Cellular Defences of the Lung: Comparative Perspectives

Abstract

'Our lungs are highly complex organs that are exquisitely specialized for gas exchange and host defense.' Rawlins (2010) 1. Introduction General considerationsThe most important function of the lung is to acquire molecular oxygen and eliminate carbon dioxide. Except probably for the gastrointestinal system, no other organ in the body interacts with the external environment as constantly and as intimately as the respiratory system. For example, during a 24 hour period, at rest, the human lung is ventilated ~25,000 times with ~20,000 L of air (e.g. Burri, 1985; Brain, 1996), it has a respiratory surface area (RSA) of ~140 m 2 (about the size of a tennis court) which is located in the acini that lie no more than 40 to 50 cm from the external environment (air) (e.g. Weibel, 1984), and the thickness of the blood-gas (tissue) barrier (BGB) (harmonic mean thickness, ht) is 0.62 μm, a value about one-fiftieth of the thickness of a foolscap paper or that of a human head hair (Gehr et al., 1978(Gehr et al., , 1990a. By weight, each day, more than 20 kg of air enters and leaves the human body, a load that far exceeds that of food and water ingested during the same time period (Brain, 1996). Depending on the level of air pollution, different types and quantities of foreign particulates and microbial pathogens are inhaled. While large RSA and thin BGB increase gas exchange, the concomitant downside to these structural properties is that they make the lung a leading portal of entry and therefore attack by pathogenic micro-organisms, damage by allergens and particulates, and injury by noxious gases (e.g. Brain, , 1992Lambrecht et al. 2001;Garn et al., 2006). The inhaled particulates are deposited on the epithelial lining of the conducting airways and that of the peripheral air spaces where they are retained for various durations before they are removed or destroyed (e.g. Geiser et al., 1988; Gehr et al., 1990 a, b). Brain (1996) observed that 'the lung is unique in that the marriage between environment and lung disease is profound. The respiratory system threfore forms a huge challenge to the body's immune integrity (e.g. von Garnier and Nicod, 2009 , 1993;Schwartz, 1994;Brunekreef et al., 1995;Ware, 2000;Pope, 2000;Nemmar et al., 2002;Pope et al., 2002;Suwa et al., 2002;Schulz et al., 2005;Kaufman, 2010;Brook et al., 2010; van den Hooven et al., 2011;Kampfrath et al., 2011 gc.ca/ccdpc-cpcmc/crdmrc/facts_gen_e.html). According to the Canadian Lung Association (CLA), the economic burden of respiratory disease in the Country is ~3 billion ($US) dollars (http://www.bukisa.com/articles/455926_lung-health-occupational-healthincidence#ixzz1I4jwMvBX). Based on net changes in gross domestic product (GDP) growth forecasts, the estimated annual cost of SARS (Sudden Avian Respiratory Syndrome) in Asia exceeds 10 billion dollars ($US) (Lee and McKibbin, 2003;Fan, 2003;McKibbin and Sidorenko, 2006) and according to the World Bank's estimates, an influenza pandemic may result in a loss (expenditure) of 800 billion dollars ($US) (Brahm...

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

“…Although the interpretation is supported by this study, the conclusion reached (by Ibe et al 59 ) in the investigation was not merited by the simple data adduced. For both C. gambianus and C. foxi, the numerous pulmonary goblet cells and alveolar macrophages should constitute an important part of the pulmonary defense system 124,135,136 : the air in the burrows is presumably highly polluted and loaded with pathogens and particulates. Interestingly, a double blood capillary arrangement was observed in some parts of the interalveolar septa of the lung of C. foxi but not in those of C. gambianus.…”

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

“…Phagocytes include the polymorphonuclear neutrophils and monocyte/macrophage cells that play an important role in host defence (Brain, ; Djaldetti et al, ; Nicod, ; Maina, ; Wynn, Chawla & Pollard, ). Functional inadequacies in these cells predispose an animal to attack by microbial and viral agents or particulate injury.…”

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?

“…It shows that the respiratory system of birds has a considerable range of highly specialised cellular defences, including tissuebased phagocytic cells which are exceptionally adept at destroying injurious agents and are mobilised from other sources and rapidly transferred to the epithelial surface where they confer additional protection (Toth & Siegel, 1993). As argued by Nganpiep & Maina (2002a,b), Maina (2002aMaina ( , 2005Maina ( , 2012 and Kiama et al (2008), the high morbidities and mortalities that characterise disease outbreaks in poultry can largely be attributed to factors such as the intense genetic manipulation that birds have undergone during their domestication, intensive genetic breeding practices for rapid weight gain and plentiful egg production, and possibly most importantly the stressful environmental conditions and severe husbandry practices by which poultry production occurs.…”

“…1E,F) leads to complicated air-flow processes that importantly should determine where, how and to what extent inhaled pathogens and particulates are deposited. While well-structured comparative investigations of the relative susceptibilities between mammalian and avian respiratory systems have not been performed, Brown et al (1997), Fedde (1998) and Maina (2002cMaina ( ,a, 2012 suggested that the intricate arrangement of the airways of the avian lung (Fig. 1C,D), the circuitous path followed by the inhaled air (Fig.…”

“…In quail and pigeon lungs, Klika et al (1999) and Scheuermann et al (2000) observed extracellular trilaminar substance material on the surface of the parabronchial epithelium and in the pigeon lung the atrial epithelium was squamous. In the smaller branches of the airways of vertebrate lungs, phagocytic cells such as polymorphonuclear neutrophilic granulocytes and macrophages comprise an important line of defence against infectious agents and injurious particulates (Green & Kass, 1964;Stearns et al, 1987;Van Alstine & Arp, 1988;Geiser et al, 2002;Nicod, 2005;Maina, 2012;Kallapura et al, 2014;Weiss & Schaible, 2015).…”