Comparative respiratory morphology: themes and principles in the design and construction of the gas exchangers

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?

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.

Perspectives on the Structure and Function of the Avian Respiratory System: Functional Efficiency Built on Structural Complexity

Among the air-breathing vertebrates, regarding respiratory efficiency, the avian respiratory system rests at the evolutionary zenith. Structurally, it is separated into a lung that serves as a gas exchanger and air sacs that mechanically ventilate the lung continuously and unidirectionally in a caudocranial direction. Largely avascular, the air sacs are delicate, transparent, compliant and capacious air-filled spaces that are not meaningfully involved in gas exchange. The avian lungs are deeply and firmly attached to the vertebrae and the ribs on the dorsolateral aspects, rendering them practically rigid and inflexible. The attachment of the lung to the body wall allowed extreme subdivision of the exchange tissue into minuscule and stable terminal respiratory units, the air capillaries. The process generated a large respiratory surface area in small lungs with low volume density of gas exchange tissue. For the respiratory structures, invariably, thin blood-gas barrier, large respiratory surface area and large pulmonary capillary blood volume are the foremost adaptive structural features that confer large total pulmonary morphometric diffusing capacities of O2. At parabronchial level, the construction and the arrangement of the airway- and the vascular components of the avian lung determine the delivery, the presentation and the exposure of inspired air to capillary blood across the blood-gas barrier. In the avian lung, crosscurrent-, countercurrent- and multicapillary serial arterialization systems that stem from the organization of the structural parts of the lung promote gas exchange. The exceptional respiratory efficiency of the avian respiratory system stems from synergy of morphological properties and physiological processes, means by which O2 uptake is optimized and high metabolic states and capacities supported. Given that among the extant animal taxa insects, birds and bats (which accomplished volancy chronologically in that order) possess structurally much different respiratory systems, the avian respiratory system was by no means a prerequisite for evolution of powered flight but was but one of the adaptive solutions to realization of an exceptionally efficient mode of locomotion.

Decellularization and Recellularization Methods for Avian Lungs: An Alternative Approach for Use in Pulmonary Therapeutics

The shortage of compatible allogeneic organs and an increase in the number of patients requiring long-term lung assist devices while waiting for lung transplantation have motivated scientists to explore alternatives to bioengineer new lungs, including through decellularization and recellularization processes. A novel approach for bioengineering an extracorporeal membrane oxygenator is based on the parenchymal structure of avian lungs which utilizes a cross-current unidirectional flow of air and blood rather than bidirectional airflow, and thus eliminates dead-space ventilation. This provides more efficient gas exchange than mammalian lungs. The novel approach utilized is to decellularize avian lungs and then to recellularize with patient-derived human lung epithelial and vascular endothelial cells with the goal of creating a fully functional structure that can be used as a gas-exchange device. Here, we present avian lung decellularization and recellularization methods for chicken and emu lungs, in order to study both small- and large-scale avian lung models. For decellularization, a detergent-based protocol is utilized, and different techniques are used to validate the de- and recellularization of those lungs, including microscopy, mass spectrometry, and immunohistochemical analyses. For recellularization, techniques for seeding different human lung cell types into the decellularized scaffolds are presented.

Hypoxia inducible factor 1-α is minimally involved in determining the time domains of the hypoxic ventilatory response in adult zebrafish (Danio rerio)

In the current study, adult zebrafish (Danio rerio) were exposed to 72 h hypoxia (90 mmHg) to assess the time domains of the hypoxia ventilatory response (HVR) and the consequence on a subsequent more severe (40 mmHg) bout of acute hypoxia. Experiments were performed on wild-type fish and mutants in which one or both paralogs of hypoxia inducible factor-1α (hif-1α) were knocked out. Although there were subtle differences among the wild-type and knockout fish, resting f_V was reestablished after 2-8 h of continuous hypoxia in both groups, a striking example of hypoxic ventilatory decline (HVD). When fish were subsequently exposed to more severe hypoxia, a rapid increase in f_V was observed, the magnitude of which was independent of genotype or prior exposure history. During recovery, fish that had been exposed to 72 h of 90 mmHg hypoxia exhibited a pronounced undershoot in f_V, which was absent in the hif-1α double knockouts. Overall, the results revealed distinct time domains of the HVR in zebrafish that were largely Hif-1α-independent.

