2.79J/3.96J/20.441/HST522JUNIT CELL PROCESSES UNDERLYING TISSUE ENGINEERING AND REGENERATIVE MEDICINEM. Spector, Ph.D.Massachusetts Institute of TechnologyHarvard Medical SchoolBrigham and Women’s HospitalVA Boston Healthcare SystemTISSUE ENGINEERING/REGENERATIVE MEDICINE Cell + ECM Prod. + Reg.Cell + ECM Prod. + Reg.Cell + ECM Prod. + Reg.MigrationMitosisSynthesis: ECMCell + ECM Prod. + Reg.ContractionUnit Cell ProcessesGenerally inhibits regenerationCell + ECM Prod. + Reg.Synthesis: EnzymeDegradationFormationReg.RemodelingTISSUE ENGINEERINGWhat is tissue engineering?• Production of tissue in vitro by growing cells in porous, absorbable scaffolds (matrices).Why is tissue engineering necessary?• Most tissues cannot regenerate when injured or diseased.• Even tissues that can regenerate spontaneously may not completely do so in large defects (e.g., bone).• Replacement of tissue with permanent implants is greatly limited.TISSUE ENGINEERINGProblems with Tissue Engineering• Most tissues cannot yet be produced by tissue engineering (i.e., in vitro).• Implantation of tissues produced in vitromay not remodel in vivo and may not become integrated with (bonded to) host tissue in the body.Solution• Use of implants to facilitate formation (regeneration) of tissue in vivo.– “Regenerative Medicine”– Scaffold-based regenerative medicineTISSUE ENGINEERING VS. REGENERATIVE MEDICINE*TISSUE ENGINEERINGRegeneration In VitroProduce the fully formed tissue in vitro by seeding cells into a biomaterial matrix, and then implant the regenerated tissue into the body.REGENERATIVE MED.Regeneration In VivoImplant the biomaterial matrix with, or without seeded cells, into the body to facilitate regeneration of the tissue in vivo.TISSUE ENGINEERING VS. REGENERATIVE MEDICINETISSUE ENGINEERINGRegeneration In VitroAdvantages• Evaluation of tissue prior to implantationDisadvantages• For incorporation, must be remodeling• Stress-induced architecture cannot yet be produced in vitroREGENERATIVE MED.Regeneration In VivoAdvantages• Incorporation and formation under the influence of endogenous regulators (including mechanical strains)Disadvantages• Dislodgment and degrad. by mech. stresses in vivoTISSUE ENGINEERINGCurrent Status• No one has yet employed Tissue Engineering methods to fully regenerate any tissue that does not have the capability for spontaneous regeneration*.– The Integra skin has no hair or glandular structures and its architecture is close to but not identical to normal dermis.– The Carticel cartilage is not articular cartilage.• Experience has taught us that full regeneration may not be necessary to achieve a meaningful clinical result (e.g., pain relief, recovery of function, esthetics)• How close to regeneration is good enough?* Many examples of bone regenerationTISSUE ENGINEERING ENDPOINTS• Morphological/Histological/Biochemical– Match the composition and architecture of the tissue.– Problem: A complete analysis is difficult and no clear relationships yet with functional and clinical endpoints.• Functional– Achieve certain functions; display certain properties (e.g., mechanical properties).– Problem: Difficult to measure all properties; Which properties are the most important?• Clinical– Pain relief.– Problems: Can only be evaluated in human subjects and the mechanisms (including the placebo effect) and kinetics of pain relief (e.g., how long it will last) are unknown.ELEMENTS* OF TISSUE ENGINEERING/ REGENERATIVE MEDICINE• MATRIX (SCAFFOLD)– Porous, absorbable synthetic (e.g., polyglycolic acid) and natural (e.g., collagen) biomaterials• CELLS (Autologous or Allogeneic)– Differentiated cells of same type as tissue– Stem cells (e.g., bone marrow-derived) – Other cell types (e.g., dermal cells)• REGULATORS– Growth factors or their genes– Mechanical loading– Static versus dynamic culture (“bioreactor”)* Used individually or in combination, but often with a scaffold)TECHNOLOGY TOOL BOXTISSUE ENGR./REGENERATIVE MED.• SCAFFOLD (MATRIX)– Porous, absorbable biomaterial; can serve to regulate cell function prior to is its absorption• CELLS• REGULATORS – Cytokines (growth factors)– Genes for growth factors– Antagonists of inhibitors– Fluid flow– Mechanical loading– Hydrostatic pressure– Shock wave and ultrasound– Electromagnetic radiation and magnetic fieldsCELL THERAPY FOR LOCAL REPAIR*Injection of Exogenous Cells; Cells Expanded in Number in Monolayer Culture• Chondrocytes for cartilage repair (FDA-approved)• Intervertebral disc cells for herniated disc (human trial)• Myoblasts and stem cells for myocardial infarction (human trial)• Cells injected into the brain (human)• Stem cells into spinal cord lesions (animal)• Cells into the retina (animal)* An alternative strategy is to implant a scaffold seeded with the cellsArthroscopicDebridementFigure by MIT OpenCourseWare.“Micro- Osteochofracture” Plug Aut(“MosaicpOsteochoPlug Auto(“Mosaicplndral ograftlasty”)Autologous chondrocytes injected under a periosteal flap (Genzyme; “Carticel”)Total Knee ReplacementMedical illustrations removed due to copyright restrictions.Current Clinical Practice30 years Osteochondral Plug Autograft (“Mosaicplasty”)CellExtracellularMatrix Figure by MIT OpenCourseWare.4 mm10 mmArticular Cartilage Articular CartilageAutologous Chondrocyte ImplantationProblems with the periosteum?This process has been commercialized by Genzyme (for $20,000).M Brittberg, et al., NEJM 33:889 (1994)Image removed due to copyright restrictions.Figure 1 in Brittberg, M., et al. “Treatment of Deep Cartilage Defects in the Knee with Autologous Chondrocyte Transplantation.” NEJM 331, no. 14 (1994): 889-895.http://content.nejm.org/cgi/content/abstract/331/14/889Collagen membrane to replace a periosteal tissue graft to contain injected autologous chondrocytes (grown in culture)DebridementImplantation of a collagen membrane to contain injected autologous chondrocytesImages removed due to copyright restrictions.Future Clinical PracticeImplementing Tissue EngineeringImplantation of the matrix alone, (or supplemented with growth factors or genes for the GFs)“Microfracture”:Stem cells from bone marrow infiltrate the defectImplantation of a cell-seeded matrixFigure by MIT OpenCourseWare.CELLS FOR TISSUE