J. Biomedical Science and Engineering, 2010, 3, 1006-1012 JBiSE
doi:10.4236/jbise.2010.310131 Published Online October 2010 (http://www.SciRP.org/journal/jbise/).
Published Online October 20 10 in SciRes. http://www.scirp.org/journal/jbise
Immunohistochemical characterization of the rabbit
tracheal cartilages
Richard D. Wemer1, Michael Detamore2, Robert A. Weatherly3
1Department of Otolaryngology-Head and Neck Surgery, Park Nicollet Clinics, St. Louis Park, USA
2Department of Chemical and Petroleum Engineering, University of Kansas, Kansas City, USA
3Department of Otolaryngology-Head and Neck Surgery, University of Kansas, Kansas City, USA
Email: wemerr@parknicollet.com; detamore@ku.edu; rweatherly@kumc.edu
Received 5 August 2010; revised 18 August 2010; accepted 27 August 2010.
The objective of this study was to immunohisto-
chemically elucidate the major extracellular matrix
constituents of rabbit tracheal cartilage. The impe-
tus for this project is the need for crucial design
and validation criteria for tissue engineering jux-
taposed with the conspicuous lack of trachea ex-
tracellular matrix data in the literature. Tracheal
tissue specimens were harvested from New Zealand
White rabbits, and were immunostained for colla-
gen I, collagen II, aggrecan and decorin; and a
Verhoeff-Van Gieson stain was performed to visu-
alize elastin. The most striking result was the highly
organized relationship between distinct fibrous
(containing collagen I, decorin and elastin) and
hyaline-like (containing collagen II and aggrecan)
regions of the tracheal wall. The tracheal cartilage
stained strongly with collagen II throughout, with
periodic bands of aggrecan in the tracheal arches,
meaning that there were areas void of aggrecan
immunostaining alternating with areas with strong
aggrecan immunostaining. In contrast, the periph-
ery of the cartilage and the perichondrium itself
exhibited strong collagen I staining and no collagen
II staining. Elastin fibers and decorin were also
detected along the periphery of the cartilage in the
perichondrium and corresponded highly with the
distribution of collagen I staining. The body of the
rabbit trachea is therefore composed of a hya-
line-cartilage structure primarily made of collagen
II and bands of aggrecan, surrounded by a fibrous
region composed of elastin and collagen I, indica-
tive of a flexible tissue with distinct regions of
compressive integrity. This information will be a
valuable reference to future tissue engineering ef-
forts in the creation of a biosynthetic substitute for
laryngotracheal reconstruction.
Keywords: Trachea; Collagen; Aggrecan; Decorin;
The treatment of subglottic stenosis is a difficult clinical
problem that may require invasive measures at both pri-
mary and donor sites. After failure of more conservative
measures, open airway surgery is often performed,
which requires donor cartilage tissue to be harvested,
usually a segment of autologous costal cartilage. Such
procedures are complicated by long healing times, donor
site complications including pain and pneumothorax,
size and shape mismatches, and prolonged hospital
Tissue-engineered cartilage offers a viable alternative
to these highly morbid procedures. Donor site complica-
tions could be eliminated, and tissue could be shaped
appropriately for specific defects in the tracheal wall.
Tissue engineering attempts for the trachea are still pre-
liminary, from a clinical perspective, at this time. Nev-
ertheless, tracheal tissue engineering is a burgeoning
field [1-13], and the level of urgency is high for provid-
ing crucial characterization data for tissue engineers,
both to develop new ideas for design strategies for reca-
pitulating the native tracheal structure, and to elucidate
validation criteria.
We have used rabbits in previous tracheal defect re-
construction studies [14-16], as rabbits are an ideal ani-
mal model for early-stage in vivo investigations. A re-
view of animal models for trachea research suggested
that large animals such as goats and sheep should be
used for tracheal tissue engineering based on size and
cell number [17]. However, the rabbit has heretofore
been a more commonly used animal model for tracheal
tissue engineering and is a logical stepping stone en
route to larger animal models. For this reason, we have
selected the rabbit for our immunohistochemical charac-
R. D. Wemer et al. / J. Biomedical Science and Engineering 3 (2010) 1006-1012
Copyright © 2010 SciRes. JBiSE
terization of the trachea.