Comparative morphometric analysis of lungs of the semifossorial giant pouched rat (Cricetomys gambianus) and the subterranean Nigerian mole rat (Cryptomys foxi)

Lungs of the rodent species, the African giant pouched rat (Cricetomys gambianus) and the Nigerian mole rat (Cryptomys foxi) were investigated. Significant morphometric differences exist between the two species. The volume of the lung per unit body mass was 2.7 times larger; the respiratory surface area 3.4 times greater; the volume of the pulmonary capillary blood 2 times more; the harmonic mean thickness of the blood-gas (tissue) barrier (τht) ~29% thinner and; the total pulmonary morphometric diffusing capacity (DLo2) for O2 2.3 times more in C. foxi. C. gambianus occupies open burrows that are ventilated with air while C. foxi lives in closed burrows. The less morphometrically specialized lungs of C. gambianus may be attributed to its much larger body mass (~6 times more) and possibly lower metabolic rate and its semifossorial life whereas the 'superior' lungs of C. foxi may largely be ascribed to the subterranean hypoxic and hypercapnic environment it occupies. Compared to other rodents species that have been investigated hitherto, the τht was mostly smaller in the lungs of the subterranean species and C. foxi has the highest mass-specific DLo2. The fossorial- and the subterranean rodents have acquired various pulmonary structural specializations that relate to habitats occupied.

Functional and evolutionary perspectives on gill structures of an obligate air-breathing, aquatic snail

Ampullariids are freshwater gastropods bearing a gill and a lung, thus showing different degrees of amphibiousness. In particular, Pomacea canaliculata (Caenogastropoda, Ampullariidae) is an obligate air-breather that relies mainly or solely on the lung for dwelling in poorly oxygenated water, for avoiding predators, while burying in the mud during aestivation, and for oviposition above water level. In this paper, we studied the morphological peculiarities of the gill in this species. We found (1) the gill and lung vasculature and innervation are intimately related, allowing alternation between water and air respiration; (2) the gill epithelium has features typical of a transporting rather than a respiratory epithelium; and (3) the gill has resident granulocytes within intraepithelial spaces that may serve a role for immune defence. Thus, the role in oxygen uptake may be less significant than the roles in ionic/osmotic regulation and immunity. Also, our results provide a morphological background to understand the dependence on aerial respiration of Pomacea canaliculata. Finally, we consider these findings from a functional perspective in the light of the evolution of amphibiousness in the Ampullariidae, and discuss that master regulators may explain the phenotypic convergence of gill structures amongst this molluscan species and those in other phyla.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Avian lungs: A novel scaffold for lung bioengineering

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.

Comparison of the tracheal systems of Anopheles sinensis and Aedes togoi larvae using synchrotron X-ray microscopic computed tomography (respiratory system of mosquito larvae using SR-μCT)

Mosquito-borne diseases, such as malaria, dengue fever, and Zika virus, are serious global health issues. Vector control may be an important strategy in reducing the mortality caused by these diseases. The respiratory system of mosquito larvae in the water has to inhale atmospheric oxygen as aquatic organisms. In this study, the three-dimensional (3D) structures of the dorsal longitudinal trunks (DLTs) of the tracheal systems of Anopheles sinensis and Aedes togoi were compared using synchrotron X-ray microscopic computed tomography. DLT respiratory frequencies were also investigated. Interestingly, the larvae of the two mosquito species exhibit tracheal systems that are both morphologically and functionally distinct. A. sinensis hangs horizontally under the water surface, and has a smaller DLT volume than A. togoi. In contrast, A. togoi hangs upside down using a siphon by fixing its tip to the water surface. The frequency of peristaltic movement in A. togoi is higher than that of A. sinensis. These differences in the structures and breathing behaviors of the respiratory systems of mosquito larvae provide new insights into the tracheal systems of mosquito larvae, which should help develop novel effective control strategies targeting mosquito larvae.

