Minireview S W I S S M E D W K LY 2 0 0 6 ; 1 3 6 : 2 9 3 – 3 0 1 · w w w . s m w . c h 293
Peer reviewed article
Biomaterials-based tissue engineering
and regenerative medicine solutions
to musculoskeletal problems
Tissue Engineering, VA Boston Healthcare System, and Orthopaedic Research Laboratory,
Brigham and Women’s Hospital, Harvard Medical School, Boston, MA, USA
Tissue engineering and regenerative medicine tunity to discover cell biological behaviour when
offer solutions to a number of compelling clinical cells grow in the three-dimensional tissue-like
problems that have not been adequately addressed environment. Selected clinical applications may
through the use of permanent replacement devices. also require the implantation of regulatory proteins
The challenge will be to select the optimal combi- such as growth factors. That the action of such
nation of a biomaterial scaffold, cells, and soluble polypeptides released from biomaterials is short-
regulators for a particular clinical problem. For lived has led to recent work wedding tissue engi-
many connective tissues of the musculoskeletal sys- neering and gene therapy. Genes can be bound to
tem, with microstructures that reflect the mechan- certain biomaterial scaffolds to be released in vivo
ical environment, it may be more advantageous to over extended periods (eg weeks) in order to genet-
regenerate the tissue in vivo than to fully engineer ically modify cells in the defect to produce the
the tissue in vitro for subsequent implantation. The desired growth factors. Thus a new role for bioma-
porous material to be used as the scaffold to facili- terials is as a delivery vehicle for genes, as well as
tate this regeneration needs to have certain pore for cells and growth factors. These endeavours are
characteristics, chemical composition and mechan- notable particularly because there is a growing con-
ical properties. One approach has been to employ sensus that the challenge of developing biomateri-
substances that serve as analogues of the extracel- als for tissue engineering, regenerative medicine,
lular matrix of the tissue to be regenerated. For se- and gene therapy exceeds the challenge that was
lected indications in which the supply of endoge- faced in the cell biological work that led to the
nous precursor cells is limited it may be more effi- proliferation of cells in vitro (in such a way that they
cacious to employ a scaffold as a delivery vehicle for retain their phenotypic characteristics) and in the
the cells rather than to inject the cells into the de- genetic engineering that has led to the production
fect. Investigations of cell-scaffold interactions in of growth factors and cloning of their genes.
vitro not only offer the opportunity for modifica-
tion of scaffold composition and structure to im- Key words: tissue engineering; regenerative medi-
prove the outcome in vivo, but also offer the oppor- cine; biomaterials; scaffolds; cells; collagen
The term “tissue engineering” has now come While tissue engineering investigations have
to encompass a wide range of strategies employing generated promising results, it is important to
cells, synthetic and processed natural materials, point out that no “tissue engineering” procedure,
tissues, cytokines and genes for the regeneration of or any other treatment, has yet been successful in
This work was tissue in vivo or the production of tissue in vitro. fully regenerating a tissue that does not have the
supported by the Cell therapies and tissue transplant procedures are capability to spontaneously regenerate (eg bone).
of Veterans Affairs thus now often considered under the rubric of tis- One such tissue that does not spontaneously
and Geistlich Bio- sue engineering. In this respect tissue engineering regenerate is articular cartilage. It has been known
is not so much a revolution in reconstructive sur- for many years that the long-term function of
which also provided gery but part of the evolutionary process that this articular cartilage is related to its composition and
the type II collagen discipline has continuously undergone since its architecture, and to the associated mechanical
inception over 100 years ago. properties. Even relatively small defects in the
Biomaterials-based tissue engineering and regenerative medicine solutions to musculoskeletal problems 294
articular surface (figure 1), which can commonly albeit only in a region of the lesion. This indicates
occur, will not heal with a tissue resembling artic- that articular cartilage regeneration is a possibility.
ular cartilage, and in some cases no reparative tis- The challenge, then, is to identify the elements of
sue will form in the defect. Left untreated, such the regeneration process that need to be supplied
defects can extend themselves, eventually (over to a particular defect: cells, matrix, cytokines, or a
decades) resulting in degeneration of the entire combination.