In contrast to the trachea, the extracellular matrix of
the vocal folds of the larynx (which lie just superior to
the trachea) continues to be a popular research topic,
including studies of collagen types, elastin, and pro-
teoglycans [18-36]. However, despite the wide variety of
trachea characterization efforts, ranging from camel tra-
chea dimensions [37] to dolphin trachea biomechanics
[38], and even speculation of airflow in dinosaur tra-
cheas [39], very little is known about the distribution of
key extracellular matrix components of the trachea.
Evans and colleagues presented a nice series of matrix
studies on the basement membrane zone associated with
the trachea in monkeys and rats [40-42]. However, only
a handful of studies have begun to elucidate the collagen
and/or elastin content of the trachea in guinea pigs, mice
and rats [43-47], in pigs [48, 49], and in humans [50, 51].
It is known that collagen fibers in the trachea run
obliquely and are intermingled [43,51,52], that collagen
I appears predominately in the perichondrium and colla-
gen II appears predominately in the cartilaginous tra-
cheal body [45-47,50]. It is also known that elastic fibers
may be abundant in the perichondrium of the trachea
[36,43,44], forming an extensive network that extends
into the posterior membranous tracheal wall [36]. With
regard to elastin, it is noteworthy that a pair of studies
found trappin-2 (also known as elafin), an elastase in-
hibitor, in the trachea [48,49].
In addition, specific glycosaminoglycans (GAGs)
were identified immunohistochemically in rat tracheas,
although the heterogeneous distributions of chondro-
itin-4-sulfate and chondroitin-6-sulfate were not in
agreement [47]. Toluidine blue O staining in humans
also revealed a heterogeneous GAG distribution [51],
although the distributions of specific proteoglycans was
not identified. It is also known that regions of the trachea
are calcified, although there is a debate as to whether the
tissue is commonly ossified [46,47,50,53].
What was not known prior to the current study was
the distribution of key proteoglycans, which have im-
portant functional roles, and whether their distribution
coincided with the distribution of collagen types I and II.
Moreover, it was unknown whether the distribution of
collagens I and II observed in the trachea of rats [45-47]
and humans [50] would be observed in the rabbit trachea
as well. Therefore, the objective of the current study was
to identify the distributions of elastin, collagen I, and
collagen II; as well as two key proteoglycans (aggrecan
and decorin) in the rabbit trachea. These matrix compo-
nents were chosen based on their relevance to tissue en-
gineering, implications for mechanical performance, and
to establish relative distribution and abundance for vali-
dation of engineered constructs. Given that collagen II
and aggrecan are associated with hyaline cartilage,
whereas elastin, collagen I and decorin are associated
primarily with fibrous tissues [54], we hypothesized that
aggrecan would coincide with collagen II distribution,
and that decorin would coincide with collagen I and
elastin distribution. The relative distributions of these
key extracellular matrix components will be crucial to
tissue engineering efforts that address surgical repair of
laryngotracheal stenosis.
2.1. Specimen Preparation
Four New Zealand White rabbits were obtained and sac-
rificed at adult age. All rabbits were male and weighed
approximately 9 lbs. They were sacrificed after under-
going a single cardiovascular physiology experiment not
relevant to tracheal protein structure or synthesis. Imme-
diately after sacrifice, the larynx-trachea complexes
were collected and stored in PBS-saturated gauze at
–20˚C prior to sectioning. Serial 10 µm frozen sections
were taken from each of the samples. Sections were
mounted on slides, fixed in chilled acetone for 20 min,
and then stored at –20˚C until further use.
2.2. Verhoeff-Van Gieson Elastic T iss ue St ain
Fixed rabbit tissue samples were stained for elastin.
The samples were first placed in an iron hematoxylin
solution for 10 min and then rinsed in distilled water
and differentiated in 2% ferric chloride. After rinsing
in distilled water and placing in 95% alcohol, the
samples were counterstained with Van Gieson’s solu-
tion (Sigma Aldrich; St. Louis, MO) for 4 min. The
samples were then dehydrated in graded alcohol and
then cleared in xylene and mounted.
2.3. Immunohistochemistry
Slides were placed in a Biogenex i6000 (San Ramon,
CA) autostainer and rehydrated for 5 min in PBS. En-
dogenous peroxidase activity was quenched with 1%
hydrogen peroxide in methanol for 30 min. Specimens
were blocked with serum from the secondary antibody
host for 20 min and followed by incubation with the
primary antibody for 60 min (dilutions provided in Ta-
ble 1). Mouse monoclonal IgG anti-collagen I and
mouse monoclonal IgG anti-collagen II antibodies were
obtained from Accurate Chemical and Scientific (West-
bury, NY) and Chondrex, LLC (Redmond, WA), respec-
tively. Mouse monoclonal IgG anti-aggrecan and sheep
polyclonal IgG anti-decorin were obtained from Abcam
(Cambridge, MA). Negative controls were assessed by
omission of the primary antibody, and the absence of
non-specific staining was verified.
