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Clinical Anatomy 22:689–697 (2009) ORIGINAL COMMUNICATION Novel Insights Into the Elastic and Muscular Components of the Human Trachea KIROLLOS SALAH KAMEL,1 LUTZ E. BECKERT,2 AND MARK D. STRINGER1* 1 Department of Anatomy and Structural Biology, Otago School of Medical Sciences, University of Otago, Dunedin, New Zealand 2 Department of Respiratory Medicine, Christchurch Hospital, Christchurch, New Zealand Despite its probable importance in health and disease, the elastic tissue in the trachea has rarely been investigated. In addition, various aspects of the trachealis muscle are controversial. The aim of this study was to clarify this clinically relevant anatomy. Ten cadaveric tracheobronchial specimens (age range 68–101 years; seven males; no major airway pathology) were qualitatively investigated by microdissection. Serial histologic sections from multiple sites in three specimens were analyzed after staining for elastin. Findings were correlated with observations from video tracheobronchoscopies. An extensive and prominent meshwork of elastic tissue was found within the trachea and bronchi. Elastic fibers were predominantly longitudinal and aggregated into discrete bundles within the membranous wall of the trachea and main bronchi; a discrete fibroelastic membrane bridging the membranous wall of the trachea; and vertical laminae connecting the ends of successive cartilages. The longitudinal elastic bundles continued into the segmental bronchi, becoming thinner and more circumferentially distributed. Trachealis consisted of a transverse layer of smooth muscle deep to the fibroelastic membrane of the membranous wall of the trachea, together with scattered longitudinal muscle bundles, mostly embedded within the fibroelastic membrane in the distal half of the trachea. In conclusion, there is an extensive but relatively neglected elastic framework within the tracheobronchial tree. This is likely to have major clinical relevance to the pathophysiology of respiratory disease and ageing. The trachealis muscle is more complex than previously stated. Clin. Anat. 22:689–697, 2009. V 2009 Wiley-Liss, Inc. C Key words: elastic fibers; trachealis; tracheal anatomy INTRODUCTION The importance of normal tracheal anatomy is highlighted by such conditions as congenital tracheal deformities, tracheobronchomegaly, and tracheomalacia. The trachea is known to be elastic, but there is a paucity of information about the detailed arrangement of these elastic fibers. Their overall distribution within the trachea has not been studied systematically. In addition, there are controversies about the anatomy of the trachealis muscle and its bronchial counterpart, such as the existence of longitudinal muscle fibers and the precise insertion of the muscle into cartilage rings. In fact, the muscle has rarely been investigated since Miller’s detailed description in 1913 (Miller, 1913). The aims of this study were C 2009 V Wiley-Liss, Inc. to qualitatively investigate the distribution of elastic fibers in the human trachea and major bronchi and the anatomy of trachealis. Grant sponsor: Asthma and Respiratory Foundation of New Zealand (Summer Research Scholarship to K.S.K.) *Correspondence to: Prof. M.D. Stringer, Department of Anatomy and Structural Biology, Otago School of Medical Sciences, University of Otago, PO Box 913, Dunedin, New Zealand. E-mail: [email protected] Received 25 March 2009; Revised 17 June 2009; Accepted 23 June 2009 Published online 27 July 2009 in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/ca.20841 690 Kamel et al. MATERIALS AND METHODS Statistics Tracheal Dissection Mean widths of tracheal membranes were compared with an unpaired t-test, expressing P as an absolute value. Ten cadaveric human tracheas were obtained under the New Zealand Human Tissue Acts (1964, 2008) and the Body Bequest programme of the Department of Anatomy and Structural Biology, University of Otago. Each specimen extended from the larynx to the main bronchi. The specimens were from seven males (age range 68–91 years) and three females (age range 70–101 years) embalmed with Crosado mix (phenoxyethanol), Dodge Anatomical mix, or Dunedin mix (Nicholson et al., 2005). None of the cadavers had any known pathology affecting the trachea or main bronchi. The membranous wall of the trachea and main bronchi were dissected using a dissecting microscope (Nikon, Model C-LEDS), optical loupes 32.5 magnification (Carl Zeiss, West Germany), and microinstruments. Histology The three best fixed tracheal specimens were selected for histological study. The regions of histological interest were as follows: the inferior onethird of the cricoid en bloc with the first four tracheal rings; the fourth to eighth tracheal rings; the ninth and tenth tracheal rings; and the last tracheal ring en bloc with the carina and first bronchial ring. In two specimens, a block consisting of the fourth and fifth cartilage