joint. A promising observation, however, has been It is also important to recognise that despite
that under certain circumstances – sometimes the absence of full regeneration of a tissue such as
those occurring in an untreated cartilage defect – articular cartilage, many patients report a dramatic
regeneration of articular cartilage can take place, relief of pain as a result of certain treatments. This
has raised fundamental questions about the crite-
Figure 1 ria for success that should be adopted in the eval-
Sketch of a human uation of new procedures. For example, a clinically
knee joint showing
the type of defect in
meaningful outcome might be one that provides
the articular surface pain relief for 5 years, and this may be achieved
that can commonly through the formation of a tissue that falls short of
occur. Healing of the
defect will not result replicating the composition and structure of artic-
in the formation of ular cartilage. Thus, is it more appropriate to use
and in many cases histological or clinical criteria for success of a new
no reparative tissue tissue engineering procedure? The problem with
will form in the de-
fect. Such defects in
using the clinical endpoint is that patients may re-
avascular tissue with port a relief of symptoms for a few years only to
limited potential experience a precipitous decline in their condition.
of the host cells to
participate in a There is no reason to expect that there would be a
reparative response gradual decline in function that signals potential
challenges that are
problems and thus allows adjustments in the pro-
addressed by tissue cedure or for it to be abandoned before large num-
engineering. bers of patients are operated on. Fundamental
questions thus remain as to how to gage the suc-
cess of a new tissue engineering procedure.
The scientific basis of tissue engineering
As with any engineering discipline the work- knowledge about the interaction of cells with ma-
ing goal of “tissue engineering” is the implemen- trices we will better be able to prepare new scaf-
tation of existing knowledge for the creation of folds to more specifically elicit the responses from
a product – tissue . At the same time, the engi- cells that best suit a tissue engineering application.
neering process often provides opportunities for The challenging engineering aspect of tissue
the discovery of new knowledge, ie the process of engineering, as with any engineering endeavour, is
science. It is becoming apparent that the unique the judicious use of existing knowledge for the pro-
circumstances related to the growth of cells in duction of a useful product, in this case, tissue.
three-dimensional scaffolds in vitro in the course There are so many physical and biological issues
of tissue engineering are revealing aspects of the related to the production of tissue in vivo or in vitro,
phenotypes of a wide variety of cells and insights and so few hard facts to guide the engineering
into cell behaviours that would have otherwise process that tissue engineering is much more of
escaped view . In this regard, tissue engineer- a demanding field than other engineering disci-
ing is likely to contribute important new knowl- plines. Moreover, that the risks of failure include
edge to cell and molecular biology, drawn from death, greatly increase the stakes of tissue engi-
the advancement of health care through the pro- neering pursuits.
duction of tissue in vitro or the facilitation of tis- Tissue engineering can now be pursued be-
sue regeneration in vivo. cause of recent advances in enabling technologies
One unique aspect of tissue engineering sci- related to the tissue engineering triad of cells,
ence is the investigation of the interactions of cells matrices, and regulators. Only recently technolo-
with absorbable matrices and environmental fac- gies have been developed that focus on the prolif-
tors (eg mechanical loading) that relate to the for- eration of cells in vitro under conditions that allow
mation of tissue. The cell responses to these inter- maintenance or recovery of the cell phenotype.
actions include cell proliferation and biosynthesis A critical aspect of most tissue engineering strate-
of matrix molecules. More recently it has been ob- gies is the expansion of cell number in culture in
served that cell contraction is another important order to generate the requisite number of cells for
aspect of the cell response to scaffolds employed the production of tissue in vitro or the implanta-
for tissue engineering . As we acquire more tion of cells alone or seeded in matrices for the re-
S W I S S M E D W K LY 2 0 0 6 ; 1 3 6 : 2 9 3 – 3 0 1 · w w w . s m w . c h 295
generation of tissue in vivo. Many cell types loose of tissue in vitro and or in vivo. Synthetic and nat-
critical phenotypic traits with increasing time ural polymers and calcium phosphates have been
in culture. Advances in cell biology allowed the developed as scaffolds for the engineering of soft
discovery of culture conditions that favour the and hard tissues. Control of the pore characteris-
proliferation of cells while a) preserving their tics including pore volume fraction, pore diameter
phenotype, or b) recovering lost phenotypic gene and pore orientation, as well as the chemical com-
expression post-expansion. There have been many position of the matrix, has played a critical role in
advances in the control of culture conditions em- the advance of tissue engineering. Another impor-
ployed for the preparation of tissue engineering tant enabling technology that has had an impact
constructs. Some of these new developments have on tissue engineering is the genetic engineering of
come from the work of Swiss investigators , selected cytokines, such as the bone morpho-
including work employing stem cells . genetic proteins. These growth and differentiation
One of the most important technological ad- factors and agents that stimulate biosynthetic
vances enabling tissue engineering regards the activity are playing important roles in efforts to
production of the porous, absorbable scaffolds that form a tissue in vitro and to facilitate regeneration
are required to contain the cells for the production in vivo.
