Light microscopic morphometry of the lung of 19 avian species

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.

A histological and immunohistochemical study on the parabronchial epithelium of the domestic fowl's (Gallus gallus domesticus) lung with special reference to its scanning and transmission electron microscopic characteristics

The current study was designed to give complete histo-and immunohistochemical features of the parabronchial epithelium of domestic fowl's (Gallus gallus domesticus) lung with special reference to Scanning electron microscope (SEM) and mean transmission electron microscope (TEM) features. The lung exhibited variable-sized atrial openings encircled by exchange tissue zones. The parabronchial atrial chambers appeared as ovoid and polygonal-shaped that separated by the well-developed interatrial septum. The deep atrial lumens had blood vessels pierced by openings that represent the infundibula. The parabronchial blood capillaries meshwork was branched and exhibited ovoid-shaped air capillaries with numerous extravasated blood vessels. By TEM, there were several air capillaries and groups of squamous and endothelial respiratory cells and the squamous cells had oval nucleus with evenly distributed chromatin. The endothelial respiratory cells had few microvilli on their free surfaces. The parabronchial tubes opened into a group of widened atria that had smooth muscle bundles at the interatrial septa. The atrial chambers led to narrow infundibula. Moreover, the lining epithelium of parabronchi, atria, infundibula, and air capillaries was formed by simple squamous epithelium. Air capillary walls were lined by two types of respiratory cells (Types-I and II). Collagen fibers were concentrated within the tunica externa layers of the parabronchial blood vessels as well as, they were observed in CT interparabronchial septa. Immunohistochemically, the elastin immunoreactivity was detected around the parabronchial blood vessels, at the base of each parabronchial atria, and in the area encircling the alveolar-capillary walls. Our work concluded that there are a relation between the fowl's lifestyle and the surrounding environmental conditions.

Morphological and morphometric specializations of the lung of the Andean goose, Chloephaga melanoptera: A lifelong high-altitude resident

High altitude flight in rarefied, extremely cold and hypoxic air is a very challenging activity. Only a few species of birds can achieve it. Hitherto, the structure of the lungs of such birds has not been studied. This is because of the rarity of such species and the challenges of preparing well-fixed lung tissue. Here, it was posited that in addition to the now proven physiological adaptations, high altitude flying birds will also have acquired pulmonary structural adaptations that enable them to obtain the large amounts of oxygen (O₂) needed for flight at high elevation, an environment where O₂ levels are very low. The Andean goose (Chloephaga melanoptera) normally resides at altitudes above 3000 meters and flies to elevations as high as 6000 meters where O₂ becomes limiting. In this study, its lung was morphologically- and morphometrically investigated. It was found that structurally the lungs are exceptionally specialized for gas exchange. Atypically, the infundibulae are well-vascularized. The mass-specific volume of the lung (42.8 cm³.kg^-1), the mass-specific respiratory surface area of the blood-gas (tissue) barrier (96.5 cm².g^-1) and the mass-specific volume of the pulmonary capillary blood (7.44 cm³.kg^-1) were some of the highest values so far reported in birds. The pulmonary structural specializations have generated a mass-specific total (overall) pulmonary morphometric diffusing capacity of the lung for oxygen (DLo₂) of 0.119 mlO₂.sec^-1.mbar^-1.kg^-1, a value that is among some of the highest ones in birds that have been studied. The adaptations of the lung of the Andean goose possibly produce the high O₂ conductance needed to live and fly at high altitude.

Critical appraisal of some factors pertinent to the functional designs of the gas exchangers

