Fisher John P. e.a. (ed.) Tissue Engineering
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17-12 Tissue Engineering
of the clinical testing, licensing, and use of new medical products is distributed among relevant divisions,
such as the Evaluation and Licensing Division (premarketing and supplemental application approvals)
and the Safety Division (adverse reaction measures). A regulatable medical product in Japan is classified
as either a medical device or a pharmaceutical.
Advice concerning the design and conduct of clinical trials, as well as the adequacy of applications for
approval of pharmaceuticals, is provided to PMDEC and to the product sponsor by the Drug Organization,
a quasi-governmental agency established in 1979 as a fund to support patients experiencing adverse drug
reactions. It is not clear whether the Drug Organization serves a similar function with respect to medical
devices, or if there exists an equivalent medical device organization. However, applications for approval of
“copy-cat” devices are referred to the Japan Association for the Advancement of Medical Equipment for a
determination of the equivalence of the new device to devices already approved for clinical use. For a more
detailed description of Japan’s general medical product approval process, see, for example, Hirayama [9]
and Yamada [10].
17.5 Other Considerations Relevant to Engineered Tissues
17.5.1 FDA Regulation and Product Liability
Protection from product liability lawsuits, in the form of an immunity from such litigation, may come
from satisfying the federal regulations which govern the design and manufacture of, as well as the warnings
to be supplied with, medical products.
By virtue of the Supremacy Clause of the Constitution, the federal government is permitted to regulate
certain affairs free of state interference. State civil litigation is a form of regulation, so it is a form
of interference. If Congress elects to exclusively regulate certain conduct, then litigation under state
law regarding the same conduct is prohibited, as it may produce inconsistent or conflicting standards
regulating that conduct.
The public policy arguments in favor of federal preemption with respect to the regulation of medical
products are readily discernible: while both state and federal regulation would have the enhancement of
public health and safety as their goal, the establishment of nationwide labeling and design criteria for
medical products will promote uniformity and regularity in the interpretation of applicable regulations
and will ensure enforcement of these regulations is conducted in the public interest, rather than through
isolated lawsuits that may produce inconsistent results. In addition, the natural preeminence of a federal
administration administering such regulations will simplify and improve communication between the
regulators and the medical product sponsors. Federal preemption, then, is not a shield for bad medical
products; rather, it protects a process of reasoned, scientific inquiry.
Congressional and FDA silence on the question of preemption in statutes and regulations governing
drugs and biologics going back to the earliest federal Food and Drug Acts has meant that sponsors of
these products have been left to argue only “implied” or “conflict” preemption with largely inconsistent
— though often disappointing — results.
The situation for sponsors of devices would seem to be decidedly different. When Congress passed
the 1976 Medical Device Amendments to the FD&C Act, it explicitly authorized preemption of any state
requirement as to the safety or effectiveness of a medical device in favor of FDA regulations governing the
same product. Because of the high degree of scrutiny to which Class III devices are subjected through
the FDA’s premarket approval (PMA) process, courts have shown a general willingness to recognize
that personal injury litigation regarding these devices has been preempted. However, the FDA may also
permit the marketing of devices which have been shown to be “substantially equivalent” to devices either
previously approved or marketed before the enactment of the Medical Device Amendments in 1976. This
“510(k)” clearance involves significantly less regulatory review of the device than would have been given
to a PMA application.
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Regulation of Engineered Tissues 17-13
In Medtronic v. Lohr, though, the U.S. Supreme Court concluded that the apparent preemption language
of the Medical Device Amendments does not actually preclude product liability actions against sellers
of “510(k)” devices. The court reasoned that the 510(k) premarket clearance process did not involve
a comprehensive, in-depth analysis of the safety, efficacy, and labeling of a medical product prior to
clinical use. While a number of justices suggested they might not reach the same conclusion regarding
“PMA” devices, the Lohr decision has weakened — but certainly has not obliterated — a major defense
of those engineered tissue products which may be classified and regulated as devices. Indeed, to the
extent the further viability of the preemption defense is predicated upon the degree to which the FDA
has given specific attention to and approval of the design of the device in question, premarket approval
by means of the PDP process would seem to offer the greatest chance of immunity from product liability
litigation.