Morphological Pulmonary Diffusion Capacity for Oxygen of Burmese Pythons (Python molurus): a Comparison of Animals in Healthy Condition and with Different Pulmonary Infections

A qualitative and quantitative morphological study of the pulmonary exchange capacity of healthy and diseased Burmese pythons (Python molurus) was carried out in order to test the hypothesis that the high morphological excess capacity for oxygen exchange in the lungs of these snakes is one of the reasons why pathological processes extend throughout the lung parenchyma and impair major parts of the lungs before clinical signs of respiratory disease become apparent. Twenty-four Burmese pythons (12 healthy and 12 diseased) were included in the study. A stereology-based approach was used to quantify the lung parenchyma using computed tomography. Light microscopy was used to quantify tissue compartments and the respiratory exchange surface, and transmission electron microscopy was used to measure the thickness of the diffusion barrier. The morphological diffusion capacity for oxygen of the lungs and the anatomical diffusion factor were calculated. The calculated anatomical diffusion capacity was compared with published values for oxygen consumption of healthy snakes, and the degree to which the exchange capacity can be obstructed before normal physiological function is impaired was estimated. Heterogeneous pulmonary infections result in graded morphological transformations of pulmonary parenchyma involving lymphocyte migration into the connective tissue and thickening of the septal connective tissue, increasing thickness of the diffusion barrier and increasing transformation of the pulmonary epithelium into a columnar pseudostratified or stratified epithelium. The transformed epithelium developed by hyperplasia of ciliated cells arising from the tip of the faveolar septa and by hyperplasia of type II pneumocytes. These results support the idea that the lungs have a remarkable overcapacity for oxygen consumption and that the development of pulmonary disease continuously reduces the capacity for oxygen consumption. However, due to the overcapacity of the lungs, this reduction does not result in clinical signs and disease can progress unrecognized for an extended period.

Hydrostatic pressure activates ATP-sensitive K+ channels in lung epithelium by ATP release through pannexin and connexin hemichannels

Lungs of air-breathing vertebrates are constantly exposed to mechanical forces and therefore are suitable for investigation of mechanotransduction processes in nonexcitable cells and tissues. Freshly dissected Xenopus laevis lungs were used for transepithelial short-circuit current (ISC) recordings and were exposed to increased hydrostatic pressure (HP; 5 cm fluid column, modified Ussing chamber). I(SC) values obtained under HP (I(5cm)) were normalized to values before HP (I(0cm)) application (I(5cm)/I(0cm)). Under control conditions, HP decreased I(SC) (I(5cm)/I(0cm)=0.84; n=68; P<0.0001). This effect was reversible and repeatable ≥30 times. Preincubation with ATP-sensitive K(+) channel (K(ATP)) inhibitors (HMR1098 and glibenclamide) prevented the decrease in I(SC) (I(5cm)/I(0cm): HMR1098=1.19, P<0.0001; glibenclamide=1.11, P<0.0001). Similar effects were observed with hemichannel inhibitors (I(5cm)/I(0cm): meclofenamic acid=1.09, P<0.0001; probenecid=1.0, P<0.0001). The HP effect was accompanied by release of ATP (P<0.05), determined by luciferin-luciferase luminescence in perfusion solution from the luminal side of an Ussing chamber. ATP release was abrogated by both meclofenamic acid and probenecid. RT-PCR experiments revealed the expression of pannexin and connexin hemichannels and KATP subunit transcripts in X. laevis lung. These data show an activation of KATP in pulmonary epithelial cells in response to HP that is induced by ATP release through mechanosensitive pannexin and connexin hemichannels. These findings represent a novel mechanism of mechanotransduction in nonexcitable cells.

