Tissue engineering Study Guide

📖 Core Concepts Tissue Engineering (TE) – Integration of cells, scaffolds (materials), and bio‑active signals to restore or replace damaged tissue. Three Main TE Strategies – (1) Cells only (cell transplantation), (2) Substances only (growth‑factor/biomolecule delivery), (3) Cells + Scaffold (most common clinical approach). Scaffold – A 3‑D structure that provides mechanical support, guides cell attachment/migration, and degrades as new tissue forms. Stem‑Cell Hierarchy – Totipotent → Pluripotent → Multipotent → Adult stem cells; each level loses differentiation potential. In‑situ TE – Implant biomaterials (± cells/biomolecules) directly into a defect; the body acts as the bioreactor. Bioreactor – Engineered culture system that supplies oxygen, nutrients, mechanical/flow cues to mature large‑scale constructs. --- 📌 Must Remember Cell source classification – Autologous (same patient) → no immune rejection; Allogenic (same species donor) → may need immunosuppression; Xenogenic (different species) → high immune/ethical concerns; Syngeneic (genetically identical). Scaffold core requirements – High porosity & appropriate pore size, biodegradability matched to tissue formation rate, mechanical strength appropriate to target tissue (e.g., cortical bone ≈ 100–150 MPa compressive). Synthetic polymer degradation – PLA → slow (lactic acid); PGA → fast; PCL → very slow; PLGA degradation rate tuned by PLA/PGA ratio. MSC phenotypic drift – Prolonged monolayer culture ↓ CD73, CD90, CD105; ↑ senescence → reduced osteogenic/chondrogenic potential. Bioreactor benefits – Overcome diffusion limits, provide shear stress or cyclic strain to direct lineage commitment. Regulatory categories (EU) – TE products may be classified as medical devices, medicinal products, or biologics; hybrid cell‑scaffold combos often sit in a gray zone. --- 🔄 Key Processes Cell Isolation Blood‑derived: centrifugation → apheresis. Solid tissue: enzymatic digestion (trypsin or collagenase) → release of cells. Scaffold Fabrication (example: Electrospinning) Dissolve polymer → load into syringe → apply high voltage → fibers deposited on collector → control fiber diameter & pore architecture. Bioprinting Workflow Prepare bio‑ink (cells + hydrogel). Load into ink‑jet/laser‑assisted printer. Deposit droplets/layers according to CAD model. Cross‑link/solidify construct (thermal, UV, chemical). In‑situ Bone Regeneration Prepare defect, place biomaterial (e.g., PLGA scaffold) ± autologous MSCs + BMP‑2. Body’s native environment supplies vasculature and mechanical cues → tissue formation. MSC Trilineage Differentiation Assay Culture MSCs in osteogenic, chondrogenic, and adipogenic media → assess mineralization (Alizarin Red), proteoglycan (Alcian Blue), lipid droplets (Oil Red O). --- 🔍 Key Comparisons Autologous vs. Allogenic Cells – Autologous: patient‑specific, no rejection; Allogenic: off‑the‑shelf, may need immunosuppression. PLA vs. PGA vs. PCL – PLA: slow degradation, stronger long‑term; PGA: fast degradation, weaker; PCL: very slow degradation, highly flexible. Natural vs. Synthetic Scaffolds – Natural (collagen, hyaluronic acid): excellent biocompatibility, limited tunability; Synthetic (PLA, PLGA): controllable mechanics & degradation, may need surface functionalization (e.g., RGD). Bioreactor Types – Rotating wall (low shear, spheroid formation) vs. Perfusion (continuous flow, vascularization) vs. Mechanical (cyclic strain/ compression). --- ⚠️ Common Misunderstandings “Scaffold = permanent implant.” Scaffolds are designed to degrade as native tissue replaces them; permanent implants are usually devices, not TE scaffolds. “All MSCs are equal.” Source (bone marrow vs. adipose) and culture duration dramatically affect phenotype and differentiation capacity. “Higher porosity always better.” Excessive porosity reduces mechanical strength; optimal pore size balances cell infiltration with load‑bearing needs. “Bioprinting automatically yields functional tissue.” Print fidelity is only the first step; post‑printing maturation in a bioreactor is essential for functionality. --- 🧠 Mental Models / Intuition “Scaffold as a temporary scaffold‑bridge.” Imagine building a bridge with wooden planks that are later replaced by concrete—scaffold supports early cells, then disappears as the “concrete” (native tissue) solidifies. “MSC culture is a ‘time‑bomb.’ The longer MSCs sit in monolayer, the more they lose stemness—think of fresh fruit vs. over‑ripe fruit losing quality. “Bioreactor = artificial body.” Treat the bioreactor as a mini‑organism that must provide oxygen, nutrients, and mechanical cues exactly as the target tissue experiences in vivo. --- 🚩 Exceptions & Edge Cases Decellularized tissue scaffolds – retain native ECM cues but may still harbor immunogenic residues; thorough decellularization is critical. Hydrogel bio‑inks – excellent for soft tissues, but often lack the stiffness needed for load‑bearing bone unless reinforced (e.g., carbon nanotubes). Xenogenic cells – rarely used clinically because of strong immune and ethical barriers, despite potential for research models. --- 📍 When to Use Which Choose autologous MSCs when immune rejection risk must be minimal (e.g., patient‑specific cartilage repair). Select PLGA scaffolds for bone TE when a controllable, moderate degradation rate is needed and you want to load growth factors (BMP‑2). Use hydrogel‑based bioprinting for soft tissue (skin, liver organoids) where high cell density and matrix mimicry are priority. Apply perfusion bioreactors for constructs > 2 mm thickness to overcome diffusion limits and promote vascular network formation. --- 👀 Patterns to Recognize “High porosity + low mechanical strength” → likely a soft‑tissue scaffold (e.g., cartilage, skin). “PLA / PGA ratio ↑ PLA → slower degradation, higher stiffness – typical for load‑bearing applications. “Loss of CD73/90/105 + extended passage → MSCs entering senescence – expect reduced differentiation in assay results. “Presence of RGD peptide on synthetic polymer → intentional enhancement of cell adhesion. --- 🗂️ Exam Traps “All scaffolds are biodegradable.” False – some synthetic polymers are designed for long‑term implantation (e.g., certain silicones). “In‑situ TE eliminates the need for any cells.” Misleading – many in‑situ approaches still embed autologous or minimally manipulated cells to boost regeneration. “PLGA always degrades faster than PLA.” True only when PGA fraction is high; a PLGA 85:15 PLA:PGA can degrade slower than pure PLA. “MSC multipotency equals pluripotency.” Incorrect – MSCs are multipotent (lineage‑restricted), whereas pluripotent cells can become any somatic cell type. ---