Tissue engineering: historical perspective
There are too many investigations that have plates also revealed the importance of having the
served as the antecedents of tissue engineering to degradation rate of the matrix, controlled by cross-
include in a review. The following provides a brief linking, match the regeneration rate in a process
summary of just a few . referred to as isomorphous replacement. Later
Perhaps the earliest successful application of work in this line of investigation  showed for
tissue engineering is the implementation of porous the first time, in a rat model, that an off-the-shelf
collagen-glycosaminoglycan matrices for the in scaffold could serve better than an autograft in the
vivo regeneration of dermis . This work that case of treating gaps in peripheral nerve.
led to the term “artificial skin” served as the basis Other early studies  that used the term “tis-
for the subsequent use of these matrices in tubular sue engineering” investigated the endothelium-
form for the regeneration of peripheral nerve . like cell layer that formed on the polymethyl
The underlying concept was to develop analogues methacrylate implants in the eye. Later important
of the extracellular matrix of the tissue to be regen- work demonstrated the ability of cell-seeded ma-
erated. In addition to demonstrating that selected trices made from a synthetic polymer to form and
analogues could facilitate the regeneration of tis- maintain a viable cartilaginous tissue of a selected
sues that did not have the capability to sponta- shape when implanted in an animal model .
neously regenerate, these studies showed that tis- These and other studies formed the basis of a
sue-specific pore characteristics (ie pore diameter review article  that established tissue engineer-
and orientation) were necessary for the optimal ing as a distinct discipline.
performance. The use of these regeneration tem-
Tissue engineering versus regenerative medicine
The term “tissue engineering” was initially in- function, and can profoundly influence the archi-
troduced to describe the technology for producing tecture of tissue as it is forming. Because the me-
tissue in vitro . More recently the term “regen- chanical environment during the formation of
erative medicine” has been used to describe the de- most musculoskeletal tissue in vivo is not well un-
velopment of technology and surgical procedures derstood, it is not yet possible to recreate such an
for the regeneration of tissue in vivo. There are ad- environment in vitro during the engineering of
vantages and disadvantages to both strategies. One most tissues. Another disadvantage of the forma-
advantage of the synthesis of tissue in vitro is the tion of musculoskeletal tissue outside of the body
ready ability to examine the tissue as it forms, and is the necessary incorporation of the tissues after
to make certain non-destructive measurements to implantation. This incorporation requires that the
establish its functions prior to implantation. How- engineered tissue be mechanically coupled to the
ever, a disadvantage, particularly in the production surrounding structures. Union of the implanted
of musculoskeletal tissue that must play a load- tissue with the host organ requires remodelling –
bearing role, is the absence of a physiological me- degradation and new tissue formation – at the
chanical environment during the formation of the interfaces of the implant with the host tissues.
tissue in vitro. It is now well established that me- That remodelling of the implanted tissue is essen-
chanical force serves as a critical regulator of cell tial for its functional incorporation.
Biomaterials-based tissue engineering and regenerative medicine solutions to musculoskeletal problems 296
Thus, for certain tissues (eg musculoskeletal), both being referred to as tissue engineering.
an effective strategy may be to facilitate tissue Just as they are the three components of tissue,
formation in vivo, under the influence of the matrix, cells and soluble regulators are the ele-
physiological mechanical environment. However, ments of strategies to engineer tissue in vivo, or
one disadvantage of this approach is that the in vitro for subsequent implantation. Decisions
regenerating tissue may be dislodged or degraded as to which elements might be required for regen-
by the mechanical forces normally acting at the eration of tissue in vivo can be guided by an under-
site before it is fully formed and incorporated. standing of the deficits of the natural (ie spon-
In most cases a distinction is not made between taneous) healing processes that prevent regene-
tissue engineering and regenerative medicine, with ration.