17.5.2 Ownership of Human Tissues
While critical to the general advance of medical research, access to human tissues for research or product
development is highly sensitive to public disclosure of practices where tissues are taken or used without
consent or under circumstances suggesting a commercial market in body parts. The absence of compre-
hensive federal or state legislation governing “research” tissues deprives the biomedical community of
clear, consistent guidelines to follow in acquiring and using tissues, while simultaneously representing a
legislative vacuum that may be filled with substantial adverse unintended consequences if done suddenly in
response to some public outcry. Absent effective coordination, the initiatives of individual federal agencies
to establish policies for research involving human tissues or subjects may impose conflicting requirements
or expectations.
Significant advances in medical research over the past several years has contributed substantially to
the commercial utility of human biological materials. Consequently, the source of such materials used in
the creation of engineered tissue products may become important for reasons beyond — and certainly
removed from — the possible transfer of adventitious agents or the management of immunological
responses. Simply put, the use of allogeneic materials raises issues of ownership, donation, and consent
not to be found with respect to autologous tissues.
The common law of the United States recognizes a severely restricted property interest in human bodies
or organs. In a broad sense, a “property interest” in something may be thought of as a “bundle of rights”
to possess, to use, to profit from, to dispose of, and to deal in that thing. Courts have granted next of kin
nothing more than a “quasiproperty” right — or right of sepulcher — in a decedent’s body for the purposes
of burial or other lawful disposition. In place of an exegesis of the religious or cultural prohibitions against
recognizing a property interest in a dead body, it is clear that the limited right which has been fashioned
by the courts has been intended to offer nothing more than that some interested person may ensure the
remains are disposed of with dignity.
Organ transplantation has certainly created the conditions for a market for human body parts. In
response, Congress and state legislatures have enacted statutes prohibiting the sale of any human organ.
The National Organ Transplant Act was passed to regulate the availability of organs for transplantation
through voluntary donation exclusively by explicitly prohibiting organ purchases. The same prohibition
has been passed into law by the 15 states, to date, which have adopted the Uniform Anatomical Gift Act
(1987) [11]. Other state statutes have imposed criminal penalties for the purchase of organs or tissue from
either living or cadaveric providers.
These federal and state statutes effectively banning purchases of human organs were enacted in the
mid-1980s in immediate response to the prospect of a widespread trade in these body parts to supply the
growing demand for transplant material. The vision of a vendor selling livers and kidneys — or worse, a
patient harvesting one of his or her own organs for money clearly hovered over the debate leading to the
passage of this legislation. But that vision imagined people selfdismantling for cash; it did not really allow
for a trade in renewable body parts, especially cells.
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17.6 Conclusion
No part of the process of bringing new biomedical products from the laboratory to the patient occurs in
isolation from or independent of all of the other aspects of organizing and maintaining that technology
development effort: including intellectual property protection and financing market and end user accept-
ance, and public perception just to mention a few. While FDA premarket approval is the most obvious
form of external control over the introduction of new medical products in the United States, it is
not the only one; healthcare reimbursement under U.S. centers for Medicare and Medicaid Services
(CMS) regulations and private insurer practices is critical, and the evidence necessary to secure reim-
bursement should be considered in planning clinical trials for FDA approval. The approach to FDA
regulatory oversight itself requires careful analysis of product classification (including special designa-
tion) options. The novelty, variety, and potential complexity of forms of tissue engineering compel
strategic analysis of external controls over the commercial development of human cellular and tissue-based
products.
