Structural and Biomechanical Properties of the Exchange Tissue of the Avian Lung

Maina, John N.

doi:10.1002/ar.23162

Cited by 15 publications

(4 citation statements)

References 101 publications

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“…) and lacks a dedicated support system such as that in the interalveolar septum of the mammalian lung (Weibel, , ; Maina, , , ,) (see insert in Fig. ), the strengths of the air and blood capillaries of the avian lung are perplexing (Maina & West, ; Maina & Sikiru, ; Maina, ,). Macklem et al () determined that the air capillaries remained open when a pressure of 20 cm H 2 O (2 kPa) was applied to the exchange tissue of a parabronchus.…”

Section: Structural Features Of the Lung And Air And Blood Capillariesmentioning

confidence: 99%

Pivotal debates and controversies on the structure and function of the avian respiratory system: setting the record straight

Maina

2016

Biological Reviews

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Among the extant air-breathing vertebrates, the avian respiratory system is structurally the most complex and functionally the most efficient gas exchanger. Having been investigated for over four centuries, some aspects of its biology have been extremely challenging and highly contentious and others still remain unresolved. Here, while assessing the most recent findings, four notable aspects of the structure and function of the avian respiratory system are examined critically to highlight the questions, speculations, controversies and debates that have arisen from past research. The innovative techniques and experiments that were performed to answer particular research questions are emphasised. The features that are outlined here concern the arrangement of the airways, the path followed by the inspired air, structural features of the lung and the air and blood capillaries, and the level of cellular defence in the avian respiratory system. Hitherto, based on association with the proven efficiency of naturally evolved and human-made counter-current exchange systems rather than on definite experimental evidence, a counter-current gas exchange system was suggested to exist in the avian respiratory system and was used to explain its exceptional efficiency. However, by means of an elegant experiment in which the direction of the air-flow in the lung was reversed, a cross-current system was shown to be in operation instead. Studies of the arrangement of the airways and the blood vessels corroborated the existence of a cross-current system in the avian lung. While the avian respiratory system is ventilated tidally, like most other invaginated gas exchangers, the lung, specifically the paleopulmonic parabronchi, is ventilated unidirectionally and continuously in a caudocranial (back-to-front) direction by synchronized actions of the air sacs. The path followed by the inspired air in the lung-air sac system is now known to be controlled by a mechanism of aerodynamic valving and not by anatomical valves or sphincters, as was previously supposed. The structural strength of the air and blood capillaries is derived from: the interdependence between the air and blood capillaries; a tethering effect between the closely entwined respiratory units; the presence of epithelial-epithelial cell connections (retinacula or cross-bridges) that join the blood capillaries while separating the air capillaries; the abundance and intricate arrangement of the connective tissue elements, i.e. collagen, elastin, and smooth muscle fibres; the presence of type-IV collagen, especially in the basement membranes of the blood-gas barrier and the epithelial-epithelial cell connections; and a putative tensegrity state in the lung. Notwithstanding the paucity of free surface pulmonary macrophages, the respiratory surface of the avian lung is well protected from pathogens and particulates by an assortment of highly efficient phagocytic cells. In commercial poultry production, instead of weak pulmonary cellular defence, stressful husbandry practices such a...

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Section: Structural Features Of the Lung And Air And Blood Capillariesmentioning

confidence: 99%

Pivotal debates and controversies on the structure and function of the avian respiratory system: setting the record straight

Maina

2016

Biological Reviews

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“…The evolutionary divergence between avian and mammalian respiratory systems has led to unique mechanisms of avian ventilation and gas exchange. Driven by selection pressures and aerobic metabolism required for flight, the avian lung maximizes gas exchange and oxygenation in part through an extremely thin blood-gas barrier [ 42 , 43 ]. Further, the avian respiratory system features a unidirectional airflow system in which inspired air is first directed to the posterior or caudal air sacs, followed by the lung itself, followed by the cranial or anterior air sacs, concluding with exhalation back through the trachea [ 44 , 45 ].…”

