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Tissue Engineering in Otorhinolaryngology
cal and microbiological aggression.
5
Fibroblasts are the main
cell type in the lamina propria. These cells are produced
and embedded in an extracellular matrix. The extracellu-
lar matrix supports all tissue cells and plays an important
role in the regulation of cell migration, proliferation and
differentiation.
6,7
Hyaluronic acid abounds particularly in
the superficial layer of the lamina propria, hydrating the
vocal folds and making them compressible. The remaining
layers are mainly composed of collagen and elastin, which
are responsible, respectively, for tensile and elastic resis-
tance of the vocal folds.
5,6
When the ultrastructure of the extracellular matrix is
altered, either by surgical interventions or factors such as
infection, trauma, or radiation, a disruption of the vibrating
function of the vocal folds may result.
6,8,9
In these situa-
tions there is usually an aberrant healing process that favors
collagen deposition and decreased production of hyaluronic
acid and elastin fibers, which ultimately, leads to scar for-
mation, responsible for dysphonia. A number of therapeutic
interventions have been described to prevent and/or treat
vocal fold scars or atrophy, invariably with limited success
due to the difficulty of restoring the mucosal wave. Thus,
the goal of regenerative medicine concerning vocal folds
is restoring the vibratory and respiratory functions of the
larynx through the reconstruction of the lamina propria’s
extracellular matrix using the elements described below.
Regulators/Growth Factors
Growth factors are the only elements that, to date, have
been successfully applied in vocal fold bioengineering.
5,10
Within these, fibroblast growth factor assumes a prominent
position since it has an important role in the regulation
of scar formation. Hirano showed that fibroblast growth
factor enhanced the production of hyaluronic acid by fibrob-
lasts of the vocal folds, while inhibiting local deposition of
collagen.
11,12
He described a clinical case in which fibrob-
last growth factor was used in the treatment of atrophic
vocal folds of a 63-year-old male, with clear improvement
of acoustic parameters only 1 week after surgery.
12
Another cytokine that has been increasingly studied in
the treatment of vocal scars is the hepatocyte growth factor,
mainly due to its anti-fibrotic and angiogenic properties.
5
Similar to the fibroblast growth factor, hepatocyte growth
factor also stimulates the production of hyaluronic acid
and elastin and inhibits collagen synthesis.
13,14
Hirano made
another experience where he injected hydrogel impreg-
nated with hepatocyte growth factor in a previously injured
canine vocal fold. Here, a structural regeneration of the
vocal fold with improvement of the mucosal wave was
observed.
3,5
Despite the promising results, the safety profile of these
newly used growth factors has not been completely defined,
and the risk of malignant transformation has been consid-
ered as a major limitation to their clinical application.
5
Scaffolds
Any scaffolds used in laryngology should have structural
features, chemical composition and mechanical properties
similar to those of the lamina propria. These should be con-
stituted by biocompatible and re-absorbable material that
promote adhesion, proliferation and cell differentiation, in
order to allow a successful restoration of the extracellular
matrix.
5,15
A wide range of materials has been used, such as
hydrogels based on hyaluronic acid (Carbylan-SX and
Carbylan-GSX).
12
These biomaterials, when applied to an
injured vocal fold (such as a deep biopsy), modulate scar for-
mation and consequently preserve the vocal fold viscoelastic
properties.
9
Acellular biological scaffolds have also been used as an
alternative to hydrogels. These structures are derived from
porcine intestinal mucosa, lamina propria of bovine vocal
folds or human umbilical vein. They are then submitted to
decellularization procedures based on the immune response
triggered by contact with human vocal fold fibroblasts.
16
Kishimoto et al.
17
conducted a study in 6 patients with a
vocal fold scar or sulcus, who were submitted to the place-
ment of such scaffolds in a subepithelial bag after excision
of scar tissue. In the 6 months following surgery, a significant
restoration of the extracellular matrix was observed, with
improvement of acoustic parameters, and total degradation
of the scaffold material. Therefore, it is a promising medical
device.
Cells
Cell therapy in laryngology is based on the injection of
cells that will produce extracellular matrix elements, resul-
ting in the reconstruction of the vocal fold microstructure.
These cells are generally fibroblasts,
18
bone marrow or adi-
pose tissue stem cells
19
(which are capable of producing,
in vitro
, all the elements of the vocal fold),
20
and embry-
onic stem cells.
21
In all cell therapy-based studies, a clear
improvement of vocal fold fibroelastic properties has been
observed.
18--21
However, the potential risk for neoplastic
transformation of targeted tissues
3,5
and many ethical issues
concerning the manipulation of embryonic cells have limited
its use.
Plastic and Reconstructive Surgery
Many head and neck surgical procedures, such as rhinoplasty,
septoplasty, correction of septal perforation, or pinna recon-
struction involve the use of autologous cartilage which is
collected from the nasal septum, ribs, or concha.
2
Despite
being a limited resource concerning its finite extension and
particular geometrical configuration, septal cartilage has
been the most commonly used, due to its structural features
and accessibility.
22,23
Tissue engineering may, therefore,
provide an alternative, with the possibility of originating a
higher amount of cartilage with the desired shape.
The first and most popular work in this area was published
in 1997 by Cao et al.
24
The production of an ear-shaped car-
tilage matrix for treating a 3-year-old child was described,
using bovine chondrocytes added to a previously molded
polyglycolic acid matrix (scaffold). These elements were
implanted in the back of a laboratory mouse, and new repli-
cating chondrocytes were observed within 12 weeks.
More recently, Yanaga et al.
25
published a series of
4 cases in which the surgical technique for reconstruction of