The FDA has adopted a cooperative approach across appropriate FDA Centers and the Office of
Commissioner in developing its regulatory strategies for engineered tissue products. These strategies
have attempted to simplify and facilitate the administrative process for evaluating products and for
resolving product regulatory jurisdiction questions. However, it is apparent that, just as the science
continues to evolve, so too must the regulatory approaches. As new and different research directions
are pursued, such as the apparent shift toward the use of stem cell technology [12] new and differ-
ent products will be developed, with the expectation of unique product-specific issues. The FDA, given
its legislative authority and regulatory responsibility will certainly continue to build on current initiat-
ives, apply lessons learned from previous products as applicable, and look to the best scientific minds
and methods to achieve innovative, flexible, and appropriate resolutions that are transparent to the
research and industrial community as well as the public. The challenge for the tissue engineering com-
munity is to maintain awareness of the regulatory landscape in the United States and abroad and to
be an active voice for articulating issues in order to maintain a productive dialogue with the regu-
latory agencies and consumers so that engineered tissue products find their proper place in clinical
medicine.
References
[1] Hellman, K.B. and Heineken, F.G. Introductory letter. In World Technology Evaluation Center
(WTEC) Panel Report on Tissue Engineering Research, McIntyre, J.L. et al. (Eds.), Academic Press,
San Diego, CA, 2003.
[2] Reinventing the Regulation of Human Tissue: A Proposed Approach to Regulation of
Cellular and Tissue-based Products (the “Proposed Approach”) (62 FR 9721) March 4,
1997.
[3] PHS Act. Effective 1999. Public Health Service Act, 42 U.S. Code Section 201. See also FDA website:
http://www.fda.gov/opacom/laws/phsvcact/phsvcact.htm
[4] U.S. Government. Food, Drug, and Cosmetic Act (as amended by the FDA Modernization Act of
1997). See also http://www.fda.gov/opacom/laws/fdcact/fdctoc.htm
[5] Establishment Registration and Listing for Manufacturers of Human Cellular and Tissue-based
Products (63 FR 26744), May 14, 1998.
[6] Suitability Determination for Donors of Human Cellular and Tissue-based Products (64 FR 52696),
September 30, 1999.
[7] Current Good Tissue Practice for Manufacturers of Human Cellular and Tissue-based Products:
Inspection and Enforcement.
[8] EMEA Biotechnology Working Party, Points to Consider on Human Somatic Cell Therapy,
CPMP/BWP/41450/98 draft.
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Regulation of Engineered Tissues 17-15
[9] Hirayama, Y. Changing the review process: the view of the Japanese Ministry of Health and Welfare.
Drug Inform. J. 1998; 32:111–117.
[10] Yamada, M. The approval system for biological products in Japan. Drug Inform. J. 1997; 3:1385–
1393.
[11] Uniform Anatomical Gift Act of 1987, Section 10. Reproduced at 8A Uniform Laws Annotated 58
(Master Edition, 1993).
[12] Lysaght, M. Tissue Engineering: Great Expectations; Presentation at Engineering Tissue Growth
Conference and Exposition, Pittsburgh, PA, March 17–20, 2003.
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18
Bioengineering of
Human Skin
Substitutes
Dorothy M. Supp
Shriners Hospital for Children
Cincinnati Burns Hospital
Steven T. Boyce
University of Cincinnati
18.1 Introduction.............................................. 18-1
18.2 Objectives of Skin Substitutes........................... 18-2
18.3 Composition of Skin Substitutes ....................... 18-2
Acellular Skin Substitutes • Allogeneic Cellular Skin
Substitutes • Autologous Cellular Skin Substitutes
18.4 Clinical Considerations.................................. 18-7
18.5 Assessment ............................................... 18-8
18.6 Regulatory Issues......................................... 18-8
18.7 Future Directions ........................................ 18-9
Pigmentation • In Vitro Angiogenesis • Cutaneous Gene
Therapy
18.8 Conclusions .............................................. 18-10
References ....................................................... 18-11
18.1 Introduction
The field of tissue engineering has grown in response to the many medical needs for tissue replacement.