The specimens were then incubated with the appro-
R. D. Wemer et al. / J. Biomedical Science and Engineering 3 (2010) 1006-1012
Copyright © 2010 SciRes. JBiSE
Table 1. Immunohistochemistry primary antibody dilutions.
ECM Constituent Primary Antibody Dilution
Collagen I
Collagen II
priate secondary antibody for 30 min (1:300 dilution),
followed by an avidin-biotinylated enzyme complex for
30 min and then 3,3’-diaminobenzidine for 4 min.
Avidin-biotin complex kits were obtained from Vector
Laboratories (Burlingame, CA). The kits consisted of a
biotinylated secondary antibody (horse anti-mouse IgG,
or rabbit anti-sheep IgG), the corresponding host animal
serum, and an avidin-biotinylated enzyme complex.
Slides were removed from the autostainer, counter-
stained with hematoxylin, dehydrated in graded ethanol
and mounted.
3.1. Collagen II and Aggrecan
The tracheal hyaline cartilage stained strongly with col-
lagen II throughout the body of the cartilage ring. Out-
side of the cartilage ring, no collagen II staining was
detected Figure 1(a). Also, within the tracheal body, we
did observe staining for aggrecan. Aggrecan, however,
had a different staining pattern within the tracheal body.
The body of the cartilage exhibited a striped effect where
there were areas void of aggrecan immunostaining, al-
ternating with areas displaying strong aggrecan immu-
nostaining. This was observed throughout all sections of
the tracheal cartilage, on multiple slides, and from all
observed animal specimens. There was also some ag-
grecan staining in the surrounding tissues outside of the
cartilage ring Fig ure 1(b).
3.2. Collagen I, Decorin, and Elastin
Collagen I immunostaining was observed around the
periphery of the tracheal body cartilage. There was no
immunostaining in the cartilage body itself Figure 2(a).
Decorin staining was also identified in the tracheal car-
tilage samples. Decorin was seen in the periphery of the
cartilage and perichondrium, as well as the surrounding
soft tissues. This distribution was very similar to the
distribution seen with collagen I staining Figure 2(b),
with no decorin staining being observed in the hyaline
cartilage itself.
Elastin fibers, detected with the Verhoeff-Van Gieson
stain, were also located along the periphery of the carti-
lage within the perichondrium. No staining was seen in
the pericellular areas or in within the cartilage extracellu-
lar matrix. Elastin staining corresponded highly with the
distribution of collagen I and decorin staining Figur e 2(c).
Figure 1. Collagen II and Aggrecan Localization. Tracheal
cartilage with collagen II (brown immunostain) demonstrated
throughout the tracheal arch (a). A striped effect (arrows) was
observed in the tracheal arch with aggrecan immunostaining
(b). Hematoxylin counterstain. Scale bars are 50 μm (a) and
100 μm (b).
To the best of our knowledge, the current study is the
first to describe the extracellular matrix of the rabbit
trachea, and more importantly, the first to investigate the
distribution of key proteoglycans in the trachea and cor-
relate their distribution with collagens I and II. The cur-
rent study was in agreement with previous findings in
other species that found collagens I and II in mutually
exclusive regions [45-47,50], with the former present in
only the surrounding perichondrium and the latter pre-
sent in only the body of the tracheal cartilage. Further-
more, the current study was in agreement with a previ-
R. D. Wemer et al. / J. Biomedical Science and Engineering 3 (2010) 1006-1012
Copyright © 2010 SciRes. JBiSE
Figure 2. Collagen I, Decorin and Elastin Localization. Colla-
gen I was peripheral to the tracheal arch (a). Decorin was also
peripheral to the tracheal arch (b). Verhoeff-Van Gieson stain
for elastin (black bands marked by arrows) (c). Counterstain
for (a) and (b) was hematoxylin. Scale bars are 100 μm (a, c)
and 50 μm (b).
ous study in guinea pigs that found elastin in the tracheal
periphery [43].
Although the distributions of three specific GAGs
(most likely chondroitin-4-sulfate, chondroitin-6 sulfate
and keratan sulfate) were elucidated in rats [47], the
relevant and functionally more significant distribution of
proteoglycans in the trachea was heretofore unknown.