Respiration acquires O₂ from the external fluid milieu and eliminates CO₂ back into the same. Gas exchangers evolved under certain immutable physicochemical laws upon which their elemental functional design is hardwired. Adaptive changes have occurred within the constraints set by such laws to satisfy metabolic needs for O₂, environmental conditions, respiratory medium utilized, lifestyle pursued and phylogenetic level of development: correlation between structure and function exists. After the inaugural simple cell membrane, as body size and structural complexity increased, respiratory organs formed by evagination or invagination: the gills developed by the former process and the lungs by the latter. Conservation of water on land was the main driver for invagination of the lungs. In gills, respiratory surface area increases by stratified arrangement of the structural components while in lungs it occurs by internal subdivision. The minuscule terminal respiratory units of lungs are stabilized by surfactant. In gas exchangers, respiratory fluid media are transported by convection over long distances, a process that requires energy. However, movement of respiratory gases across tissue barriers occurs by simple passive diffusion. Short distances and large surface areas are needed for diffusion to occur efficiently. Certain properties, e.g., diffusion of gases through the tissue barrier, stabilization of the respiratory units by surfactant and a thin tripartite tissue barrier, have been conserved during the evolution of the gas exchangers. In biology, such rare features are called Bauplans, blueprints or frozen cores. That several of them (Bauplans) exist in gas exchangers almost certainly indicates the importance of respiration to life.

Protective roles of free avian respiratory macrophages in captive birds

In the mammalian lung, respiratory macrophages provide front line defense against invading pathogens and particulate matter. In birds, respiratory macrophages are known as free avian respiratory macrophages (FARM) and a dearth of the cells in the avian lung has been purported to foreordain a weak first line of pulmonary defense, a condition associated with high mortality of domestic birds occasioned by respiratory inflictions. Avian pulmonary mechanisms including a three tiered aerodynamic filtration system, tight epithelial junctions and an efficient mucociliary escalator system have been known to supplement FARM protective roles. Current studies, however, report FARM to exhibit an exceptionally efficient phagocytic capacity and are effective in elimination of invading pathogens. In this review, we also report on effects of selective synthetic peroxisome proliferator activated receptor gamma (PPAR γ) agonists on non phlogistic phagocytic properties in the FARM. To develop effective therapeutic interventions targeting FARM in treatment and management of respiratory disease conditions in the poultry, further studies are required to fully understand the role of FARM in innate and adaptive immune responses.

Evolution of air breathing: oxygen homeostasis and the transitions from water to land and sky

Life originated in anoxia, but many organisms came to depend upon oxygen for survival, independently evolving diverse respiratory systems for acquiring oxygen from the environment. Ambient oxygen tension (PO2) fluctuated through the ages in correlation with biodiversity and body size, enabling organisms to migrate from water to land and air and sometimes in the opposite direction. Habitat expansion compels the use of different gas exchangers, for example, skin, gills, tracheae, lungs, and their intermediate stages, that may coexist within the same species; coexistence may be temporally disjunct (e.g., larval gills vs. adult lungs) or simultaneous (e.g., skin, gills, and lungs in some salamanders). Disparate systems exhibit similar directions of adaptation: toward larger diffusion interfaces, thinner barriers, finer dynamic regulation, and reduced cost of breathing. Efficient respiratory gas exchange, coupled to downstream convective and diffusive resistances, comprise the "oxygen cascade"-step-down of PO2 that balances supply against toxicity. Here, we review the origin of oxygen homeostasis, a primal selection factor for all respiratory systems, which in turn function as gatekeepers of the cascade. Within an organism's lifespan, the respiratory apparatus adapts in various ways to upregulate oxygen uptake in hypoxia and restrict uptake in hyperoxia. In an evolutionary context, certain species also become adapted to environmental conditions or habitual organismic demands. We, therefore, survey the comparative anatomy and physiology of respiratory systems from invertebrates to vertebrates, water to air breathers, and terrestrial to aerial inhabitants. Through the evolutionary directions and variety of gas exchangers, their shared features and individual compromises may be appreciated.

Recent advances into understanding some aspects of the structure and function of mammalian and avian lungs