Ventilation patterns of the songbird lung/air sac system during different behaviors

Unidirectional, continuous airflow through the avian lung is achieved through an elaborate air sac system with a sequential, posterior to anterior ventilation pattern. This classical model was established through various approaches spanning passively ventilated systems to mass spectrometry analysis of tracer gas flow into various air sacs during spontaneous breathing in restrained ducks. Information on flow patterns in other bird taxa is missing, and these techniques do not permit direct tests of whether the basic flow pattern can change during different behaviors. Here we use thermistors implanted into various locations of the respiratory system to detect small pulses of tracer gas (helium) to reconstruct airflow patterns in quietly breathing and behaving (calling, wing flapping) songbirds (zebra finch and yellow-headed blackbird). The results illustrate that the basic pattern of airflow in these two species is largely consistent with the model. However, two notable differences emerged. First, some tracer gas arrived in the anterior set of air sacs during the inspiration during which it was inhaled, suggesting a more rapid throughput through the lung than previously assumed. Second, differences in ventilation between the two anterior air sacs emerged during calling and wing flapping, indicating that adjustments in the flow pattern occur during dynamic behaviors. It is unclear whether this modulation in ventilation pattern is passive or active. This technique for studying ventilation patterns during dynamic behaviors proves useful for establishing detailed timing of airflow and modulation of ventilation in the avian respiratory system.

Study of nebulization delivery of aerosolized fluorescent microspheres to the avian respiratory tract

This study investigated the delivery of an aerosol of monodisperse microspheres to the respiratory tract of birds following aerosol exposure. Adult domestic pigeons (Columbia livia domestica, n = 5 birds per timed treatment) were exposed to an aerosol of fluorescent 1.0 microm diameter carboxylate microspheres for 0.5, 1, 2, or 4 hr. During the aerosolization period, the birds were free-standing in a plexiglass treatment chamber and the aerosol was delivered using a commercial nebulizer. Immediately following aerosol exposure, the birds were euthanatized and the carcasses were intravenously infused with a modified paraformaldehyde/gluteraldehyde fixative. Evaluation of microsphere distribution was performed using a stereoscopic microscope with an epifluorescent module. The results from this study revealed that the amount of aerosolized particles delivered using a commercial nebulizer was proportional to exposure periods. Aerosol exposure periods of 0.5 hr or 1 hr did not result in a readily observable distribution of 1.0 microm fluorescent microspheres to the cranial thoracic, caudal thoracic, or abdominal air sac membranes. This was partly attributed to the relatively low concentration of the individual monodisperse microspheres in the aerosolized suspension. The 2- and 4-hr exposure periods resulted in readily observable deposition of the 1.0 mirom fluorescent microspheres in the cranial thoracic, caudal thoracic, or abdominal air sac membranes, with the 4-hr exposure period resulting in the greatest number of particles on the membrane surfaces. For each of the exposure periods, there was individual animal variation regarding the distribution and relative number of spheres deposited. This study demonstrates the widespread deposition of particles that had an aerodynamic equivalent diameter of approximately 1 microm and provides a better understanding of particle deposition efficiency within the respiratory system following aerosol exposure in birds.

Why Do We have to Move Fluid to be Able to Breathe?

The ability to breathe air represents a fundamental step in vertebrate evolution that was accompanied by several anatomical and physiological adaptations. The morphology of the air-blood barrier is highly conserved within air-breathing vertebrates. It is formed by three different plies, which are represented by the alveolar epithelium, the basal lamina, and the endothelial layer. Besides these conserved morphological elements, another common feature of vertebrate lungs is that they contain a certain amount of fluid that covers the alveolar epithelium. The volume and composition of the alveolar fluid is regulated by transepithelial ion transport mechanisms expressed in alveolar epithelial cells. These transport mechanisms have been reviewed extensively. Therefore, the present review focuses on the properties and functional significance of the alveolar fluid. How does the fluid enter the alveoli? What is the fate of the fluid in the alveoli? What is the function of the alveolar fluid in the lungs? The review highlights the importance of the alveolar fluid, its volume and its composition. Maintenance of the fluid volume and composition within certain limits is critical to facilitate gas exchange. We propose that the alveolar fluid is an essential element of the air-blood barrier. Therefore, it is appropriate to refer to this barrier as being formed by four plies, namely (1) the thin fluid layer covering the apical membrane of the epithelial cells, (2) the epithelial cell layer, (3) the basal membrane, and (4) the endothelial cells.