Scaffolds for tissue engineering and regenerative medicine
For most of the decades of the 20th century, bio- its influence on cell adhesion and the phenotypic
materials have played a critical role in enabling the expression of the infiltrating cells. Moreover, be-
fabrication of a large number and wide variety of cause the objective is the regeneration of the orig-
medical implants. Except for a few examples, how- inal tissue, the scaffold needs to be absorbable. The
ever, these were permanent devices meant to fix or degradation rate of the material generally may be
replace the function of tissues and organs. Stainless determined based on the rate of new tissue forma-
steel devices were developed for the fixation of frac- tion and the normal period for remodelling of the
tures and to fix allografts to host bone. Implants tissue at the site of implantation. Of course, it is im-
fashioned from metallic, ceramic and polymeric portant to consider the effects of moieties released
materials facilitated life-saving procedures in many during degradation of the matrix on the host and
patients (eg vascular prostheses and artificial heart regenerating tissue. Finally, the mechanical prop-
valves) and profoundly improved the quality of the erties of the biomaterial employed as a scaffold for
lives of other individuals (eg joint replacement tissue engineering are important for providing
prostheses). Despite these remarkable successes, temporary support of applied loading in vivo dur-
the new roles for biomaterials in medicine will ing the regeneration process and for resisting the
likely exceed these achievements. The new roles contractile forces that may be exerted by the seeded
include the use of porous, absorbable biomaterial cells prior to implantation and by cells infiltrating
(sponge-like) scaffolds in tissue engineering, regen- the scaffold in vivo.
erative medicine, and gene therapy. There have been numerous reviews on the
Scaffolds for engineering bone and the soft tis- characteristics of the biomaterial scaffolds gener-
sues have been synthesised from an array of syn- ally employed for tissue engineering [1, 17]. Using
thetic and natural calcium phosphates and myriad a specific biomaterial system (a poly(ethylene gly-
synthetic (eg polylactic acid and polyglycolic acid) col)-terephthalate-poly(butylene)-terephthalate
and natural (eg collagen and fibrin) polymers. Scaf- block copolymer), Swiss investigators and their col-
folds for engineering tissue in vitro, or to be used as laborators recently provided an example of how the
implants to facilitate regeneration in vivo, need to composition and architecture of the scaffold can af-
have a microstructure and chemical composition fect the behaviour of the cells grown in the scaffold
able to accommodate cells and their functions. In . A comprehensive review of biomaterial scaf-
this regard a porous structure is generally neces- folds is outside the scope of this article. Rather, the
sary. The required porosity and pore diameter, pore author will draw from his personal experience em-
distribution, and pore orientation, might be ex- ploying collagen-based biomaterials (figure 2) to
pected to vary with tissue type. The chemical com- address certain issues related to the use of scaffolds
position of the matrix is important with respect to for specific tissue engineering applications.
micrograph of a
for tissue engineer-
1 mm 500 mm
S W I S S M E D W K LY 2 0 0 6 ; 1 3 6 : 2 9 3 – 3 0 1 · w w w . s m w . c h 297
Roles of a scaffold in tissue engineering and regenerative medicine
There are many roles that a scaffold can play to maintain their effectiveness during the remod-
in the tissue regeneration process: elling phase that ensues a few weeks to months
– The scaffold can serve as a framework to sup- after the initial repair procedure. The limited
port cell migration into the defect from sur- efficacy of the bolus dosing of growth factors may
rounding tissues; especially important when a be due to its inherent inability to maintain thera-
fibrin clot is absent. peutic levels of the cytokine for prolonged periods.
– Before it is absorbed, a scaffold can serve as The transitory effects of bolus dosing of poly-
a matrix for endogenous or exogenous cell peptide growth factors are a consequence of
adhesion, and can facilitate/regulate certain their relatively short in vivo half-lives (minutes
cell processes including mitosis, synthesis, and to hours), the temporal nature of growth factor
migration. This may be mediated by ligands signalling on cellular differentiation and metabo-
for cell receptors (integrins), on the biomate- lic function, and the fact that many exogenous
rial and/or the biomaterial may selectively ad- cytokines do not stimulate endogenous produc-
sorb cell adhesion proteins. tion.
