Persistent structural adaptation in the lungs of guinea pigs raised at high altitude

Parabronchial remodeling in chicks in response to embryonic hypoxia

The embryonic development of parabronchi occurs mainly during the second half of incubation in precocious birds, which makes this phase sensitive to possible morphological modifications induced by O₂ supply limitation. Thus, we hypothesized that hypoxia during the embryonic phase of parabronchial development induces morphological changes that remain after hatching. To test this hypothesis, chicken embryos were incubated entirely (21 days) under normoxia or partially under hypoxia (15% O₂ during days 12 to 18). Lung structures, including air capillaries, blood capillaries, infundibula, atria, parabronchial lumen, bronchi, blood vessels larger than capillaries and interparabronchial tissue, in 1- and 10-day-old chicks were analyzed using light microscopy-assisted stereology. Tissue barrier and surface area of air capillaries were measured using electron microscopy-assisted stereology, allowing for calculation of the anatomical diffusion factor. Hypoxia increased the relative volumes of air and blood capillaries, structures directly involved in gas exchange, but decreased the relative volumes of atria in both groups of chicks, and the parabronchial lumen in older chicks. Accordingly, the surface area of the air capillaries and the anatomical diffusion factor were increased under hypoxic incubation. Treatment did not alter total lung volume, relative volumes of infundibula, bronchi, blood vessels larger than capillaries, interparabronchial tissue or the tissue barrier of any group. We conclude that hypoxia during the embryonic phase of parabronchial development leads to a morphological remodeling, characterized by increased volume density and respiratory surface area of structures involved in gas exchange at the expense of structures responsible for air conduction in chicks up to 10 days old.

Comparative analysis of the mechanical signals in lung development and compensatory growth

This review compares the manner in which physical stress imposed on the parenchyma, vasculature and thorax and the thoraco-pulmonary interactions, drive both developmental and compensatory lung growth. Re-initiation of anatomical lung growth in the mature lung is possible when the loss of functioning lung units renders the existing physiologic-structural reserves insufficient for maintaining adequate function and physical stress on the remaining units exceeds a critical threshold. The appropriate spatial and temporal mechanical interrelationships and the availability of intra-thoracic space, are crucial to growth initiation, follow-on remodeling and physiological outcome. While the endogenous potential for compensatory lung growth is retained and may be pharmacologically augmented, supra-optimal mechanical stimulation, unbalanced structural growth, or inadequate remodeling may limit functional gain. Finding ways to optimize the signal-response relationships and resolve structure-function discrepancies are major challenges that must be overcome before the innate compensatory ability could be fully realized. Partial pneumonectomy reproducibly removes a known fraction of functioning lung units and remains the most robust model for examining the adaptive mechanisms, structure-function consequences and plasticity of the remaining functioning lung units capable of regeneration. Fundamental mechanical stimulus-response relationships established in the pneumonectomy model directly inform the exploration of effective approaches to maximize compensatory growth and function in chronic destructive lung diseases, transplantation and bioengineered lungs.

Lung Structure and the Intrinsic Challenges of Gas Exchange

Structural and functional complexities of the mammalian lung evolved to meet a unique set of challenges, namely, the provision of efficient delivery of inspired air to all lung units within a confined thoracic space, to build a large gas exchange surface associated with minimal barrier thickness and a microvascular network to accommodate the entire right ventricular cardiac output while withstanding cyclic mechanical stresses that increase several folds from rest to exercise. Intricate regulatory mechanisms at every level ensure that the dynamic capacities of ventilation, perfusion, diffusion, and chemical binding to hemoglobin are commensurate with usual metabolic demands and periodic extreme needs for activity and survival. This article reviews the structural design of mammalian and human lung, its functional challenges, limitations, and potential for adaptation. We discuss (i) the evolutionary origin of alveolar lungs and its advantages and compromises, (ii) structural determinants of alveolar gas exchange, including architecture of conducting bronchovascular trees that converge in gas exchange units, (iii) the challenges of matching ventilation, perfusion, and diffusion and tissue-erythrocyte and thoracopulmonary interactions. The notion of erythrocytes as an integral component of the gas exchanger is emphasized. We further discuss the signals, sources, and limits of structural plasticity of the lung in alveolar hypoxia and following a loss of lung units, and the promise and caveats of interventions aimed at augmenting endogenous adaptive responses. Our objective is to understand how individual components are matched at multiple levels to optimize organ function in the face of physiological demands or pathological constraints.