Pulmonary acini exhibit complex changes during postnatal rat lung development

Pulmonary acini represent the functional gas-exchanging units of the lung. Due to technical limitations, individual acini cannot be identified on microscopic lung sections. To overcome these limitations, we imaged the right lower lobes of instillation-fixed rat lungs from postnatal days P4, P10, P21, and P60 at the TOMCAT beamline of the Swiss Light Source synchrotron facility at a voxel size of 1.48 μm. Individual acini were segmented from the three-dimensional data by closing the airways at the transition from conducting to gas exchanging airways. For a subset of acini (N = 268), we followed the acinar development by stereologically assessing their volume and their number of alveoli. We found that the mean volume of the acini increases 23 times during the observed time-frame. The coefficients of variation dropped from 1.26 to 0.49 and the difference between the mean volumes of the fraction of the 20% smallest to the 20% largest acini decreased from a factor of 27.26 (day 4) to a factor of 4.07 (day 60), i.e. shows a smaller dispersion at later time points. The acinar volumes show a large variation early in lung development and homogenize during maturation of the lung by reducing their size distribution by a factor of 7 until adulthood. The homogenization of the acinar sizes hints at an optimization of the gas-exchange region in the lungs of adult animals and that acini of different size are not evenly distributed in the lungs. This likely leads to more homogeneous ventilation at later stages in lung development.

Effects of Pulmonary Fibrosis and Surface Tension on Alveolar Sac Mechanics in Diffuse Alveolar Damage

Diffuse alveolar damage (DAD) is a characteristic histopathologic pattern in most cases of acute respiratory distress syndrome and severe viral pneumonia, such as COVID-19. DAD is characterized by an acute phase with edema, hyaline membranes, and inflammation followed by an organizing phase with pulmonary fibrosis and hyperplasia. The degree of pulmonary fibrosis and surface tension is different in the pathological stages of DAD. The effects of pulmonary fibrosis and surface tension on alveolar sac mechanics in DAD are investigated by using the fluid-structure interaction (FSI) method. The human pulmonary alveolus is idealized by a three-dimensional honeycomb-like geometry, with alveolar geometries approximated as closely packed 14-sided polygons. A dynamic compression-relaxation model for surface tension effects is adopted. Compared to a healthy model, DAD models are created by increasing the tissue thickness and decreasing the concentration of the surfactant. The FSI results show that pulmonary fibrosis is more influential than the surface tension on flow rate, volume, P-V loop, and resistance. The lungs of the disease models become stiffer than those of the healthy models. According to the P-V loop results, the surface tension plays a more important role in hysteresis than the material nonlinearity of the lung tissue. Our study demonstrates the differences in air flow and lung function on the alveolar sacs between the healthy and DAD models.

Characterization of air flow and lung function in the pulmonary acinus by fluid-structure interaction in idiopathic interstitial pneumonias

Background and objective

The idiopathic interstitial pneumonias (IIPs) are diffuse parenchymal lung disorders that are associated with substantial morbidity and mortality. Early diagnosis and disease stratification of IIP patients are important because these are related with the treatment and prognosis. Idiopathic pulmonary fibrosis (IPF) and nonspecific interstitial pneumonia (NSIP) are two major distinctive pathologic patterns of pulmonary fibrosis. We researched the application of the fluid-structure interaction (FSI) to the respiratory system and compared the pulmonary acinus mechanics and functions in healthy and IIP models.

Methods

The human pulmonary alveolus is idealized by a three-dimensional honeycomb-like geometry, and a fluid-structure interaction analysis is performed to study the normal and diseased breathing mechanics. The computational domain consists of two generations of alveolar ducts within the pulmonary acinus, with alveolar geometries approximated as closely packed 14-sided polygons.

Findings

In a normal breathing cycle, the flow rate of the healthy model is significantly larger than that of the NSIP and IPF models. Similar trends are observed for the volume change and the maximum pressure drop. The flow rate and the volume change of the NSIP are almost the same as those of IPF. The maximum pressure drop of NSIP is 5.5% larger than that of IPF. There is a 47% decrease in the pulmonary acinus compliance for the NSIP and IPF compared with that of the healthy model. The acinus resistances of NSIP and IPF are higher than those of the healthy lung by 6.4~11.2%. In particular, the pulmonary acinus resistance of the NSIP lung is higher than that of the IPF lung by 4.5%.

Conclusions

Our study demonstrates the differences of air flow and lung function in the pulmonary acinus between the healthy and the IIP models. These changes in the lung are important considerations for early diagnosis and disease stratification in patients. Patient-based geometry can to be included in the computational models in future studies.