– The scaffold may serve as a delivery vehicle Transfer of the gene for a selected cytokine to
for exogenous cells, growth factors, and genes. the cells involved in the reparative process using
This activity is enabled by a large surface a scaffold as the delivery vehicle is one means
area for attachment and the possible control of maintaining therapeutic levels of the protein
of the density of the agents (ie agents/unit through the later phases of the cartilage repair
volume). process . Non-viral vector systems offer the
– The scaffold may structurally reinforce the de- advantages of low immunogenicity, simplicity of
fect to maintain the shape of the defect and vector design, and relative ease of large-scale pro-
prevent distortion of surrounding tissue. duction [12, 25]. The major disadvantage of
– The scaffold can serve as a barrier to prevent this approach is related to the lower efficiency
the infiltration of surrounding tissue that may of transfection. However, for some reparative
impede the process of regeneration. processes (eg articular cartilage) even relatively
small amounts of the cytokine produced by a few
The potential role of the scaffold as a delivery transfected cells may be of significant value. This
vehicle for exogenous cells has become increas- approach has provided promising results in recent
ingly important in a wide variety of tissues and studies directed toward enhancing bone regenera-
organs in the light of recent advances in the inves- tion using a collagen matrix as a carrier for selected
tigation of cell therapy for local repair. Injection genes [2, 14].
of exogenous cells, expanded in number in mono- Prolonged release (over several weeks or
layer culture, is being studied for the treatment months) of DNA from an implant is necessary
of defects and degenerative conditions in many in cases where there is a benefit in transfecting
tissues: selected cells that only appear at the implant site
– chondrocytes for the repair of defects in artic- days or weeks post-operatively, and in which there
ular cartilage on the surface of joints , is a rapid loss of expression in transfected cells or
– intervertebral disc cells for herniated disc , in which transfected cells migrate from the defect
– stem cells into spinal cord lesions , site.
– myoblasts and stem cells for myocardial infarc- In one recent study, porous gene-supple-
tion , and mented collagen-GAG (GSCG) matrices were
– cells into the retina . loaded with plasmid DNA coding for the luciferase
reporter gene, and the effects of cross-linking and
An alternative to injection of cells is implan- pH (during gene loading) on release kinetics and
tation a cell-seeded scaffold. As noted above, the DNA integrity were determined . The optimal
large surface area of porous scaffolds allows the conditions showed luciferase expression in chon-
delivery of an exceedingly large number of at- drocyte-seeded GSCG constructs up to 28 days
tached cells, and facilitates the retention of the demonstrating continuous transfection of articu-
cells at the implant site. lar chondrocytes throughout the culture period.
Another potential role of the scaffold is the In a prior study investigating release of plasmid
delivery of genes for selected growth factors . DNA from copolymers of D,L-lactide and gly-
The regeneration of tissue may in some circum- colide, less than 10% of the DNA remained in
stances require the administration of certain ther- the synthetic polymer construct after 28 days
apeutic factors (eg growth factors). For example, in leaching studies performed using Tris-EDTA
selected growth factors, given as a single bolus dose buffer . Other matrix materials may lend them-
at the beginning of the cartilage repair process, selves to modification for gene-supplementation
have been shown to accelerate the production of a for more prolonged release of genes.
hyaline-like reparative cartilage matrix . How-
ever, none of these growth factors have been able
Biomaterials-based tissue engineering and regenerative medicine solutions to musculoskeletal problems 298
Methods for the production of scaffolds and design rationale
Many methods have been used for the produc- terials designed specifically for tissue engineering
tion of porous materials to be used as scaffolds for scaffolds. Alternatively the driving force for the de-
tissue engineering and regenerative medicine. sign of scaffolds may be the precision (computer)
These include a) the manipulation of fibers into multi-scale control of material, architecture, and
non-woven and woven structures , b) incorpo- cells: solid free-form fabrication technologies.
ration of sacrificial pore-forming agents including This has become possible with the introduction of
ice (through freeze-drying ; figure 2) and a wide array of solid free-form fabrication tech-
soluble particles (eg NaCl and sucrose), c) self-as- niques and apparatus .
semblying molecules (eg certain peptides  and As noted above, one design approach has been
collagen-hydroxyapatite composites ), and d) to employ materials that can serve as analogues of
solid free-form fabrication. the extracellular matrix of the tissue to be engi-
The underlying concepts guiding the develop- neered . This concept recognises that the mo-
ment of scaffolds can be predicated on the selected lecular composition and architecture of the extra-
biomaterial or on the method of production of cellular matrix displays chemical and mechanical
the scaffold. Examples of biomaterials-based ap- properties required by the parenchymal cells and
proaches include 1) use of biomaterials that have the physiological demands of the tissue. For scaf-
been frequently used for other implant applica- folds for regeneration of bone, this approach has
tions (eg PLA-PGA) , 2) treated natural extra- led to the use of natural bone mineral produced by
cellular matrix materials (eg anorganic bone ), removing the organic matter of bovine bone .
