, and fewer alveoli (39
), based on histologic studies of fatal cases. There have been no studies of alveolarization in survivors of neonatal chronic lung disease and it is possible that there is catch-up alveolarization. There may be hope of recovery from diseases that result in diminished alveolar number at birth (e.g., pulmonary hypoplasia caused by severe oligohydramnios or congenital diaphragmatic hernia) or later in life (e.g., surgical lung resection). Adverse environmental exposures can affect alveolar structure. Systemic corticosteroids in early postnatal life can inhibit alveolarization (40
). If alveolarization is not confined to early life, it is a possible that inhaled corticosteroids may be deleterious throughout childhood. Passive tobacco smoke exposure in childhood is linked to adult chronic obstructive pulmonary disease (42
). Impaired alveolarization may be a potential mechanism linking passive smoking during childhood to increased lung senescence and emphysema, and therefore chronic obstructive pulmonary disease. Finally, the advent of potential “alveolar therapy” to restore damaged alveolar structure (43
) requires safe, noninvasive repeatable measurements to study the outcome of future therapeutic trials. Hyperpolarized 3
HeMR provides such a method.
We conclude that there is evidence for continuing alveolarization through childhood and adolescence in humans. The exciting possibility of late alveolarization implies that the lung may be able to recover from damage occurring in early life. Emerging therapies have the potential to enhance this process. Conversely, environmental exposures during childhood could have a longer window of opportunity to impair the process of alveolarization, with potential adverse consequences throughout later life.