As hypothesized, aggrecan and collagen II distribution
coincided, and decorin distribution was consistent with
collagen I and elastin. However, an interesting and un-
expected finding was the observation of a striped or
“layered” effect with regard to the staining pattern of
aggrecan (Figure 3). This observation is the first of its
kind and its significance is not yet clear. Nevertheless, it
is likely that this pattern of aggrecan distribution may
have an important functional role. For example, periodic
regions of increased compressive integrity, interspersed
with regions of more flexibility, may provide the tra-
cheal cartilaginous arch with a balance of flexibility and
rigidity. The need for aggrecan to promote such a bal-
ance in a tissue engineered construct is a hypothesis
worth exploring in the future.
Both collagen I and elastin were found to stain around
the periphery of the tracheal body. Decorin, which has
been described to “decorate” collagen, was also found in
the extracellular matrix surrounding the tracheal body.
Previous studies have demonstrated a decorin-collagen
interaction, concluding that decorin plays a role in regu-
lating collagen fiber formation [55]. One study demon-
strated a decrease in collagen fiber diameter, as well as
altered collagen morphology in decorin-deficient mice
[56]. It may, therefore, be possible that the close alliance
of decorin and collagen I are necessary for structural
integrity of the tracheal arches, while the relationship of
aggrecan and collagen II are primarily responsible for
optimal flexibility of the tracheal cartilages.
Figure 3. Schematic of Aggrecan Staining in Tracheal Carti-
lage. Aggrecan was located in such a way as to demonstrate a
“layered” or “striped” appearance within the body of the tra-
cheal cartilagenous arch. See also Figure 1(b).
R. D. Wemer et al. / J. Biomedical Science and Engineering 3 (2010) 1006-1012
Copyright © 2010 SciRes. JBiSE
The biochemical characterization of the rabbit trachea
is an important step for a tissue engineering solution to
the treatment of airway stenosis. This animal model has
been employed for reasons presented earlier, as well as
for the rabbit laryngotracheal segment’s size closely ap-
proximating that of a young pediatric population, of high
relevance to tissue engineering for use in treating human
disease. In demonstrating the composition of a portion of
the laryngotracheal complex, we can then attempt to
engineer a tissue that possesses properties similar to that
of the native tissue. Future studies will be required to
elucidate whether any part of the tracheal body possesses
an osseous character (e.g., alkaline phosphatase activity
and collagen I in addition to calcium) [46,47,50,53].
Moreover, other trachea characterization efforts will be
crucial for construct design and analysis, in particular,
studies of the biomechanics of the trachea [51,57-59],
the fluid dynamics of air flow through the trachea
[60-65], and characterization of surrounding tissues such
as the tracheal smooth muscle.
The identification of native tracheal proteins and pro-
teoglycans is critical to the actual construction of tis-
sue-engineered implants. The identification of proteins
such as collagen I and II, as well as proteoglycans like
decorin and aggrecan, may ultimately lend information
as to what substrates are required to produce a suitable
cartilage graft. We must, however, remain cautious when
considering the need to replicate native tracheal cartilage.
The biochemical composition of cartilage is complex,
and an exact reproduction of the structure may be neither
feasible nor warranted to successfully reconstruct the
trachea after injury to a tracheal segment. For example, a
newly formed cartilage segment may merely need to
possess rigidity, but not maximal flexibility, depending
on the size and length of the diseased tracheal segment
to be replaced. Additional aspects of tracheal reconstruc-
tion such as mucosal resurfacing and vascular supply to
large segments, as highlighted by the Leuven Tracheal
Transplant Group from Belgium [66], will also need to
be addressed as the process of engineered cartilage re-
placement for tracheal defects becomes more clearly
In the current study, we have demonstrated that the
body of the tracheal ring is composed of collagen II and
bands of aggrecan (hyaline-like tissue), surrounded by a
supporting matrix composed of collagen I, elastin, and
decorin (fibrous-like tissue). Looking forward, the in-
formation obtained in the current study can be applied to
the construction of a bioengineered tracheal graft. In an
endeavor to replicate the tracheal cartilage ultrastructure,
an ideal graft would possess enough rigidity to withstand
stresses, but remain flexible enough to allow for motion
and other functional measures. Future studies as de-
scribed above will be required to correlate the structure
of the tracheal cartilage to its function (fluid mechanics
of airflow and biomechanical integrity).
All of the authors in this study had full access to all the
data in the study and take responsibility for the integrity
of the data and the accuracy of the data analysis.
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