Fibrous Network of the Lung and its Change with Age

Abstract

The remarkable elastic behaviour of the lung entails both continuous tissue stress and a cyclic stress produced by the fluctuating difference between intrathoracic and intra-alveolar pressures throughout the entire span of life. In spite of these potentially disruptive forces, lung structure is preserved by the fibrous connective-tissue proteinscollagen, elastin, and reticulin. These extracellular proteins are characterized by their insolubility, resistance to destruction, and high tensile strength. Collagen and elastin both exhibit elastic behaviour, but great extensibility is a property only of elastin. The most reasonable assumption is that the elastic fibres, therefore, are principally responsible for the elastic behaviour of the lung tissue, but sure proof is lacking.Recently, Carton, Dainauskas, Tews, and Hass (1960) and Wright (1961) have described the nature of the network of elastic tissue in the lung. We sought to describe more fully the roles of collagenous and elastic fibres in the terminal air spaces of the lung, and thus their respective contributions to tissue elasticity and the general elastic performance of the lung. As the study developed, it seemed important to measure the amounts of collagen and elastin in the lung parenchyma and pleura. These findings have been correlated with the age of the subjects and interpreted according to knowledge about tissue elasticity. METHODSThe study was based on 36 human lungs taken from patients who had died of a variety of diseases. No lungs were taken from patients with a history of cough, sputum production, dyspnoea, or chronic pulmonary disease, or from patients whose clinical history was incomplete. Of these 36, 27 lungs were dried in an inflated state by the extraction process using dilute (0-1 N) sodium hydroxide solution (Pierce, Hocott, and Ebert, 1959). Nine more lungs were also dried in the inflated state but were fixed with formalin vapour. The advantage of the alkaline extraction method is that it permits the morphology of the lung 1 Supported in part by grants HTS5333 and HE04031 from the U.S. Public Health Service to be examined and also the quantities of collagen and elastin to be measured.The lungs were cut into thin slices and examined grossly and with a stereomicroscope. Slices of tissue were variously stained (Lillie, 1954) but not embedded. Elastic tissue was stained by the Taenzer-Unna acidorcein method (in absolute ethanol); Van Gieson's picric acid-acid fuchsin mixture was used to stain collagen; and Fraenkel's method of using orcein and indigo-carmine was employed to stain both collagen and elastin in the same slice. All slices were rapidly dehydrated in alcohol and then in ether, and dried in vacuo to prevent collapse. This procedure enabled one to study the structure of the alveoli, alveolar ducts, and respiratory bronchioles as well as to visualize the relationship of the fibres in three dimensions. Preparations extracted with sodium hydroxide provided a more clearly defined fibrous network than did the formalin-vapour preparation...

Effects of Elastase, Collagenase, and Papain on Structure and Function of Rat Lungs In Vitro

“…The fragility of collagenase-treated lungs prevented the investigation of changes occurring at high transpulmonary pressures in the present study. Consideration of the anatomical association of collagen and elastin in the lung has led to the view that collagen plays the major role in maintaining the structural integrity of the lung (13). In contrast, the present study suggests that major alterations in lung structure and elastic behavior are associated with alterations in elastin.…”

“…From such studies it has been suggested that collagen is the major determinant of lung structure (13) and that elastin plays little role in lung elastic behavior (14). In contrast, the present study indicates that enzymatic attack of elastin is associated with marked architectural changes and altered elastic behavior of the lung.…”

Effects of a molecular change in collagen on lung structure and mechanical function.

“…Consistent with this, the extensibility of elastin is much greater than collagen (22). Also, even though the uncoiling of these fibrous elements may affect lung elasticity more than their linear extension (3). lung compliance is increased after exposure to elastase but not collagenase (8).…”

“…The separate functions of these fibrous proteins have been deduced from histological studies of their arrangement in the lung parenchyma (1)(2)(3) and from the mechanical behavior of elastin and collagen fibers dissected out from other tissues (4,5). Further knowledge has been gained Received for publication 6 November 1974 and in revzised forni 6 February 1975. by studying the effects of elastolytic and collagenolytic enzymes on lung structure and function (6)(7)(8).…”

Elastin metabolism and chemistry: potential roles in lung development and structure.

“…Once elastin is synthesized and stabilized by crosslinks, it does not undergo rates of turnover common to other proteins (10,23,28,97,(105)(106)(107)(108). For example, the data in Figure 8 indicate that the elastin in whole lung turns over very slowly.…”