Section: Discussionmentioning

confidence: 99%

Avian lungs: A novel scaffold for lung bioengineering

et al. 2018

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Allogeneic lung transplant is limited both by the shortage of available donor lungs and by the lack of suitable long-term lung assist devices to bridge patients to lung transplantation. Avian lungs have different structure and mechanics resulting in more efficient gas exchange than mammalian lungs. Decellularized avian lungs, recellularized with human lung cells, could therefore provide a powerful novel gas exchange unit for potential use in pulmonary therapeutics. To initially assess this in both small and large avian lung models, chicken (Gallus gallus domesticus) and emu (Dromaius novaehollandiae) lungs were decellularized using modifications of a detergent-based protocol, previously utilized with mammalian lungs. Light and electron microscopy, vascular and airway resistance, quantitation and gel analyses of residual DNA, and immunohistochemical and mass spectrometric analyses of remaining extracellular matrix (ECM) proteins demonstrated maintenance of lung structure, minimal residual DNA, and retention of major ECM proteins in the decellularized scaffolds. Seeding with human bronchial epithelial cells, human pulmonary vascular endothelial cells, human mesenchymal stromal cells, and human lung fibroblasts demonstrated initial cell attachment on decellularized avian lungs and growth over a 7-day period. These initial studies demonstrate that decellularized avian lungs may be a feasible approach for generating functional lung tissue for clinical therapeutics.

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“…The air capillaries (ACs) (Fig. 2A,B), which are the terminal respiratory units of the avian lung, are rotund structures that interconnect across extremely narrow passageways (Maina, 2015) (Fig. 2A,B) while the blood capillaries (BCs) comprise spatially interconnected segments (Woodward & Maina, 2005, 2008; Maina, 2005, 2015; Maina & Woodward, 2009) (Fig.…”

Section: The Complex Structure Of the Avian Respiratory Systemmentioning

confidence: 99%

A critical assessment of the cellular defences of the avian respiratory system: are birds in general and poultry in particular relatively more susceptible to pulmonary infections/afflictions?

Maina

2023

Biological Reviews

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In commercial poultry farming, respiratory diseases cause high morbidities and mortalities, begetting colossal economic losses. Without empirical evidence, early observations led to the supposition that birds in general, and poultry in particular, have weak innate and adaptive pulmonary defences and are therefore highly susceptible to injury by pathogens. Recent findings have, however, shown that birds possess notably efficient pulmonary defences that include: (i) a structurally complex three‐tiered airway arrangement with aerodynamically intricate air‐flow dynamics that provide efficient filtration of inhaled air; (ii) a specialised airway mucosal lining that comprises air‐filtering (ciliated) cells and various resident phagocytic cells such as surface and tissue macrophages, dendritic cells and lymphocytes; (iii) an exceptionally efficient mucociliary escalator system that efficiently removes trapped foreign agents; (iv) phagocytotic atrial and infundibular epithelial cells; (v) phagocytically competent surface macrophages that destroy pathogens and injurious particulates; (vi) pulmonary intravascular macrophages that protect the lung from the vascular side; and (vii) proficiently phagocytic pulmonary extravasated erythrocytes. Additionally, the avian respiratory system rapidly translocates phagocytic cells onto the respiratory surface, ostensibly from the subepithelial space and the circulatory system: the mobilised cells complement the surface macrophages in destroying foreign agents. Further studies are needed to determine whether the posited weak defence of the avian respiratory system is a global avian feature or is exclusive to poultry. This review argues that any inadequacies of pulmonary defences in poultry may have derived from exacting genetic manipulation(s) for traits such as rapid weight gain from efficient conversion of food into meat and eggs and the harsh environmental conditions and severe husbandry operations in modern poultry farming. To reduce pulmonary diseases and their severity, greater effort must be directed at establishment of optimal poultry housing conditions and use of more humane husbandry practices.

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Structural and Biomechanical Properties of the Exchange Tissue of the Avian Lung

Cited by 15 publications

References 101 publications

Pivotal debates and controversies on the structure and function of the avian respiratory system: setting the record straight

Pivotal debates and controversies on the structure and function of the avian respiratory system: setting the record straight

Avian lungs: A novel scaffold for lung bioengineering

A critical assessment of the cellular defences of the avian respiratory system: are birds in general and poultry in particular relatively more susceptible to pulmonary infections/afflictions?

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