Ion transport by pulmonary epithelia

The lung surface of air-breathing vertebrates is formed by a continuous epithelium that is covered by a fluid layer. In the airways, this epithelium is largely pseudostratified consisting of diverse cell types such as ciliated cells, goblet cells, and undifferentiated basal cells, whereas the alveolar epithelium consists of alveolar type I and alveolar type II cells. Regulation and maintenance of the volume and viscosity of the fluid layer covering the epithelium is one of the most important functions of the epithelial barrier that forms the outer surface area of the lungs. Therefore, the epithelial cells are equipped with a wide variety of ion transport proteins, among which Na⁺, Cl⁻, and K⁺ channels have been identified to play a role in the regulation of the fluid layer. Malfunctions of pulmonary epithelial ion transport processes and, thus, impairment of the liquid balance in our lungs is associated with severe diseases, such as cystic fibrosis and pulmonary oedema. Due to the important role of pulmonary epithelial ion transport processes for proper lung function, the present paper summarizes the recent findings about composition, function, and ion transport properties of the airway epithelium as well as of the alveolar epithelium.

Evidence for basolateral Cl- channels as modulators of apical Cl- secretion in pulmonary epithelia of Xenopus laevis

Pulmonary epithelia of air-breathing vertebrates are covered by a thin, fluid layer that is essential for immune defense and gas diffusion. The composition of this layer is maintained by ion transport mechanisms, including Cl(-) transport. The present study focuses on the function of basolateral Cl(-) channels in Xenopus pulmonary epithelia, since knowledge concerning this issue is limited. Therefore, Ussing chamber measurements were performed, and transepithelial short-circuit currents (I(SC)) were monitored. Basolateral application of the Cl(-) channel inhibitor N-phenylanthranilic acid (DPC) resulted in an increase of the I(SC), indicating a DPC-sensitive Cl(-) conductance. This observation was confirmed in experiments using an apical-to-basolateral Cl(-) gradient, with and without nystatin (apical side) to permeabilize the epithelia as well as by establishing an iodide gradient. The DPC-sensitive Cl(-) conductance was influenced by procedures interfering with apical Cl(-) secretion. For example, the effect of forskolin was increased when basolateral Cl(-) channels were blocked by the simultaneous application of DPC. Activation of apical Cl(-) secretion by forskolin/IBMX and subsequent DPC application resulted in a significantly reduced DPC effect. Accordingly, DPC led to an increased apical Cl(-) secretion estimated by an increased 5-nitro-2-(3-phenylpropylamino)benzoic acid-sensitive I(SC). Furthermore, inhibition of basolateral anion exchangers responsible for Cl(-) uptake resulted in a decreased DPC-sensitive current. Taken together, we have evidence concerning the function of basolateral Cl(-) channels in Xenopus pulmonary epithelium and that these channels play a significant role in mediating apical Cl(-) secretion involving a novel Cl(-) recycling mechanism across the basolateral membrane.