For the skin, two distinctly different types of wounds have stimulated the development of various tissue-
engineered skin substitutes. On one end of the spectrum are burn wounds, which represent an acute
injury to the skin. Over 1 million burn injuries occur annually, resulting in over 50,000 acute hospital
admissions and more than 5,000 deaths [1]. Partial-thickness wounds have the capacity to heal without
the need for tissue replacement from stem cells present in skin appendages. However, full-thickness burn
wounds require grafting of skin to replace the destroyed tissue. The grafting of split-thickness autologous
skin has been the prevailing standard for permanent closure of excised full-thickness burn wounds. This
can be accomplished in patients with relatively small wounds, but in patients with massive burns involving
a large total body surface area (TBSA), permanent wound closure is problematic because of the lack of
donor sites for skin autografting. Delayed wound coverage increases the likelihood of infection and sepsis,
major causes of burn mortality [2].
On the other end of the spectrum are chronic wounds, which tend to involve a relatively small area
of skin, but represent a major medical need because they affect a large population of patients. The most
18-1
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18-2 Tissue Engineering
common chronic wounds include pressure ulcers and leg ulcers, which are estimated to affect over
2 million people in the United States alone [3]. This figure may be expected to rise as the average age of the
population increases. In some patients, wound closure can be enhanced with topical agents, such as growth
factors to stimulate healing and antimicrobial agents to minimize infection. However, a large percentage
of patients require grafting for permanent closure of chronic wounds. Autograft may not be a feasible
option in these patients due to underlying deficiencies in wound healing, which compromise healing of
donor sites.
The need for timely wound closure in these diverse clinical settings has led to the development of
skin substitutes as alternatives to split-thickness or full-thickness autograft. Although none of the skin
substitutes currently available can replace all of the structures and functions of native skin, they can be
used to provide wound coverage and facilitate healing of both acute and chronic wounds. Further, the
technologies that have yielded skin substitutes for grafting have also been used to produce materials for
in vitro irritancy and toxicology studies.
18.2 Objectives of Skin Substitutes
Normal human skin performs a wide variety of protective, perceptive, and regulatory functions that help
the body maintain homeostasis. The outermost layer of the skin, the epidermis, is comprised mainly
of keratinocytes, which provide the barrier function of the skin. Other epidermal cells and structures
include adnexal cells that comprise the glands, hair, and nails; melanocytes, which contribute pigment;
Langerhans cells of the immune system; and sensory structures of nerves. The epidermis contains only very
small amounts of extracellular matrix, predominantly carbohydrate polymers and stratum corneum lipids
that form a barrier to permeability of aqueous fluids [4–6]. In contrast, the underlying dermis consists
mainly of extracellular matrix molecules, including collagens, elastin, reticulin, and polysaccharides, that
give mechanical strength to the skin. Dermal cells include fibroblasts, which synthesize collagen and other
matrix components; vascular components, such as endothelial cells and smooth muscle cells; nerve cells;
and mast cells of the immune system.
To be useful in a clinical setting, the primary goal for any skin substitute is restoration of skin barrier,
normally a function of the epidermis, which helps to minimize protein and fluid loss and prevent infection
[7]. As outlined in Table 18.1, there are many products that meet this need, but most provide only
temporary wound coverage or partial skin replacement. Currently, limitations of bioengineered skin
substitutes, compared with native skin grafts, include: reduced rates of engraftment, increased microbial
contamination, mechanical fragility, increased time to healing, increased requirement for regrafting, and
very high cost [59–63]. These complications may increase, rather than decrease, the risks to patients and
delay recovery. Therefore, the use of skin substitutes may be suitable as an adjunctive therapy in cases
without other alternatives, such as very large burns or chronic wounds that have failed to respond to
conventional treatments.
Ideally, skin replacements should promote permanent engraftment without the need for regrafting;
allow rapid healing to replace both dermal and epidermal layers; be ready to use when needed; achieve
acceptable functional and cosmetic outcome; and be free from risk of disease transmission and immun-
ological reaction [2]. These many goals have not yet been satisfied by any skin substitute, but ongoing
research in biomedical and genetic engineering may someday make an “off-the-shelf” skin replacement
a reality.
18.3 Composition of Skin Substitutes
Skin substitutes currently available vary in complexity, ranging from temporary synthetic wound dressings
to permanent skin replacements, either with or without incorporation of cultured skin cells (Table 18.1).