Recent findings are reported about certain aspects of the structure and function of the mammalian and avian lungs that include (a) the architecture of the air capillaries (ACs) and the blood capillaries (BCs); (b) the pulmonary blood capillary circulatory dynamics; (c) the adaptive molecular, cellular, biochemical, compositional, and developmental characteristics of the surfactant system; (d) the mechanisms of the translocation of fine and ultrafine particles across the airway epithelial barrier; and (e) the particle-cell interactions in the pulmonary airways. In the lung of the Muscovy duck Cairina moschata, at least, the ACs are rotund structures that are interconnected by narrow cylindrical sections, while the BCs comprise segments that are almost as long as they are wide. In contrast to the mammalian pulmonary BCs, which are highly compliant, those of birds practically behave like rigid tubes. Diving pressure has been a very powerful directional selection force that has influenced phenotypic changes in surfactant composition and function in lungs of marine mammals. After nanosized particulates are deposited on the respiratory tract of healthy human subjects, some reach organs such as the brain with potentially serious health implications. Finally, in the mammalian lung, dendritic cells of the pulmonary airways are powerful agents in engulfing deposited particles, and in birds, macrophages and erythrocytes are ardent phagocytizing cellular agents. The morphology of the lung that allows it to perform different functions-including gas exchange, ventilation of the lung by being compliant, defense, and secretion of important pharmacological factors-is reflected in its "compromise design."

Implicit mechanistic role of the collagen, smooth muscle, and elastic tissue components in strengthening the air and blood capillaries of the avian lung

To identify the forces that may exist in the parabronchus of the avian lung and that which may explain the reported strengths of the terminal respiratory units, the air capillaries and the blood capillaries, the arrangement of the parabronchial collagen fibers (CF) of the lung of the domestic fowl, Gallus gallus variant domesticus was investigated by discriminatory staining, selective alkali digestion, and vascular casting followed by alkali digestion. On the luminal circumference, the atrial and the infundibular CF are directly connected to the smooth muscle fibers and the elastic tissue fibers. The CF in this part of the parabronchus form the internal column (the axial scaffold), whereas the CF in the interparabronchial septa and those associated with the walls of the interparabronchial blood vessels form the external, i.e. the peripheral, parabronchial CF scaffold. Thin CF penetrate the exchange tissue directly from the interparabronchial septa and indirectly by accompanying the intraparabronchial blood vessels. Forming a dense network that supports the air and blood capillaries, the CF weave through the exchange tissue. The exchange tissue, specifically the air and blood capillaries, is effectively suspended between CF pillars by an intricate system of thin CF, elastic and smooth muscle fibers. The CF course through the basement membranes of the walls of the blood and air capillaries. Based on the architecture of the smooth muscle fibers, the CF, the elastic muscle fibers, and structures like the interparabronchial septa and their associated blood vessels, it is envisaged that dynamic tensional, resistive, and compressive forces exist in the parabronchus, forming a tensegrity (tension integrity) system that gives the lung rigidity while strengthening the air and blood capillaries.

Spectacularly robust! Tensegrity principle explains the mechanical strength of the avian lung

Among the air-breathing vertebrates, the respiratory system of birds, the lung-air sac system, is remarkably complex and singularly efficient. The most perplexing structural property of the avian lung pertains to its exceptional mechanical strength, especially that of the minuscule terminal respiratory units, the air- and the blood capillaries. In different species of birds, the air capillaries range in diameter from 3 to 20 micro m: the blood capillaries are in all cases relatively smaller. Over and above their capacity to withstand enormous surface tension forces at the air-tissue interface, the air capillaries resist mechanical compression (parabronchial distending pressure) as high as 20 cm H(2)O (2 kPa). The blood capillaries tolerate a pulmonary arterial vascular pressure of 24.1 mmHg (3.2 kPa) and vascular resistance of 22.5 mmHg (3 kPa) without distending. The design of the avian respiratory system fundamentally stems from the rigidity (strength) of the lung. The gas exchanger (the lung) is uncoupled from the ventilator (the air sacs), allowing the lung (the paleopulmonic parabronchi) to be ventilated continuously and unidirectionally by synchronized bellows like action of the air sacs. Since during the ventilation of the lung the air capillaries do not have to be distended (dilated), i.e., surface tension force does not have to be overcome (as would be the case if the lung was compliant), extremely intense subdivision of the exchange tissue was possible. Minuscule terminal respiratory units developed, producing a vast respiratory surface area in a limited lung volume. I make a case that a firm (rigid) rib cage, a lung tightly held by the ribs and the horizontal septum, a lung directly attached to the trunk, specially formed and compactly arranged parabronchi, intertwined atrial muscles, and tightly set air capillaries and blood capillaries form an integrated hierarchy of discrete network system of tension and compression, a tensegrity (tensional integrity) array, which absorbs, transmits, and dissipates stress, stabilizing (strengthening) the lung and its various structural components.

