Beyond simply improving tissue integration for synthetic implants, functional tissue regeneration within artificial matrices or on artificial surfaces is now possible. This is where the new science of tissue engineering diverges from conventional biomaterials research. Tissue engineering combines elements of engineering and materials science with genetics, molecular, cell, and developmental biology in organ replacement and organ regeneration.40 41 Engineered replacement tissue constructs are already in development for a variety of tissues including skin, cartilage, nerve, liver, kidney, muscle, heart valves, and blood vessels.42-50
Many organs have regenerative capacity and will regenerate rather than scar in the absence of matrix destruction. Good examples include liver, lung, and epithelial surfaces throughout the body including skin and the ocular surface. One strategy in tissue engineering with possible relevance to ophthalmology is the replacement of damaged tissue with engineered matrices to restore a normal cell adhesion environment. Good regenerative responses have been observed clinically after extensive burns using artificial skin constructs based on collagen/proteoglycan coprecipitates.51 Enhanced axonal regeneration has also been demonstrated in a rat model,45 in which a portion of the sciatic nerve is replaced by a similar collagen/proteoglycan coprecipitate within a collagen tube. These matrices are degraded and replaced by autologous matrix from regenerating cells. Collagen cross linkage density is varied to match the rate of matrix degradation with the rate of healing for the tissue to be regenerated. Pore size and directionality are also controlled to optimise results in the target tissue.52
In some circumstances it may be desirable to preseed the matrix with donor cells in order to normalise the initial cell signalling environment, rather than waiting for autologous cells to populate an engineered matrix. For epithelial surfaces, the key to regeneration appears to be a normalised substrate. In skin, for example, dermal replacement promotes epidermal regeneration. Allogeneic dermal fibroblasts are only weakly antigenic. Neonatal foreskins, discarded at circumcision are used as a source of fibroblasts for cell seeded artificial skin constructs. These young cells have immense replicative potential. Incredibly, an area of artificial skin construct the size of a football pitch can be seeded from a single donor foreskin.53 Cells seeded within a collagen matrix produce proteoglycans, adhesion molecules, and growth factors when the matrix is enriched before use. Clinical trials of these skin constructs in conditions of retarded healing (diabetic foot ulcers) appear to indicate that cell viability within the engineered replacement dermis is an important determinant of successful regeneration.53
Resorbable matrices for ocular surface regeneration analogous to current artificial skin constructs may have applications in external disease, refractive surgery, oculoplastics, and glaucoma.
Progressing from essentially two dimensional constructs (for example, skin or conjunctival replacement) to solid organ replacement requires careful consideration of the nutrient environment. Most cells are unable to survive in a matrix at greater than approximately 500 µm from a diffusible nutrient source (blood, aqueous, synovial or cerebrospinal fluid).54 This limitation for non-vascular tissues immediately suggests the cornea as a realistic target for tissue engineered replacement. Perfusion culture systems, or "bioreactors", developed for seeding artificial cartilage matrices could be modified and applied to the development of a true replacement cornea. Early studies have already demonstrated normal morphology and expression of phenotypic markers for engineered corneal constructs (Fig 6) with an epithelial and endothelial layer.55 56 Significant problems relating to source materials and the optimisation of matrix clarity remain; but at the current rate of progress, conventional corneal transplantation may be obsolete within quarter of a century. Theoretical advantages of tissue engineered corneal replacement could include no tissue supply problems, no rejection, and no iatrogenic disease transmission.
Improving on nature?
Collagen for artificial tissue matrices is currently derived from animal sources. Mammalian collagen alters little between species. Processing techniques are available to reduce antigenicity, but prion disease remains a significant concern. It is likely that recombinant collagen sources will soon be available. Synthetic resorbable matrices incorporating integrin ligand peptide sequences are also in development.41
Whilst the concept of a resorbable matrix replaced by autologous tissue is seductive in its "ultimate biocompatibility", resorbable matrices to guide tissue regeneration could also have disadvantages. The final result will leave the tissue, at best, no less resistant to injury or any underlying disease process than before the original insult. It may be possible to improve tissue performance using a permanent synthetic matrix. An artificial cornea, for example, could have tailored refractive power in addition to enhanced resistance to enzymatic matrix degradation.
Rapid evolution in cell sources for artificial tissue proceeds in tandem with advances in matrix engineering. Where regenerative potential is lost, or did not originally exist, cloned autologous tissue derived from embryonic stem cells57 may be available. Reprogramming of adult stem cells may also be possible.58
Stem cell reprogramming and cloning techniques avoid tissue rejection by producing autologous or genetically identical cell populations for tissue replacement. An alternative tissue engineering strategy with some exciting potential ocular applications is immunoisolation, in which foreign cells are protected from immune attack by encapsulation within a porous membrane59; with a pore size large enough to allow permeability to nutrients and smaller molecular species but small enough to prevent immunoglobulin and immunological effector cell access. Long term survival of allogeneic human and animal cells has been demonstrated for encapsulated cell/matrix constructs.59 60 Current uses include liver support devices and gene therapy. In contrast with other gene therapy protocols, delivery of an engineered protein product can be measured before implantation for encapsulated cells. Sustained intraocular delivery of a variety of cytokines could potentially be achieved using immunoisolation technology.
Biomaterials research spans the full spectrum of possibilities for restoring tissue function from entirely synthetic, non-degradable implants and prostheses, through hybrid cell/matrix constructs, to fully resorbable matrix templates for organ regeneration. Developments throughout this exciting spectrum will change the landscape of medical practice in the coming century. The immediate challenge for ophthalmology is to translate existing bioinert, bioactive, and tissue engineering biomaterial concepts into applications relevant to the prevention of visual loss.