rings of the right and left main bronchi was processed. In one of the three specimens, lobar and segmental bronchi were also available for histological examination. All specimens were postfixed overnight in 10% neutral buffered formalin and embedded in paraffin. Blocks were sectioned transversely with the exception of the specimen containing the ninth and tenth tracheal rings, which was sectioned sagittally. All sections (5 lm) were stained with Hematoxylin and Eosin, VerhöeffVan Gieson for elastic fibers, and Masson’s Trichrome stain, and examined under an Olympus BX51 microscope. Photographs were taken using The MicroPublisher 5.0 RTV Q imaging system (JH Technologies, USA). Tracheobronchoscopies With the patients’ consent, recordings of five anonymized archival video tracheobronchoscopies were reviewed. These were from three males and two females with a mean age of 57 years (range 47–68); none had gross pathology affecting the proximal major airways. All procedures were performed by a single operator (L.B.) using a flexible fiberoptic bronchoscope of either 4.9 or 6 mm external diameter (Olympus New Zealand, Auckland). Endoscopies were performed without sedation after spraying the vocal cords with local anesthetic. All study material was independently reviewed by two researchers (K.S.K. and M.D.S.). RESULTS Dissection and Histology Figure 1 shows the basic arrangement of the layers of the membranous wall of the trachea observed in this study. Microdissection and histological studies revealed three morphologic features that have previously been poorly described. Elastic fibers in the trachea and bronchi. An extensive meshwork of elastic fibers was found within the trachea and main bronchi. The fibers were organized into discrete longitudinal bundles within the submucosa of the membranous wall of the trachea and main bronchi (Fig. 2). The bundles ran continuously from the inferior border of the cricoid into and beyond the main bronchi. In the sub-cricoid region, they were denser toward the centre of the membranous wall of the trachea. Fine oblique strands interconnected adjacent longitudinal bundles. At the tracheal bifurcation, the majority of longitudinal elastic bundles in the membranous trachea continued into the right main bronchus; those in the left main bronchus tended to originate from near the ends of cartilage rings in the distal trachea (Fig. 2). In photomicrographs, the longitudinal elastic fiber bundles were clearly visible in the submucosa (Figs. 1 and 3a). In addition, there was a thin layer of elastic fibers running transversely beneath the basement membrane of the mucosa in the membranous wall of the trachea (Fig. 3a). In the nonmembranous part of the trachea, i.e., internal to the cartilage rings, the longitudinal elastic fibers were more evenly distributed in smaller, less discrete bundles, and the transverse elastic fiber layer was incomplete or absent (Fig. 3b). Elastic tissue was also aggregated in other regions of the trachea: i. within the dominantly collagenous posterior membrane spanning the gap between the ends of the cartilage rings; the fibers were mostly vertical with some transverse elements. ii. between cartilage rings there was a dense concentration of mostly vertical elastic fibers connecting adjacent perichondrial surfaces (Fig. 4a). iii. in a dense laminated arrangement of predominantly vertical elastic fibers connecting the ends of cartilage rings, deep to the fibroelastic membrane (Fig. 4b). iv. at the tracheal bifurcation where elastic fibers were noted in the carinal. More peripherally, in lobar and segmental bronchi, the elastic fibers in the submucosa were arranged as a thin incomplete circumferential layer with more prominent longitudinal fibers organized into small bundles. In the lobar bronchi, the longitudinal elastic bundles were more discrete in membranous parts of Microanatomy of the Human Trachea Fig. 1. Low-power photomicrograph of a transverse section of a human trachea (Verhöeff–Van Gieson stain). The following layers are visible from the lumen outwards: pseudostratified ciliated columnar respiratory epithelium (RE) on a basal lamina under which there is a thin lamina propria and thicker submucosa containing bundles of black elastin fibers, trachealis muscle (TM), a fibroelastic membrane (M), and an adventitial layer (A) of loose connective tissue. Distributed within the submucosa and fibroelastic membrane are serous and mucous glands (G), nerve fibers, and blood vessels. the bronchial wall and more diffuse in areas deep to cartilage plates. In segmental bronchi, the elastic fibers were in even smaller and less discrete bundles Fig. 2. Posterior view of the membranous wall of a human trachea showing longitudinal elastic fiber bundles. There are only a few remaining fibers of trachealis, the transversely orientated muscle external to the elastic bundles. a: Complete specimen. b: Close-up showing the arrangement of discrete longitudinal elastic