Some of the acellular materials have components that incorporate into the wound bed and may become
populated with dermal cells from the host. These dermal substitutes replace the dermal component of the
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Skin Substitutes 18-3
TABLE 18.1 Bioengineered Skin Substitutes
Skin substitute Dermal component Epidermal component Indications for use Refs.
AlloDerm®
(LifeCell Corp.)
Allogeneic acellular
human dermis
None Burn wounds; repair of
soft tissue defects
[8–11]
Apligraf® (Novartis) Collagen gel with
allogeneic fibroblasts
Allogeneic keratinocytes Burn wounds; chronic
foot ulcers; venous leg
ulcers; epidermolysis
bullosa
[12–20]
Biobrane® (Bertek
Pharmaceuticals)
Nylon mesh coated with
collagen
Semipermeable silicone
membrane
Partial-thickness burn
wounds and donor sites
[21–24]
Cultured Skin
Substitutes (Univ.
Cincinnati/Shriners
Hospitals
Collagen-based polymer
with autologous
fibroblasts
Autologous keratinocytes Burn wounds; congenital
nevi; chronic wounds
(using allogeneic cells)
[25–29]
Dermagraft® (Smith
and Nephew)
Polyglactin mesh scaffold
with allogeneic
fibroblasts
None Chronic/diabetic foot
ulcers
[30–32]
Epicel® (Genzyme
Biosurgery)
None Autologous keratinocyte
sheet
Burn wounds; congenital
nevi
[33–36]
Epiderm™(MatTek
Corporation)
Collagen coated cell
culture insert
Allogeneic keratinocytes In vitro assessment for
irritancy and toxicology
testing
[37–40]
EpidermFT™ (MatTek
Corporation)
Allogeneic fibroblasts on
cell culture insert
Allogeneic keratinocytes In vitro testing model for
assessment of
dermal–epidermal
interactions
Epidex™ (Modex
Therapeutics)
None Autologous keratinocytes
with silicone membrane
support
Chronic leg ulcers [41,42]
Integra® (Integra Life
Sciences)
Collagen-chondroitin-6-
sulfate matrix
Silicone polymer coating Burn wounds [43–49]
Melanoderm™(MatTek
Corporation)
Collagen coated cell
culture insert
Allogeneic keratinocytes
and melanocytes
In vitro testing model for
assessment of
pigmentation
OrCel® (OrTec
International)
Collagen sponge with
allogeneic fibroblasts
Allogeneic keratinocytes Donor sites in burn
patients; epidermolysis
bullosa
[50,51]
SkinEthic® (SkinEthic
Laboratories)
Cell culture insert Allogeneic keratinocytes
(without or with
melanocytes)
In vitro testing model [52–54]
SureDerm (Hans
Biomed)
Allogeneic acellular
human dermis
None Burn wounds; repair of
soft tissue defects
TranCell (CellTran
Limited)
None Autologous keratinoctyes
on acrylic acid polymer
Chronic diabetic foot
ulcers
[55]
TransCyte® (Smith
and Nephew)
Nylon mesh with dermal
matrixsecretedby
allogeneic fibroblasts
(cells nonviable at
grafting)
None Burn wounds [56–58]
skin, but may require grafting of autologous epidermis in a second surgical procedure. Skin substitutes that
contain allogeneic cells have greater similarity to native skin at grafting than acellular materials, but these
are considered temporary skin substitutes [64]. The allogeneic cells are eventually replaced by host-derived
cells [7]. Allogeneic skin substitutes containing cells can secrete extracellular matrix and growth factors
that improve healing, and can provide wound coverage until autograft is available. For small wounds,
such as chronic wounds, allogeneic skin substitutes may stimulate healing from the wound bed and
margins, without further grafting. These characteristics, coupled with essentially immediate availability,
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18-4 Tissue Engineering
are advantages of the allogeneic skin substitutes. However, for permanent closure of large wounds grafting
of autologous skin or engineered skin containing autologous cells is required. A disadvantage of bioengin-
eered skin substitutes containing autologous cells is the increased time required for cell culture and graft
preparation [65]. However, they can act as permanent skin replacements once engraftment is achieved.
