Massachusetts Institute of Technology Harvard Medical School Brigham and Women’s Hospital VA Boston Healthcare System 2.79J/3.96J/BE.441/HST522J TISSUE ENGINEERING: OVERVIEW I.V. Yannas, Ph.D. and M. Spector, Ph.D. TISSUE ENGINEERING What 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 ENGINEERING Problems with Tissue Engineering ? Most tissues cannot yet be produced by tissue engineering (i.e., in vitro). ? Implantation of tissues produced in vitro may 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 medicine TISSUE ENGINEERING/REGEN. MED. Historical Perspective; Selected Milestones 1980 Yannas: Collagen-GAG matrix for dermal regeneration (“artificial skin”); Integra 1984 Wolter/Meyer: 1st use of the term, TE; endothel.- like layer on PMMA in the eye 1991 Cima/Vacanti/Langer: Chondrocytes in a PGA scaffold; the ear on the nude mouse 1993 Langer/Vacanti: Science paper on TE; cells in matrices for tissue formation in vitro; PGA 1994 Brittberg/Peterson: NEJM paper on human autologous chondrocyte implantation; Carticel Arthroscopic Debridement “Microfracture” Osteochondral Autograft Autologous chondrocytes injected under a periosteal flap (ACT) Total Knee Replacement Current Clinical Practice Image removed due to copyright considerations Image removed due to copyright considerations Image removed due to copyright considerations Image removed due to copyright considerations Future Clinical Practice Implementing Tissue Engineering Implantation of a cell-seeded matrix Image removed due to copyright considerations “Tissue engineered” cartilage implanted in a rabbit model did not remodel (Advanced Tissue Sciences, Inc.). Future Clinical Practice Implementing Tissue Engineering Implantation of the matrix alone “Microfracture”: Stem cells from bone marrow infiltrate the defect 500μm Image removed due to copyright considerations. Image removed due to copyright considerations TISSUE 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. Which Tissues Can Regenerate? Yes No Connective Tissues ? Bone  ? Articular Cartilage, Ligament, Intervertebral Disc, Others  Epithelia (e.g., epidermis)  Muscle ? Cardiac, Skeletal  ? Smooth  Nerve  FACTORS THAT CAN PREVENT REGENERATION ? Size of defect – e.g., bone does not regenerate in large defects ? Collapse of surrounding tissue into the defect – e.g., periodontal defects ? Excessive strains in the reparative tissue – e.g., unstable fractures 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) ? SOLUBLE REGULATORS – Growth factors or their genes ? ENVIRONMENTAL FACTORS – Mechanical loading – Static versus dynamic (“bioreactor”) CELL-MATRIX INTERACTIONS REQUIRED FOR TISSUE ENGINEERING Connective Tissues (Musculoskeletal) Mitosis 1 Migration 2 Synthesis 3 Contract. 4 + + + + + + - ? ? ? + - + ? ? Bone + Articular Cartilage - Ligament/Tendon + Intervertebral Disc ? Meniscus ? 1 Inadequate mitosis requires exogenous cells. 2 Inadequate migration may require a scaffold. 3 Inadequate biosynthesis require growth factors or their genes. 4 Contraction ? TISSUE ENGINEERING Current 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 regeneration TISSUE ENGINEERING Risks Exercise caution that the tissue engineering solution does not create larger problems that being solved. ? Tissue harvest for the isolation of cells places the donor site and surrounding tissue at risk of degeneration. ? Implants that accelerate the breakdown of surrounding tissues. EFFECTS OF THE CARTILAGE REPAIR PROCEDURES ON UNINVOLVED CARTILAGE ? Effects of Harvest (Canine Model) ? Changes in the mechanical properties of AC at sites away from the harvest, 4-mo post-op (up to 3- fold). ? Changes were consistent with hypertrophy, predisposing to osteoarthritis. Harvest Sites CR Lee, et al., JOR, 2000;18:790-799