bundles and fine oblique interconnecting elastic fibers in the dis- 691 distributed circumferentially. The connective tissue connecting cartilaginous plates was rich in elastic fibers, which were variably orientated. The posterior fibroelastic membrane. In the posterior membranous wall of the human trachea was a distinct fibroelastic membrane lying outside the transverse muscle fibers of trachealis (Figs. 1 and 5). Embedded within this membrane, particularly in its distal half, were a few scattered longitudinal bundles of trachealis muscle (Fig. 6). The membrane was dominantly collagenous but contained a variable amount of elastic fibers. It was consistently thicker in the subcricoid region and thinner more distally; mean thickness was 1,044 6 71 lm in the subcricoid region, 368 6 248 lm in the mid trachea, and 108 6 26 lm just above the carina (n ¼ 3). In the distal main bronchi, it was even thinner. The mean width of the membrane, calculated from a minimum of 14 serial measurements along the entire length of each of ten tracheal specimens, was 17.7 6 4.4 mm in men (n ¼ 7) and 11.8 6 3 mm in women (n ¼ 3) (two-tailed t-test P value 0.07, difference between means 5.9 mm, 95% CI ¼ 0.7 to 12.5 mm). Scattered within the membrane were nerve fibers, seromucous glands, and blood vessels. At the junction between the membranous wall of the trachea and the cartilage rings, the fibroelastic membrane was continuous with a layer of predominantly fibrous tissue on the outside of the tracheal cartilages. It was not inserted into the posterior ends of the tracheal rings. Trachealis muscle. The trachealis muscle consisted of a transverse layer of smooth muscle in the membranous wall of the trachea between the submucosa and the fibroelastic membrane. Muscle fibers tal trachea. c: The membranous wall of the trachea has been excised and reflected to reveal a luminal view of the longitudinal elastic fiber bundles (right). Note the fine elastic bundles in the nonmembranous part of the left main bronchus (arrowheads). RMB, right main bronchus. 692 Kamel et al. extended for a variable distance (*1 cm) vertically upward from the carina (Fig. 7b). There was no evidence of a muscular deficiency, the ‘‘posterior spur triangle,’’ reported by Miller (1913, 1947). In one specimen, the lower part of the short raphe contained a spur of cartilage extending up from the tracheal bifurcation into which the trachealis muscle fibers also inserted. In some histological sections, the transverse component of trachealis appeared to consist of an inner transverse layer (tTM) and an outer oblique layer (oTM) (Fig. 6b), but these were not distinguishable on microdissection. As noted above, trachealis also had a poorly developed longitudinal component manifest as a few sparsely distributed bundles of smooth muscle embedded in the posterior fibroelastic membrane. Fig. 3. a: High-power photomicrograph of a transverse section of human trachea showing black transverse elastic fibers (small arrowheads) beneath the basal lamina and longitudinal elastic fiber bundles (large arrowheads) in the membranous wall of the trachea. Verhöeff–Van Gieson stain. b: Low-power photomicrograph of a transverse section of human trachea showing longitudinal black elastic fibers organized into less discrete bundles and more diffusely scattered throughout the submucosa in the nonmembranous part. Verhöeff– Van Gieson stain. TM, trachealis muscle; C, cartilage; RE, respiratory epithelium. were easily seen on gross dissection (Fig. 7a). The muscle inserted bilaterally into the dense fibroelastic tissue running vertically and connecting the ends of the cartilage rings, and into the perichondrium on the inner aspect of the posterior end of each ring. The transverse muscle layer was visible from the subcricoid region to the bifurcation of the trachea and continued distally as an equivalent muscle layer in the membranous parts of the walls of the main bronchi; it was thickest in the subcricoid region. Just above the tracheal bifurcation, the transverse fibers of trachealis became slightly oblique, and inserted into a short fibrous raphe that Fig. 4. Low-power photomicrographs of sections of the human trachea stained with Verhöeff–Van Gieson. a: Sagittal section showing longitudinal black elastic fibers (arrowheads) connecting adjacent tracheal rings. b: Transverse section showing the densely aggregated black elastic fibers (arrowheads) around the posterior end of a cartilage ring. C, cartilage; TM, trachealis muscle; M, fibroelastic membrane; RE, respiratory epithelium. Microanatomy of the Human Trachea 693 These longitudinal fibers were discontinuous, variable in length (up to 3–4 cm), and had no consistent pattern between individuals, except that they were more obvious in the lower half of the trachea. In Vivo Observations (Video Tracheobronchoscopy) In all video tracheobronchoscopy recordings, longitudinal elastic fiber bundles could be clearly visualized beneath uninflamed