Sprouting and intussusceptive angiogenesis in postpneumonectomy lung growth: mechanisms of alveolar neovascularization

Abstract

In most rodents and some other mammals, the removal of one lung results in compensatory growth associated with dramatic angiogenesis and complete restoration of lung capacity. One pivotal mechanism in neoalveolarization is neovascularization, because without angiogenesis new alveoli can not be formed. The aim of this study is to image and analyze three-dimensionally the different patterns of neovascularization seen following pneumonectomy in mice on a sub-micron-scale. C57/BL6 mice underwent a left-sided pneumonectomy. Lungs were harvested at various timepoints after pneumonectomy. Volume analysis by microCT revealed a striking increase of 143 percent in the cardiac lobe 14 days after pneumonectomy. Analysis of microvascular corrosion casting demonstrated spatially heterogenous vascular densitities which were in line with the perivascular and subpleural compensatory growth pattern observed in anti-PCNA-stained lung sections. Within these regions an expansion of the vascular plexus with increased pillar formations and sprouting angiogenesis, originating both from pre-existing bronchial and pulmonary vessels was observed. Also, type II pneumocytes and alveolar macrophages were seen to participate actively in alveolar neo-angiogenesis after pneumonectomy. 3D-visualizations obtained by high-resolution synchrotron radiation X-ray tomographic microscopy showed the appearance of double-layered vessels and bud-like alveolar baskets as have already been described in normal lung development. Scanning electron microscopy data of microvascular architecture also revealed a replication of perialveolar vessel networks through septum formation as already seen in developmental alveolarization. In addition, the appearance of pillar formations and duplications on alveolar entrance ring vessels in mature alveoli are indicative of vascular remodeling. These findings indicate that sprouting and intussusceptive angiogenesis are pivotal mechanisms in adult lung alveolarization after pneumonectomy. Various forms of developmental neoalveolarization may also be considered to contribute in compensatory lung regeneration.Electronic supplementary materialThe online version of this article (doi:10.1007/s10456-013-9399-9) contains supplementary material, which is available to authorized users.

Remodeling of alveolar septa after murine pneumonectomy

“…We estimate that 20 -30% of alveolar ducts in the cardiac lobe demonstrated retracted septa on day 3 after pneumonectomy. These results are consistent with previous findings of noncontiguous regions of alveolar angiogenesis in the subpleural cardiac lobe (1,23).…”

“…In contrast, highenergy imaging, such as the synchrotron, provides less than 1-m resolution (29,31). In studying fixed or casted lungs, the synchrotron has been effectively used to produce 3D imaging of the lung microstructure (1,21).…”

Repair and Regeneration of the Respiratory System: Complexity, Plasticity, and Mechanisms of Lung Stem Cell Function

“…Despite these intriguing observations, the precise cellular mechanisms that result in the dramatic increase in alveolar number and lung capacity remain unclear. Detailed anatomical studies, including scanning electron microscopy of vascular casts, suggest that alveolar number increases by the ingrowth of crests of tissue containing capillaries and stromal cells from the walls of preexisting alveoli, a process that mimics normal postnatal development (Figure 2) (Ackermann et al, 2013). Elucidation of the mechanisms that drive regrowth, and how they are attenuated as lung capacity is restored, may suggest potential therapeutic interventions for when repair does not occur properly.…”

Can We Understand the Pathobiology of Bronchopulmonary Dysplasia?

“…Specific recent advances include methodology to assess the number 186 and formation 187 of alveolar capillaries in the lung, and assembling serial histology sections to create digital three-dimensional reconstructions of the alveolo-capillary network 188, 189 , to permit visualization of perturbations that occur during aberrant lung development. It remains desirable to also study these changes by radiological imaging, such as that described to study alveolarization in human subjects with hyperpolarized gas in 3 He magnetic resonance imaging 190 ; however, this approach has not yet been applied to experimental animal models.…”