A systematic study of the development of the airway (bronchial) system of the avian lung from days 3 to 26 of embryogenesis: a transmission electron microscopic study on the domestic fowl, Gallus gallus variant domesticus

In the embryo of the domestic fowl, Gallus gallus variant domesticus, the lung buds become evident on day 3 of development. After fusing on the ventral midline, the single entity divides into left and right primordial lungs that elongate caudally while diverging and shifting towards the dorsolateral aspects of the coelomic cavity. On reaching their definitive topographical locations, the lungs rotate along a longitudinal axis, attach, and begin to slide into the ribs. First appearing as a solid cord of epithelial cells that runs in the proximal-distal axis of the developing lung, progressively, the intrapulmonary primary bronchus begins to canalize. In quick succession, secondary bronchi sprout from it in a craniocaudal sequence and radiate outwards. On reaching the periphery of the lung, parabronchi (tertiary bronchi) bud from the secondary bronchi and project into the surrounding mesenchymal cell mass. The parabronchi canalize, lengthen, increase in diameter, anastomose, and ultimately connect the secondary bronchi. The luminal aspect of the formative parabronchi is initially lined by a composite epithelium of which the peripheral cells attach onto the basement membrane while the apical ones project prominently into the lumen. The epithelium transforms to a simple columnar type in which the cells connect through arm-like extensions and prominently large intercellular spaces form. The atria are conspicuous on day 15, the infundibulae on day 16, and air capillaries on day 18. At hatching (day 21), the air and blood capillaries have anastomosed profusely and the blood-gas barrier become remarkably thin. The lung is well developed and potentially functionally competent at the end of the embryonic life. Thereafter, at least upto day 26, no further consequential structures form. The mechanisms by which the airways in the avian lung develop fundamentally differ from those that occur in the mammalian one. Compared with the blind-ended bronchial system that inaugurates in the mammalian lung, an elaborate, continuous system of air conduits develops in the avian one. Further studies are necessary to underpin the specific molecular factors and genetic processes that direct the morphogenesis of an exceptionally complex and efficient respiratory organ.

Composite cellular defence stratagem in the avian respiratory system: functional morphology of the free (surface) macrophages and specialized pulmonary epithelia

Qualitative and quantitative attributes of the free respiratory macrophages (FRMs) of the lung--air sac systems of the domestic fowl (Gallus gallus variant domesticus) and the muscovy duck (Cairina moschata) were compared with those of the alveolar macrophages of the lung of the black rat (Rattus rattus). The birds had significantly fewer FRMs compared to the rat. In the birds, the FRMs were found both in the lungs and in the air sacs. Under similar experimental conditions, the most robust FRMs were those of the domestic fowl followed by those of the rat and the duck. Flux of macrophages onto the respiratory surface from the subepithelial compartment and probably also from the pulmonary vasculature was observed in the birds but not in the rat. In the duck and the domestic fowl, a phagocytic epithelium that constituted over 70% of the surface area of the blood-gas (tissue) barrier lines the atrial muscles, the atria and the infundibulae. The epithelial cells of the upper respiratory airways contain abundant lysosomes, suggesting a high lytic capacity. By inference, the various defence strategies in the avian lung may explain the dearth of FRMs on the respiratory surface. We counter-propose that rather than arising directly from paucity of FRMs, an aspect that has been over-stressed by most investigators, the purported high susceptibility of birds (particularly table birds) to respiratory ailments and afflictions may be explained by factors such as inadequate management and husbandry practices and severe genetic manipulation for fast growth and high productivity, manipulations that may have weakened cellular and immunological defences.

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.