mucosa, at least to the level of intrasegmental bronchi (Fig. 8). These longitudinal bands were most prominent posteriorly in the distal trachea and main bronchi, but were distributed circumferentially in more peripheral bronchi. DISCUSSION Elastic Fibers in the Trachea and Bronchi Fig. 5. Posterior view of a human trachea in which the fibroelastic membrane of the membranous wall has been dissected free. Note the discrete nature of this membrane and the presence of longitudinal muscle fibers embedded within it. The longitudinal elastic fiber bundles in the membranous wall of the trachea are slightly less obvious in this specimen, because most of the transverse muscle fibers of trachealis have been retained. RMB, right main bronchus. Fig. 6. a: A close-up view of the fibroelastic membrane in the membranous wall of the trachea and main bronchi seen from behind. Note the sparsely distributed islands of longitudinally orientated trachealis muscle fibers within the membrane. RMB ¼ right main bron- Our study demonstrates that the elastic tissue in the human trachea is abundant and elaborately organized. Firstly, there are discrete longitudinal bundles of elastic fibers in the submucosa of the membranous wall of the trachea. Fine interconnections pass between these bundles. In the cadaver, the bundles produce longitudinal corrugations in the tracheal mucosa (Monkhouse and Whimster, 1976) (Fig. 2c). Longitudinal elastic fibers are also present in the nonmembranous parts of the trachea, deep to the cartilage rings, but here they are arranged more diffusely. The longitudinal elastic fibers continue into the bronchi where they form thinner bundles, which become distributed in a progressively more circumferential pattern with each bronchial subdivision. Secondly, there is a thin layer of transverse elastic fibers just beneath the basal lamina of the membranous wall of the trachea. Bock and Stockinger chus. b: Medium power photomicrograph of a transverse section of human trachea stained with Verhöeff– Van Gieson. lTM, longitudinal muscle fiber bundles of trachealis; oTM, oblique fibers of trachealis; tTM, transverse muscle fibers; M, fibroelastic membrane. 694 Kamel et al. Fig. 7. a: Posterior view of the subcricoid region in which the fibroelastic membrane of the posterior wall has been reflected superiorly together with the underlying transverse fibers of trachealis. Note the vertically running elastic bundles deep to the trachealis muscle and most pronounced toward the center of the posterior wall. b: Posterior view of a human trachea in which the fibroelastic membrane of the posterior wall has been partially removed to expose the underlying fibers of trachealis. Note the oblique fibers of trachealis in the region of the tracheal bifurcation (small arrowheads) and the short fibrous median raphe extending superiorly from the carina (arrow). CC, cricoid cartilage; TM, trachealis muscle; E, elastic bundles; RMB, right main bronchus. (1984) showed that these subepithelial fibers consist of elaunin and oxytalan fibers in contrast to the longitudinal elastic fibers in the trachea, which are composed of elastin. A third, dense aggregate of elastic fibers vertically connects the ends of successive C-shaped cartilages, forming a column of elastic tissue along the length of the trachea posteriorly on each side. Finally, elastic fibers are distributed within the fibroelastic membrane of the membranous wall of the trachea and within the submucosa at the carina. The videobronchoscopic images further highlight the existence of longitudinal bands of elastic fibers throughout the major airways. The elastic fibers in the bronchi are continuous with those surrounding the alveoli and within the interalveolar septa (Miller, 1947; Monkhouse and Whimster, 1976), creating a system analogous to a network of fine bungy cords. Fig. 8. Endobronchial views of the elastic bundles within (a) a lobar bronchus and (b) an intrasegmental bronchus, where they were distributed more circumferentially. Microanatomy of the Human Trachea While considerable attention has been focused on the elastic properties of the lung, there is comparatively little research on the elastic network in the major airways, which is surprising given its potential importance in health and disease. Elastic fibers are known to exist in the lamina propria and submucosa of the trachea and bronchi, but their precise architecture has not been described. The trachea has no muscularis mucosae, and some histology texts define the boundary between the mucosa and submucosa by a layer of elastic fibers (Ross and Pawlina, 2006; Eroschenko, 2008). However, this is variously referred to as a ‘‘distinct band’’ or ‘‘longitudinal membrane,’’ neither of which is accurate. In our specimens, this layer most closely corresponds to the thin and sometimes incomplete transversely running elastic fibers below the basement membrane (Fig. 3). Most histology texts simply refer to the presence of