Therefore, the increased time required for preparation may be offset by a reduction in the number of
surgical procedures required for permanent wound closure [66].
Skin replacements can be used as adjunctive therapies in patients who are also receiving conven-
tional skin grafts to facilitate wound closure [25–27,67,68]. By increasing availability of skin grafts, skin
substitutes can provide several advantages over conventional therapy, including reduced donor site area
required to close wounds permanently, decreased number of surgical procedures and hospitalization time,
and reduced scarring [66,69].
18.3.1 Acellular Skin Substitutes
Biobrane, an acellular biocomposite dressing, has been used for over two decades for treatment of partial-
thickness burn wounds [21,70]. It has been shown to control pain and reduce frequency of dressing
changes, decrease the length of hospitalization, and improve healing times [21,23,70]. Biobrane is com-
posed of a collagen peptide-coated nylon mesh material attached to a semipermeable silicone membrane.
It is applied to clean, debrided wounds with the mesh side down; the mesh adheres to the wound, and
the material is removed after the underlying tissue has healed. Integra is also a synthetic acellular skin
replacement, but one that combines both temporary and permanent components. It consists of a cross-
linked collagen–glycosaminoglycan membrane as a dermal matrix, and a silastic coating that provides
a synthetic epidermal structure for barrier replacement [43,44]. It has been widely used for coverage of
excised burn wounds. The artificial dermis does not contain cells at the time of grafting, but within a
few weeks it becomes populated with cells from the wound bed and is vascularized. Thus, the dermal
component incorporates into the wound, generating a neodermis; the temporary silastic coating can then
be removed and replaced with a thin sheet of split-thickness skin [45,46].
Another example of a bilayer temporary skin substitute is TransCyte, which has been used for coverage
of partial-thickness wounds during re-epithelialization, or for full-thickness wounds prior to autograft
placement. Transcyte is comprised of a synthetic semipermeable epidermal layer, and a dermal nylon
mesh layer seeded with allogeneic human fibroblasts [56–58]. The fibroblasts are allowed to proliferate
within the synthetic dermal construct, where they secrete extracellular matrix components and growth
factors that can facilitate wound healing. Prior to grafting, the material is frozen, without cryoprotection
of the cells. Thus, at the time of grafting, Transcyte does not contain any viable cells.
AlloDerm is an acellular dermal matrix derived from donated human skin [8]. The skin is processed
to remove cells, circumventing tissue rejection but preserving much of the dermis’s three-dimensional
dermal structure. Vascular channels with basement membrane are present, even though the cells have been
destroyed, facilitating graft vascularization [10]. It has been most useful for soft tissue replacement, such
as abdominal wall reconstruction [10]. For treatment of burn wounds, it has been used in conjunction
with split-thickness autograft to yield results similar to split-thickness grafts of greater thickness [8].
18.3.2 Allogeneic Cellular Skin Substitutes
Other temporary wound covers, such as Dermagraft, Apligraf, and OrCel, contain viable cells at the time
of grafting. Because the cells are allogeneic, typically isolated from donated human neonatal foreskin, these
products are considered temporary skin substitutes rather than permanent skin replacements. Dermagraft
is composed of a polymer mesh scaffold seeded with allogeneic fibroblasts [30–32]. Apligraf and OrCel
both contain allogeneic keratinocytes and fibroblasts, coupled with a biopolymer consisting of either a
bovine type I collagen gel or collagen sponge, respectively [12,13,51]. Grafts that contain allogeneic cells
provide wound coverage and supply growth factors that facilitate wound repair, but the allogeneic cells
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Skin Substitutes 18-5
do not persist on the patient after healing is complete. It is generally believed that allogeneic cells are
replaced within 1 to 6 weeks after grafting by the patient’s own cells.