A qualitative and quantitative study of the lung of an ostrich,Struthio camelus

The ostrich lung, with its lack of interparabronchial septa, the presence of very shallow atria and exceptional morphometric refinement, structurally resembles those of small, energetic flying birds, whereas it also displays features characteristic of the flightless ratites in which the neopulmo is relatively poorly developed and a segmentum accelerans may be generally lacking. The large size of the bronchial system of the ostrich may help explain the unique shifts in the airflow pathways that must occur from resting to panting breathing, explaining its insensitivity to acid–base imbalance of the blood during sustained panting under thermal stress. The mass-specific volume of the lung is 39.1 cm3kg−1 and the volume density of the exchange tissue is remarkably high (78.31%). The blood–gas (tissue) barrier is relatively thick (0.56μm) but the plasma layer is very thin (0.14μm). In this flightless ratite bird, the mass-specific surface area of the tissue barrier (30.1 cm2g−1), the mass-specific anatomical diffusing capacity of the tissue barrier for oxygen (0.0022mlO2s−1Pa−1kg−1), the mass-specific volume of pulmonary capillary blood (6.25 cm3kg−1) and the mass-specific total anatomical diffusing capacity for oxygen (0.00073mlO2s−1Pa−1kg−1) are equivalent to or exceed those of much smaller highly aerobic volant birds. The distinctive morphological and morphometric features that seem to occur in the ostrich lung may explain how it achieves and maintains high aerobic capacities and endures long thermal panting without experiencing respiratory alkalosis.

Morphometrics of the avian lung. 4. The structural design of the charadriiform lung

The lungs of five charadriiform species of bird, two of which are good divers and three predominantly flyers (soarers and gliders) have been analysed by morphometric techniques. Largely the morphometric structural values in the divers significantly exceeded those of the flyers (gulls). The average weight specific surface area of the blood-gas (tissue) barrier in the divers (28.45 +/- 2.05 cm2 X g-1 SD) surpassed that of the flyers (23.5 +/- 3.61 cm2 X g-1 SD). The divers had a higher volume of the pulmonary capillary blood per unit body weight (4.42 +/- 0.11 cm3 X kg-1 SD) than the flyers (2.84 +/- 0.58 cm3 X kg-1 SD). The weight specific volume of the lung in the divers (34.90 +/- 3.11 cm3 X kg-1 SD) exceeded that of the flyers (26.94 +/- 3.15 cm3 X kg-1 SD). The total morphometric pulmonary diffusing capacity per unit body weight in the divers (4.73 +/- 0.05 ml O2 X (min X mm Hg X kg)-1 SD) was higher than that of the flyers (3.09 +/- 0.47 ml O2 X (min X mm Hg X kg)-1 SD). The divers, however, had a notably thicker blood-gas (tissue) barrier with a harmonic mean thickness of 0.212 +/- 0.03 micron SD compared to that of the flyers (0.138 +/- 0.02 micron SD). The data acquired here commensurate the modes of life exhibited by these two groups of bird. The divers, which are relatively energetic birds, expend a lot of energy to move and stay underwater, concomitantly undergoing prolonged asphyxia during submergence and may hence need to extract as much of the oxygen in the pulmonary air as possible to prolong a dive. These birds appear in general to have structurally better adapted lungs than those of the gulls, birds which to a large extent exhibit relatively less energetic soaring and gliding flights.

Lung growth of the turkey, Meleagris gallopavo: I. Morphologic and morphometric description

To describe lung growth qualitatively and quantitatively from prehatch to adulthood of an unselected line of turkey, a precocial avian species, 36 male turkeys, three in each age group, were killed at 22 and 25 days of incubation, on hatch day, and at 1, 4, 7, 10, 14, 21, 28, 112, and 420 days of age. Body weight and lung volume were measured. A three-level cascade sampling system was used to prepare lung tissue for morphologic and morphometric observation by light microscopy. Point and intersection counting were used to estimate volume and surface densities of lung compartments relative to lung volume. Absolute volumes and surfaces of lung compartments were calculated. Bilogarithmic regressions provided allometric equations to describe growth of the lung in three phases: Tissue proliferation--explosive growth of lung volume relative to body weight and of the gas-exchange compartment within the lung. At 22 days of incubation there were few air and blood capillaries and a great deal of tissue that looked like mesenchyme between the parabronchi. Within the 6 days prior to hatch, the surface area of air capillaries increased 11-fold and of blood capillaries 27-fold, whereas the volume of interparabronchial tissue decreased 58%. Equilibrated growth--from hatch day to 28 days of age, most lung compartments grew evenly with lung volume. Regulated growth--from 28 days of age to adult, all lung compartments, except large vessels and exchange compartment, grew more slowly than the entire lung. Interatrial septa lengthened and their epithelial covering thinned, infundibula became more apparent, and interparabronchial connective tissue reached a minimal volume density in the adult lung.