elastin fibers, an ‘‘elastic lamina’’ or a layer of fibroelastic tissue in the deeper layer of the lamina propria (Young et al., 2006; Eroschenko, 2008). A few authors state that elastic fibers are organized into longitudinal bands (Stevens and Lowe, 2005; Ovalle and Nahirney, 2008). Of the anatomy texts, the most detailed description is in Gray’s Anatomy (Standring, 2008), which refers to ‘‘broad, longitudinal bands of elastin within the submucosa’’ following the course of the respiratory tree and connecting with the elastin network of the interalveolar septa. Modern texts on respiratory medicine and pathology offer little more detail; one of the better descriptions consists of a single paragraph outlining the longitudinal elastin bundles in the trachea and bronchi, but states that those in the left main bronchus originate from beyond the carina (Corrin and Nicholson, 2006). Most of the detailed anatomical studies of the major airways were performed more than half a century ago (Miller, 1913, 1947). Of the more recent publications, a few have studied the tracheobronchial cartilages (Reid, 1976), trachealis (Hakansson et al., 1976), the membranous trachea (Wailoo and Emery, 1980), the longitudinal elastic fiber bundles (Monkhouse and Whimster, 1976), and the microstructure of the elastic fibers in the trachea and bronchi (Bock and Stockinger, 1984). There has been no systematic study of the distribution of elastic fibers throughout the trachea and proximal bronchi. What are the functional correlates of this network of elastic fibers? In young adults, tracheal length increases by about 20% and the carina descends by about 2.5 cm in deep inspiration (Harris, 1959). The bronchi also elongate by up to 25% of their resting length (Holden and Ardran, 1957). The trachea also lengthens with neck movements. In a study of adult patients undergoing endotracheal intubation, laryngotracheal length from the vocal cords to carina increased by 13% between cervical flexion and extension; the airway between the vocal cords and midtrachea stretched by 38% (Wong et al., 2008). Linear extensibility and elastic recoil of the isolated human trachea is remarkably age-dependent, decreasing from a maximum of about 40–50% in children to 695 about 20–25% at 70 years of age (Harris, 1959; Croteau and Cook, 1961). For this reason, children have higher anastomotic complication rates from excessive tension as compared to adults undergoing equivalent tracheal resections (Wright et al., 2002). Several examples illustrate the potential clinical importance of the elastic network. The role of elastin destruction in the pathogenesis of emphysema is well known (Davidson and Bai, 2005), but there appears to be little discussion about the consequences of leukocyte-mediated elastase-induced degradation of elastin in the major airways. The abundance of elastin we observed in the tracheobronchial tree, even in elderly cadavers, suggests that this deserves further study. Tracheobronchomegaly is characterized by severe dilatation of the trachea and main bronchi and is believed to be due to degeneration of elastic tissue in the trachea (Hasleton, 1996). A rare congenital form of tracheobronchomegaly (Mounier–Kuhn syndrome) has been reported in children with cutis laxa (Aaby and Blake, 1966; Wanderer et al., 1969), and in association with Marfan syndrome (Shivaram et al., 1990)—both of these conditions are genetic disorders of elastin synthesis. Finally, the effect of elastin degeneration with advancing age may be important. With an estimated half-life of 70 years, elastin is the most stable and persistent protein in the human body (Vieth et al., 2007), but it does degenerate in the elderly. Robert et al. (2008) have highlighted the striking parallel between elastin degradation and declining respiratory function with advancing age. The Posterior Fibroelastic Membrane The presence of fibroelastic tissue in the membranous wall of the human trachea is known (Stevens and Lowe, 2005), but the existence of a macroscopically discrete membrane separate to the transverse fibers of trachealis is a novel finding. Furthermore, unlike previous descriptions, our study revealed that this fibroelastic membrane within the membranous wall of the trachea was continuous with a layer of connective tissue extending around the outside of the cartilage rings; in no case was it attached to the inner perichondrium. Longitudinal fibers of trachealis were embedded within the membrane. The transverse (and oblique) fibers of trachealis were internal to and separate from the membrane. The membrane was dominantly collagenous in the specimens in this study, but may well be more elastic in younger individuals. It was wider in men. The membrane may have numerous functions including strengthening the posterior wall of the trachea, resisting collapse and overdistension, and adding to elastic recoil after vertical stretch. Trachealis Muscle Despite numerous uncertainties about the anatomy