3) biomimetics and analogues of extracellular ma- For soft tissue applications collagen-based bioma-
trix (eg collagen-glycosaminoglycan  (figure 2) terials have been employed . There are special
and collagen-hydroxyapatite scaffolds ), 4) bio- issues that need to be considered in the selection
polymers for nanoscale matrix (eg self-assem- of scaffold materials for the engineering of specific
blying peptides) , and 5) new types of bioma- tissues.
Investigations of cell-scaffold interactions in vitro:
contraction of connective tissue cells
Investigations of cell-scaffold interactions in phenotype as the result of expression of the mus-
vitro can inform the rationale formulation of scaf- cle actin isoform, a-smooth muscle actin (SMA),
fold composition and structure for improved per- chondrocytes were examined for their expression
formance in tissue engineering and regenerative of this cytoskeletal protein. This led to a series of
medicine applications. These investigations of findings that adult canine and human articular
cells in three-dimensional scaffolds that may chondrocytes and many other connective tissue
mimic certain aspects of the natural extracellular cells and their mesenchymal stem cell progenitor
matrix in vivo can also provide insights into, and express SMA and can contract . These findings
discoveries of, cell biology. In this respect, studies have suggested roles for the contractile behaviour
of the behaviour of cells in collagen-GAG ana- of connective tissue cells in the control of the ar-
logues of extracellular matrix (figure 2) have been chitecture of the extracellular matrix and in the re-
particularly informative. An advantage of this ma- sponse of the tissue to injury. The contribution of
terial system is the ability to alter selected proper- muscle actin-expressing and contracting connec-
ties through cross-linking: mechanical behaviour, tive tissue cells to the process of dermal wound
degradation rate, and alteration of ligands for the closure has been recognised for three decades.
integrins of cells. Prior work has demonstrated Muscle actin-enabled cell contraction may also be
the effects of cross-link density on the mitosis and playing important roles in many other connective
synthesis of matrix molecules by chondrocytes in tissues including those comprising the muscu-
type I collagen-GAG scaffolds with increasing loskeletal system: tendon, ligament, meniscus, in-
cross-link density . tervertebral disc, articular cartilage, and bone.
Several years ago, in the course of investiga- In the context of tissue engineering and regen-
tions on the behaviour of articular chondrocytes erative medicine, the mechanical stiffness of the
in collagen-GAG scaffolds, the observation was scaffold is important in resisting SMA-enabled
made that the disc-shaped scaffolds were decreas- cell-mediated contraction that can alter the shape
ing in size. The reduction in volume did not of the implant and compress the pores. Recent
appear to be due to dissolution of the scaffold. work, however, has demonstrated that the cell-me-
Subsequent histological studies demonstrated a diated contraction of a scaffold can be employed to
reduction in the pore diameter of the matrices favour chondrogenesis in vitro . Chondrocyte
and suggested a cell-mediated process. Because seeded-scaffolds of varying cross-link densities
fibroblasts were known to adopt a contractile were cultured for 2 weeks to evaluate the effect of
S W I S S M E D W K LY 2 0 0 6 ; 1 3 6 : 2 9 3 – 3 0 1 · w w w . s m w . c h 299
scaffolds with cross-
link density increas-
ing from (A) to (D),
following a 2-week
culture period. The
thick sections paraf-
mens were stained
with a Safranin-O/
fast green stain
that labels proteogly- 100 µm 100 µm
cans, a major con- A B
stituent of cartilage,
red. The arrows in
panels (C) and (D)
show the residual
It appeared that a
higher rate of resorp-
tion of the scaffold
was associated with
a higher rate of chon-
100 µm 100 µm
cross-link density on scaffold contraction and gineering. In this approach scaffolds would have an
chondrogenesis. Scaffolds with low cross-link den- initial pore diameter large enough to facilitate cell
sities experienced cell-mediated contraction, in- seeding and a mechanical stiffness low enough to
creased cell number densities, and a greater degree allow cell-mediated contraction to yield a reduced
of chondrogenesis and an apparent increase in the pore volume favouring chondrogenesis. This ap-
rate of degradation of the scaffold compared to proach may provide a useful alternative to tradi-
more highly cross-linked scaffolds that resisted tional means of increasing cell number density and
cellular contraction (figure 3). The results of this retention of synthesised molecules that promote
study suggest the promise of “dynamic pore reduc- cartilage formation in tissue engineered con-
tion” of scaffolds for articular cartilage tissue en- structs.