Leptin stimulates Xenopus lung development: evolution in a dish

The transition from uni- to multicellular organisms required metabolic cooperativity through cell-cell interactions mediated by soluble growth factors. We have empirically demonstrated such an integrating mechanism by which the metabolic hormone leptin stimulates lung development, causing the thinning of the gas exchange surface and the obligate increase in lung surfactant synthesis. All of these processes have occurred both phylogenetically and developmentally during the course of vertebrate lung evolution from fish to man. Here we show the integrating effects of the environmentally sensitive, pleiotropic hormone leptin on the development of the Xenopus laevis tadpole lung. The process described in this study provides a mechanistically integrated link between the metabolic regulatory hormone leptin and its manifold downstream effects on a wide variety of physiologic structures and functions, including locomotion and respiration, the cornerstones of land vertebrate evolution. It provides physiologic selection pressure at multiple levels to progressively generate Gene Regulatory Networks both within and between organs, from cells to systems. This model provides a cipher for understanding the evolution of complex physiology.

Exploiting cellular-developmental evolution as the scientific basis for preventive medicine

In the post-genomic era, we must make maximal use of this technological advancement to broaden our perspective on biology and medicine. Our understanding of the evolutionary process is undermined by looking at it retrospectively, perpetuating a descriptive rather than a mechanistic approach. The reintroduction of developmental biologic principles into evolutionary studies, or evo-devo, allows us to apply embryologic cell-molecular biologic principles to the mechanisms of phylogeny, obviating the artificial space and time barriers between ontogeny and phylogeny. This perspective allows us to consider the continuum between the proximate and ultimate causes of speciation, which was unthinkable when looked at from the descriptive perspective. Using a cell-cell interactive 'middle-out' approach, we have gained insight to the evolution of the lung from the swim bladder of fish based on gene regulatory networks that generate both lung ontogeny and phylogeny, i.e. decreased alveolar size, decreased alveolar wall thickness, and increased alveolar wall strength. Vertical integration of cell-cell interactions predicts the adaptivity and maladaptivity of the lung, leading to novel insights for chronic lung disease. Since we have employed principles involved in all of development, this approach is amenable to all biologic structures, functions, adaptations, maladaptations, and diseases, providing an operational basis for preventive medicine.

The evolutionary continuum from lung development to homeostasis and repair

A functional, developmental, and comparative biological approach is probably the most effective way for arranging gene regulatory networks (GRNs) in their biological contexts. Evolutionary developmental biology allows comparison of GRNs during development across phyla. For lung evolution, the parathyroid hormone-related protein (PTHrP) GRN exemplifies a continuum from ontogeny to phylogeny, homeostasis, and repair. PTHrP signaling between the lung endoderm and mesoderm stimulates lipofibroblast differentiation by downregulating the myofibroblast Wnt signaling pathway and upregulating the protein kinase A-dependent cAMP signaling pathway, inducing the lipofibroblast phenotype. Leptin secreted by the lipofibroblast, in turn, binds to its receptor on the alveolar type II cell, stimulating surfactant synthesis to ensure alveolar homeostasis. Failure of the PTHrP/PTHrP receptor signaling mechanism causes transdifferentiation of lipofibroblasts to myofibroblasts, which are the hallmark for lung fibrosis. We have shown that by targeting peroxisome proliferator-activated receptor gamma, the downstream target for lipofibroblast PTHrP signaling, we can prevent lung fibrosis. We speculate that the recapitulation of the myofibroblast phenotype during transdifferentiation is consistent with lung injury as lung evolution in reverse. Repair recapitulates ontogeny because it is programmed to express the cross talk between epithelium and mesoderm through evolution. This model demonstrates how epithelial-mesenchymal cross talk, when seen as a recapitulation of ontogeny and phylogeny (in both a forward and reverse direction), predicts novel, effective diagnostic and therapeutic targets.