Cultured skin models containing allogeneic cells have been developed for in vitro testing purposes
[37,40,52–54]. SkinEthic Laboratories has developed a Reconstituted Human Epidermis model, com-
prised of stratified epidermal keratinocytes supplied on cell culture inserts, for in vitro test applications
[52–54]. In addition, several tissue-specific models of Reconstructed Human Epithelium have been pre-
pared. These include models of oral, vaginal, corneal, and alveolar epithelium. Similar models designed
for in vitro toxicology testing include EpiDerm, a differentiated model of human epidermis, and Melan-
oderm, a stratified coculture of human melanocytes and keratinocytes [37]. EpiDermFT (EpiDerm “Full
Thickness”) is comprised of neonatal foreskin-derived fibroblasts and keratinocytes grown on cell culture
inserts. These skin models closely parallel human skin at the ultrastructural level, and can be reproducibly
manufactured, offering attractive alternatives to in vivo animal testing for irritancy, toxicology, and gene
expression studies.
18.3.3 Autologous Cellular Skin Substitutes
Autologous keratinocytes and fibroblasts have generally been derived from biopsies of the patient’s
uninjured skin [71–73]. Recently, keratinocytes have been isolated from the outer root sheath of plucked
hair follicles [41,42]. The cells can be expanded in culture and used to populate skin substitutes in vitro.
Grafted as epithelial sheets or as composites of biopolymers and cells, these are theoretically permanent
skin substitutes once engraftment is achieved [25–27,33,41].
Relatively few commercial skin substitutes contain autologous cells. Epicel was the first commercial
product comprised of cultured autologous cells for wound transplantation. It is indicated for use in burn
patients with very large deep partial-thickness or full-thickness wounds and in congenital nevi patients
[33,36,74]. Also referred to as cultured epithelial autograft (CEA), Epicel consists of a sheet of autologous
keratinocytes that are grown in culture and transplanted to patients with a petrolatum gauze backing.
The keratinocytes are isolated from a full-thickness biopsy of the patient’s uninjured skin, and grafts
are generally ready within 3 weeks time [74]. Epicel can be grafted on top of vascularized allogeneic
dermis or directly onto debrided wounds; however, engraftment is higher when used in conjunction
with allodermis [34,74]. A major problem encountered with Epicel has been epidermal blistering. Small
blisters may resolve spontaneously, but larger blisters result in graft failure [34]. This phenomenon has
been attributed to absence of dermal–epidermal junction components at grafting. Despite this limitation,
the availability of Epicel has provided a much needed treatment option for patients with massive burns
where donor skin for autograft is extremely limited [34].
A similar product, EpiDex, is a cultured epidermal sheet graft that is indicated for the treatment of
small chronic wounds. The unique aspect of EpiDex is that the keratinocytes are not cultured from skin
biopsies, but from scalp hair follicles [42]. For preparation of grafts, hair is plucked from the patient
and the outer root sheath cells are placed in explant culture [42]. The keratinocytes that are cultured in
this fashion are highly proliferative, apparently regardless of donor age [41]. After approximately 5 weeks
of cell expansion, a secondary culture is initiated for preparation of epithelial sheet grafts. These are
transplanted to wounds as discs measuring approximately 1 cm diameter, attached to a silicone backing to
facilitate handling. Early clinical results have been favorable, though the wound areas treated with EpiDex
are relatively small [41].
A limitation of these autologous skin replacements is that they contain only keratinocytes, and thus
they supply only an epidermal layer. For large full-thickness wounds, such as burns, replacement of both
dermal and epidermal layers is beneficial, to reduce scarring and improve the functional and cosmetic out-
come. This can be accomplished by combining dermal substitutes and epidermal skin replacements, but
this generally requires multiple surgical procedures. A composite cultured skin substitute (CSS) that con-
tains both autologous keratinocytes and fibroblasts in vitro is currently in clinical trials [26,27,67,68,75].
CSS are comprised of bovine collagen-glycosaminoglycan sponges that are populated with autologous
fibroblasts and keratinocytes (Figure 18.1) [76,77]. The most extensive experience with autologous CSS