![stereology-mayhew stereology-mayhew](https://www.frontiersin.org/files/Articles/158767/fnagi-07-00196-HTML/image_m/fnagi-07-00196-g006B.jpg)
![stereology-mayhew stereology-mayhew](https://els-jbs-prod-cdn.jbs.elsevierhealth.com/cms/attachment/533356/3674492/gr1.jpg)
Structural remodeling and airspace enlargement in aging human lungs was shown qualitatively ( D’Errico et al., 1989) as well as quantitatively, by estimation of mean linear intercept ( Verbeken et al., 1992a). Changes in micromechanics are furthermore closely related to structural alterations in the human lung. Investigations on age-related changes in lung micromechanics have shown that the elastic recoil pressure decreases in the elderly ( Janssens et al., 1999 Turner et al., 2017), possibly due to changes in tissue composition and remodeling ( Mercer and Crapo, 1990 Toshima et al., 2004 Subramaniam et al., 2017). The mechanical properties of the lung tissue that influence lung function are described by different parameters, such as lung stiffness and elastance ( Suki et al., 2011 Suki, 2014 Suki and Bartolák-Suki, 2014). These changes lead to increased respiratory impairment in the elderly ( Vaz Fragoso and Gill, 2012). Moreover, static lung compliance was found to increase with age ( Wahba, 1983 Levitzky, 1984).
![stereology-mayhew stereology-mayhew](https://i1.rgstatic.net/publication/283931890_Basic_Application_of_Stereology_in_Histology_and_Medical_Sciences/links/568a780e08aebccc4e19fc8e/largepreview.png)
For example, studies have shown an increased residual volume (RV) and functional reserve volume (FRV), as well as decreased values of forced expiratory volume in one second (FEV1) in elderly individuals ( Kerstjens et al., 1997 Janssens et al., 1999 Turner et al., 2017). Lung function is described by parameters of physiological breathing ( Becklake and Crapo, 1991 Pellegrino et al., 2005) that were observed to decline in the elderly ( Janssens et al., 1999). For better comprehension of pathological changes with age, it is therefore important to understand the changes of lung function, micromechanics, and structure during normal aging. These pathologies are also associated with alterations in pulmonary structure and function. In the elderly (>65 years of age), lung diseases like chronic obstructive pulmonary disease (COPD Ito and Barnes, 2009 Provinciali et al., 2011), fibrosis/IPF ( Raghu et al., 2006 Collard, 2010 Fell et al., 2010 Thannickal, 2013), cancer ( Howlader et al., 2019) or acute respiratory distress syndrome (ARDS Ely et al., 2002 Rubenfeld et al., 2005) occur more frequently and with greater severity than in younger individuals. Lung aging is accompanied by functional, micromechanical and structural alterations ( Chan and Welsh, 1998 Sharma and Goodwin, 2006 Pinkerton et al., 2014). Interestingly, despite age-related lung remodeling, the number of ATII cells per alveolus showed a tightly controlled relation in all age groups. These changes may partly explain the functional alterations during aging. In conclusion, alveolar size declined significantly in old mice concomitant with a widening of alveolar ducts and late alveolarization. Lung compliance and inspiratory capacity increased, whereas tissue elastance and tissue resistance decreased with age, showing greatest changes between young and middle-aged mice. The ATII cell number increased from middle-aged (8.8 × 10 6) to old (11.8 × 10 6) mice, along with the alveolar number, resulting in a constant ratio of ATII cells per alveolus in all age groups (1.4 ATII cells per alveolus). While the alveolar number decreased from young (7.5 × 10 6) to middle-aged (6 × 10 6) and increased again in old (9 × 10 6) mice, the mean alveolar volume and mean septal surface area per alveolus conversely first increased in middle-aged and then declined in old mice. Parenchymal volume, total and ductal airspace volume increased in old (18 and 24 months) compared with middle-aged (6 and 12 months) and young (3 months) mice. Lung parenchymal volume, total, ductal and alveolar airspace volume, alveolar volume and number, septal volume, septal surface area and thickness were quantified by stereology as well as surfactant producing alveolar epithelial type II (ATII) cell volume and number. Lung function analysis was performed in 3, 6, 12, 18, and 24 months old C57BL/6 mice ( n = 7–8/age), followed by lung fixation and stereological sample preparation. To improve our understanding of the structure-function relationships leading to this decline, we investigated structural alterations in the lung and their impact on micromechanics and lung function in the aging mouse. Lung function declines with advancing age. 3Biomedical Research in Endstage and Obstructive Lung Disease Hannover (BREATH), Member of the German Center for Lung Research (DZL), Hanover, Germany.2Cluster of Excellence REBIRTH (From Regenerative Biology to Reconstructive Therapy), Hanover, Germany.1Institute of Functional and Applied Anatomy, Hannover Medical School, Hanover, Germany.Henri Schulte 1, Christian Mühlfeld 1,2,3 and Christina Brandenberger 1,2,3*