The avian lung-associated immune system: a review

The lung is a major target organ for numerous viral and bacterial diseases of poultry. To control this constant threat birds have developed a highly organized lung-associated immune system. In this review the basic features of this system are described and their functional properties discussed. Most prominent in the avian lung is the bronchus-associated lymphoid tissue (BALT) which is located at the junctions between the primary bronchus and the caudal secondary bronchi. BALT nodules are absent in newly hatched birds, but gradually developed into the mature structures found from 6-8 weeks onwards. They are organized into distinct B and T cell areas, frequently comprise germinal centres and are covered by a characteristic follicle-associated epithelium. The interstitial tissue of the parabronchial walls harbours large numbers of tissue macrophages and lymphocytes which are scattered throughout tissue. A striking feature of the avian lung is the low number of macrophages on the respiratory surface under non-inflammatory conditions. Stimulation of the lung by live bacteria but not by a variety of bacterial products elicits a significant efflux of activated macrophages and, depending on the pathogen, of heterophils. In addition to the cellular components humoral defence mechanisms are found on the lung surface including secretory IgA. The compartmentalisation of the immune system in the avian lung into BALT and non BALT-regions should be taken into account in studies on the host-pathogen interaction since these structures may have distinct functional properties during an immune response.

Functional and structural optimization of the respiratory system of the batTadarida brasiliensis(Chiroptera, Molossidae): does airway geometry matter?

We studied structure and function of the respiratory system in the bat Tadarida brasiliensis and compared it with those of two species of rodents, Abrothrix andinus and A. olivaceus. Tadarida brasiliensis had lower resting oxygen consumption, but higher maximum oxygen consumption and aerobic scope, than the rodents. The blood–gas barrier of the bat was thinner and its relative lung size was larger; however,alveolar surface density was similar among the three species. In consequence, T. brasiliensis has an oxygen diffusion capacity two or three times higher than that of the rodents. In Tadarida brasiliensis the characteristics of the lung were accompanied by geometrical changes in the proximal airway, such as high physical optimization as a consequence of small variations in the symmetry and the scaling ratio of the bronchial diameters. These may constitute an efficient way to save energy in respiratory mechanics and are the first report of airway adjustments to decrease entropy generation in bats.

Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs

Birds are unique among living vertebrates in possessing pneumaticity of the postcranial skeleton, with invasion of bone by the pulmonary air-sac system. The avian respiratory system includes high-compliance air sacs that ventilate a dorsally fixed, non-expanding parabronchial lung. Caudally positioned abdominal and thoracic air sacs are critical components of the avian aspiration pump, facilitating flow-through ventilation of the lung and near-constant airflow during both inspiration and expiration, highlighting a design optimized for efficient gas exchange. Postcranial skeletal pneumaticity has also been reported in numerous extinct archosaurs including non-avian theropod dinosaurs and Archaeopteryx. However, the relationship between osseous pneumaticity and the evolution of the avian respiratory apparatus has long remained ambiguous. Here we report, on the basis of a comparative analysis of region-specific pneumaticity with extant birds, evidence for cervical and abdominal air-sac systems in non-avian theropods, along with thoracic skeletal prerequisites of an avian-style aspiration pump. The early acquisition of this system among theropods is demonstrated by examination of an exceptional new specimen of Majungatholus atopus, documenting these features in a taxon only distantly related to birds. Taken together, these specializations imply the existence of the basic avian pulmonary Bauplan in basal neotheropods, indicating that flow-through ventilation of the lung is not restricted to birds but is probably a general theropod characteristic.

Structure, function and evolution of the gas exchangers: comparative perspectives

Over the evolutionary continuum, animals have faced similar fundamental challenges of acquiring molecular oxygen for aerobic metabolism. Under limitations and constraints imposed by factors such as phylogeny, behaviour, body size and environment, they have responded differently in founding optimal respiratory structures. A quintessence of the aphorism that 'necessity is the mother of invention', gas exchangers have been inaugurated through stiff cost-benefit analyses that have evoked transaction of trade-offs and compromises. Cogent structural-functional correlations occur in constructions of gas exchangers: within and between taxa, morphological complexity and respiratory efficiency increase with metabolic capacities and oxygen needs. Highly active, small endotherms have relatively better-refined gas exchangers compared with large, inactive ectotherms. Respiratory structures have developed from the plain cell membrane of the primeval prokaryotic unicells to complex multifunctional ones of the modern Metazoa. Regarding the respiratory medium used to extract oxygen from, animal life has had only two choices--water or air--within the biological range of temperature and pressure the only naturally occurring respirable fluids. In rarer cases, certain animals have adapted to using both media. Gills (evaginated gas exchangers) are the primordial respiratory organs: they are the archetypal water breathing organs. Lungs (invaginated gas exchangers) are the model air breathing organs. Bimodal (transitional) breathers occupy the water-air interface. Presentation and exposure of external (water/air) and internal (haemolymph/blood) respiratory media, features determined by geometric arrangement of the conduits, are important features for gas exchange efficiency: counter-current, cross-current, uniform pool and infinite pool designs have variably developed.