Microscopic and submicroscopic anatomy of the parabronchi, air sacs, and respiratory space of the budgerigar (Melopsittacus undulatus)

The normal microscopic and submicroscopic structure of the lower respiratory tract of the budgerigar (Melopsittacus undulatus) is described and compared with other birds and mammals. Granular (type II) pneumocytes are confined to linings of air sacs, parabronchi, and their atria; however, their secretions (surfactant) cover the surfaces of the infundibula and respiratory space. Infundibula extend from the atria and give rise to the air capillaries, which branch and anastomose freely with those of adjacent infundibula and other parabronchi (interparabronchial septa are not found). Infundibula and the respiratory labyrinth are lined by a continuous epithelium of squamous pneumocytes, whose perikarya are concentrated in the infundibula and whose peripheral cytoplasm is markedly attenuated. The squamous pneumocytes of the respiratory labyrinth share a basal lamina with the blood capillaries that they envelop.

Morphometrics of the avian lung. 2. The wild mallard (Anas platyrhynchos) and graylag goose (Anser anser)

The lungs of 5 wild mallard ducks (Anas platyrhynchos) and 5 feral graylag geese (Anser anser) of mean body weight 1.04 and 3.84 kg, respectively, were fixed in situ by intratracheal infusion of 2.3% glutaraldehyde, pH 7.4 and total osmolarity 350 mOsm, at a pressure head of 25 cm, and analysed by standard morphometric techniques. The following data apply to both lungs together, in the fixed state, the first value relating to Anas and the second to Anser in each case: lung volume, 30.4 and 95.3 cm3; volume of exchange tissue, 12.32 and 38.50 cm3; volume of capillary blood, 4.06 and 12.49 cm3; surface area of blood-gas (tissue) barrier per unit body weight, 28.56 and 23.10 cm2/g; surface area of the blood-gas (tissue) barrier per unit volume of lung, 977 and 932 cm2/cm3; surface area of blood-gas (tissue) barrier per unit volume of exchange tissue, 241 and 230 mm2/mm3; harmonic mean thickness of tissue barrier, 0.133 and 0.118 microns; arithmetic mean thickness of tissue barrier, 0.903 and 0.887 microns; harmonic mean thickness of plasma layer, 0.369 and 0.322 microns; mean total morphometric pulmonary diffusing capacity per unit body weight, 3.85 and 3.59 ml O2/min/mm Hg/kg. These morphometric parameters of Anas and Anser are compared with those reported in the literature for the domestic fowl (Gallus gallus), the budgerigar (Melopsittacus undulatus), the house sparrow (Passer domesticus), and the violet-eared hummingbird (Colibri coruscans). The lungs of these six avian species show progressively advancing adaptations, from Gallus, through Anser, Anas, Melopsittacus and Passer, to Colibri, which appear to be consistent with the energetic characteristics of these birds.

A morphometric analysis of the lung of a species of bat

The lungs of five adult Epauleted Fruit-bats (Epomophorus wahlbergi) of mean body weight 96 g were analysed morphometrically. The lung volume per unit body weight was 0.043 cm3/g, the surface area of the tissue barrier (i.e., the effective alveolar surface area) component of the blood-gas pathway per unit body weight was 138 cm2/g, and the surface density of the tissue barrier (surface area of the tissue barrier per unit volume of parenchyma) was 121 mm2/mm3. The harmonic mean thickness of the tissue barrier was between 0.267 and 0.349 micron. The morphometric pulmonary diffusing capacity per unit body weight (DLO2/W) was 0.02 ml O2 per min per mm Hg per g. These values are compared with those of shrews and birds. It is suggested that in bats enlargement of the lungs, small subdivisions of the air spaces, and a thin blood-gas barrier, could be linked with previously reported circulatory adaptations to account for the high oxygen consumption during flight.