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Clinical Anatomy 22:689–697 (2009)
Novel Insights Into the Elastic and Muscular
Components of the Human Trachea
Department of Anatomy and Structural Biology, Otago School of Medical Sciences, University of Otago,
Dunedin, New Zealand
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.
Key words: elastic fibers; trachealis; tracheal anatomy
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
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. DOI 10.1002/ca.20841
Kamel et al.
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
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
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).
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.).
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-
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
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
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
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.
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.
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
(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
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
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
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
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,
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.
We wish to thank Amanda Fisher for preparation
of histological slides, Mr. Chris Smith for assistance
with photography, and Mr. Andrew McNaughton for
advice with microscopy.
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