Some recent advances on the study and understanding of the functional design of the avian lung: morphological and morphometric perspectives

The small highly aerobic avian species have morphometrically superior lungs while the large flightless ones have less well-refined lungs. Two parabronchial systems, i.e. the paleopulmo and neopulmo, occur in the lungs of relatively advanced birds. Although their evolution and development are not clear, understanding their presence is physiologically important particularly since the air- and blood flow patterns in them are different. Geometrically, the bulk air flow in the parabronchial lumen, i.e. in the longitudinal direction, and the flow of deoxygenated blood from the periphery, i.e. in a centripetal direction, are perpendicularly arranged to produce a cross-current relationship. Functionally, the blood capillaries in the avian lung constitute a multicapillary serial arterialization system. The amount of oxygen and carbon dioxide exchanged arises from many modest transactions that occur where air- and blood capillaries interface along the parabronchial lengths, an additive process that greatly enhances the respiratory efficiency. In some species of birds, an epithelial tumescence occurs at the terminal part of the extrapulmonary primary bronchi (EPPB). The swelling narrows the EPPB, conceivably allowing the shunting of inspired air across the openings of the medioventral secondary bronchi, i.e. inspiratory aerodynamic valving. The defence stratagems in the avian lung differ from those of mammals: fewer surface (free) macrophages (SMs) occur, the epithelial cells that line the atria and infundibula are phagocytic, a large population of subepithelial macrophages is present and pulmonary intravascular macrophages exist. This complex defence inventory may explain the paucity of SMs in the avian lung.

Is the sheet-flow design a 'frozen core' (a Bauplan) of the gas exchangers? Comparative functional morphology of the respiratory microvascular systems: illustration of the geometry and rationalization of the fractal properties

The sheet-flow design is ubiquitous in the respiratory microvascular systems of the modern gas exchangers. The blood percolates through a maze of narrow microvascular channels spreading out into a thin film, a "sheet". The design has been convergently conceived through remarkably different evolutionary strategies. Endothelial cells, e.g. connect parallel epithelial cells in the fish gills and reptilian lungs; epithelial cells divide the gill filaments in the crustacean gills, the amphibian lungs, and vascular channels on the lung of pneumonate gastropods; connective tissue elements weave between the blood capillaries of the mammalian lungs; and in birds, the blood capillaries attach directly and in some areas connect by short extensions of the epithelial cells. In the gills, skin, and most lungs, the blood in the capillary meshwork geometrically lies parallel to the respiratory surface. In the avian lung, where the blood capillaries anastomose intensely and interdigitate closely with the air capillaries, the blood occasions a 'volume' rather than a 'sheet.' The sheet-flow design and the intrinsic fractal properties of the respiratory microvascular systems have produced a highly tractable low-pressure low-resistance region that facilitates optimal perfusion. In complex animals, the sheet-flow design is a prescriptive evolutionary construction for efficient gas exchange by diffusion. The design facilitates the internal and external respiratory media to be exposed to each other over an extensive surface area across a thin tissue barrier. This comprehensive design is a classic paradigm of evolutionary convergence motivated by common enterprise to develop corresponding functionally efficient structures. With appropriate corrections for any relevant intertaxa differences, use of similar morphofunctional models in determining the diffusing capacities of various gas exchangers is warranted.