of trachealis and its function, the muscle has rarely been studied since Miller in 1913 (Miller, 1913). Accounts in modern anatomy and histology texts are generally limited to a statement about it being a transversely orientated smooth muscle within the membranous wall of the trachea (Stevens 696 Kamel et al. and Lowe, 2005); a few authors mention longitudinal muscle fibers (Macleod and Heard, 1969; Young et al., 2006; Standring, 2008). Our study contradicts several anatomical features reported by Miller (1913, 1947). He largely dismissed the presence of longitudinal fibers, which were clearly visible within the fibroelastic membrane in our investigation. Indeed, longitudinal muscle bundles were seen in 84% of pediatric tracheas and in 90% of adult tracheas in two other studies (Macleod and Heard, 1969; Wailoo and Emery, 1980); the latter authors also noted that the longitudinal muscle bundles were more consistently present in the distal half of the trachea. We do not agree with Miller’s description of the arrangement of trachealis at the tracheal bifurcation. Indeed, the short fibrous raphe that we observed has not been previously described. Miller also stated that trachealis does not insert into the connective tissue between the tracheal rings, which is at odds with our observations. We found that the transverse fibers of trachealis inserted into the dense fibroelastic tissue connecting the ends of the cartilages, as well as into the perichondrium on the inner aspect of the posterior end of each ring. In this context, it is interesting that the mucosal herniation that develops with acquired tracheal diverticula occurs most commonly in the posterolateral region of the trachea at the junction between the cartilaginous and membranous parts (Soto-Hurtado et al., 2006). However, the concept that diverticula occur at this site because the transverse muscle fibers of trachealis diverge to insert into adjacent cartilage rings [rather than inserting into the connective tissue in the inter-ring spaces (Reid, 1976)] is not supported by our findings. Our study was qualitative. One previous quantitative study of trachealis in 30 cadavers, nine of which had a history of chronic bronchitis, demonstrated no significant difference in the cross-sectional area of the muscle along the length of the trachea, but the subcricoid region was not specifically investigated (Macleod and Heard, 1969). The function of trachealis and its counterpart in the main bronchi is controversial. Galen suggested that trachealis exists to accommodate esophageal dilatation during swallowing, but this would not need muscle tissue and the esophagus is only closely approximated to the trachea in its upper third (Miller, 1913). Another function commonly attributed to trachealis is narrowing of the tracheal lumen to increase airflow velocity during coughing (Olsen et al., 1967). When the glottis is closed and intrathoracic pressure rises at the beginning of a cough, the membranous wall of the trachea bulges inwards (Rayl, 1965; Olsen et al., 1967). As the glottis opens, the intrathoracic pressure continues to compress the trachea (Rayl, 1965). If trachealis contracted during this phase, it could serve several functions: (i) it would narrow the tracheal lumen by approximating the ends of tracheal cartilages, thereby increasing airflow velocity and cough efficiency (Olsen et al., 1967); (ii) it would increase the rigidity of the trachea by making the cartilage rings more circular, thus increasing the ability of the trachea to withstand extrinsic compression (Olsen et al., 1967; Cullen et al., 2002); and (iii) it would resist the tendency for anteroposterior airway collapse induced by a high intrathoracic pressure and a low intratracheal pressure secondary to rapid air flow (Bernoulli effect). Some evidence for the latter comes from high-speed film images in dogs during simulated coughing (Proctor, 1977). It is also possible that trachealis plays a role in lung fluid dynamics in the fetus (Cullen et al., 2002). The major limitation of our study was that microdissection and histology were limited to ten tracheas, all of which were obtained from elderly cadavers. Although satisfactory histological preparations can be obtained from cadavers embalmed with the reagents used in this study (Nicholson et al., 2005), age may well affect the density of elastic fibers. However, since these are likely to be even more prominent in younger subjects, they may be of even greater physiological and clinical importance. Despite this limitation, it is unlikely that our qualitative observations would change substantially with more specimens. In conclusion, the novel findings concerning the elastic tissue within the tracheobronchial tree are likely to be of clinical relevance to various respiratory disorders and may offer a new paradigm for understanding pathophysiologic mechanisms of selected respiratory disorders. 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