Chondrocyte-seeded collagen-gag scaffolds for cartilage repair
Studies demonstrating the potential benefit of ACI, the procedure has been introduced into wide-
injection of culture-expanded chondrocytes for spread clinic use  with promising symptomatic
cartilage repair date back to rabbit studies first per- relief in many patients [8, 24].
formed in the mid nineteen eighties [9, 16]. Sub- Current efforts in many laboratories around
sequent experiments in a canine model [6, 7] found the world are being directed to determine whether
significantly more hyaline cartilage in the auto- the results of ACI can be improved when the cells
logous chondrocyte implantation (ACI)-treated are implanted as a cell-seeded scaffold rather than
group after 3 and 6 months compared to the un- delivered by injection. One recent series of studies
treated control. At 6 months there was a promis- compared the reparative tissue in chondral defects
ing amount of defect filling with articular carti- in adult dogs implanted with cultured autologous
lage-like tissue. However, by 1 year there were no chondrocytes (CACs) alone, ie ACI , and CAC-
significant differences among the treated and con- seeded type II collagen-GAG scaffolds cultured
trol (periosteum alone and non-treated defects) for 24 hours  and 4 weeks  prior to implan-
groups. By 18 months neither complete filling, tation. The cell-seeded scaffolds yielded a greater
nor the restoration of the architecture was found amount of reparative tissue than the sites im-
. Moreover, cartilage surrounding the defect planted with the CACs alone. The cell-seeded
showed degenerative changes, some of which were scaffolds cultured for 24 hours induced more
related to suturing of the periosteal flap. Despite reparative tissue formation than the injection of
the absence of compelling animal findings using cells alone. However, this tissue consisted of fibro-
Biomaterials-based tissue engineering and regenerative medicine solutions to musculoskeletal problems 300
cartilage and fibrous tissue with virtually no hya- this group also demonstrated the same amount of
line cartilage. The question remains as to the rel- hyaline and articular cartilage as found in defects
ative importance of the amount versus composi- implanted with the cells alone . Although these
tion of the reparative tissue with respect to provid- studies on implementing tissue engineering scaf-
ing symptomatic relief for individuals with focal folds for cartilage repair are promising, there are
cartilage defects. Related to this point is the fact potential problems and significant expenses asso-
that the hyaline cartilage found at sites treated by ciated with culturing a cell-seeded scaffold for
CACs alone and in the collagen scaffolds did not 4 weeks prior to implantation. This draws atten-
display the architecture of articular cartilage. Of tion to the implementation of growth factors to
note was that the greatest amount of reparative tis- accelerate cell proliferation and matrix synthesis
sue was induced by the CAC-seeded scaffold cul- in the scaffolds prior to implantation .
tured for 4 weeks prior to implantation, and that
The future of tissue engineering and regenerative medicine
Just as several technologies enabled the devel- implants employed for strategies to facilitate tissue
opment of tissue engineering as a viable discipline, regeneration in vivo.
new technologies will provide continuation of its The proliferation of cells in monolayer culture
growth and maturation. One of these emerging and their subsequent growth in three-dimensional
technologies is the isolation and expansion of stem scaffolds for tissue engineering continues to pro-
cells and the identification of the signals required vide unique opportunities to observe selected cell
for their differentiation into specific cell types. An- behaviour. Tissue engineering science will thus
other related technology is the genetic modifica- provide critical new knowledge that will deepen
tion of cells in vitro or in vivo. These technologies our understanding of the phenotype of many cell
will address the difficulties that are often encoun- types and this knowledge will likely enable mean-
tered in obtaining a sufficient amount of tissue for ingful advances in tissue engineering and regener-
the isolation of autologous cells. ative medicine.
New matrix materials will likely be developed
with selected chemical compositions that allow Correspondence:
them to better serve as insoluble regulators of cell Myron Spector, Ph.D.
function. Finally, methods will likely be introduced VA Boston Healthcare System
to control the mechanical environment of the cells Boston Campus, Room D1-152
in vitro to better regulate their biosynthetic behav- Mail Stop: 151 Research
iour. Collectively these approaches will enable the 150 S. Huntington Ave.
synthesis of tissue in vitro that better replicates the Boston, MA 02130, USA
native material and will be of value in preparing E-Mail: firstname.lastname@example.org
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