Stem Cells and Tissue Engineering

1. Stem Cells and Tissue Engineering Eleni Antoniadou

2. Background • Critical-sized bone defects • Do not heal spontaneously • 500,000 bone repair procedures annually • Trauma • Resection • Abnormal development • Current clinical approaches • Autograft • Allograft • Metallic implants Limitations 1. extended surgical time, 2. limited availability, 3. variable bone quality, 4. significant blood loss 5. donor-site morbidity

3. Background Osteogenic Biological ability to directly create new bone. i.e. mesenchymal stem cells • Osteoconductive materials • Calcium phosphate, hydroxyapatite, Bioglass • Osteoinductive materials • Collagen, PLA, PLGA, Bioglass • Materials usually conductive or inductive • Bone is a collagen-hydroxyapatite composite • Not both, so composites needed • VEGF promotes angiogenesis • May speed bone healing Osteoconductive Scaffold for supporting the attachment of osteogenic precursor cells. Osteoinductive stimulate the proliferation and differentiation of mesenchymal stem cells into bone-forming cells.

4. Hypothesis Cell Source Mesenchymal Stem Cells Biomaterials approach Signals VEGF ECM PLGA + Bioglass coating Enhance bone regeneration 1. Improve vascularization 2. Better integration with native tissues

5. Reasoning • PLGA • Tailorable degradation properties • Controlled growth factor release • Bioglass • Osteoconductive and inductive • Mimics mineral composition in bone • VEGF • Promotes angiogenesis

6. Scaffold fabrication • 3D, porous PLGA (85:15) • VEGF incorporation • Gas-foaming/particulate-leaching • Bioglass coating • Soak in slurry and dry overnight • Scanning electron microscope • In vitro release kinetics • Radiolabeled VEGF • In PBS, measure amount released over time

7. In vitro characterization Osteoconductive surface Controlled growth factor release ~60% release @ 14 days Good integration, + maintain surface 50% @7 days Low error + 0.1 mg Matches PLGA degradation PLGA Mimic bone collagen Bioglass (note crystal structure) Mimics bone hydroxyapatite ~40% initial release diffusion outwards

8. Endothelial Cell proliferation • Endothelial cell culture • Growth factor removal • Insert 4 different groups of scaffolds • bioglass-coated or uncoated scaffolds • VEGF-releasing or blank • Culture 72 hours • Trypsinize and count cells • Move scaffolds to new pre-seeded wells • Repeat 72 hour cycle four times

9. Endothelial cell proliferation Comparable proliferation Additive effect? Dissolution of bioglass? PLGA control + VEGF +bioglass +VEGF +bioglass

10. MSC Differentiation • Culture to passage 6 • Statically seed onto sterilized scaffolds (4 groups) with Matrigel and α-MEM • Add osteogenic supplements • 10 mM β-glycerophosphate • 50 ug/ml ascorbic acid • 0.1 uM dexamethasone • Culture on orbital shaker at 25 rpm • Lyse cells and assay either after 1,2, or 4 weeks • Alkaline phosphatase (spectrophotometer) • Normalized by DNA (Hoechst dye + flourometer) • Osteocalcin (ELISA)

11. Alkaline Phosphatase ~20% variation In general, no major effects PLGA control + VEGF +bioglass +VEGF +bioglass Bioglass trends lower

12. Osteocalcin Again, in general, no major effects PLGA control + VEGF +bioglass +VEGF +bioglass

13. In vivo critical defect model • 9 mm diameter circular cranial defect in rats • Full thickness (1.5-2 mm) • Bioglass or bioglass + VEGF scaffolds implanted • Euthanized after 2 or 12 weeks • Fixation in formalin • Scanned using micro-CT • Bone volume fraction • Bone mineral density • Resolution 9 um

14. Analysis of blood vessel ingrowth • Samples bisected, decalcified, parafin embedded • Sectioned for histology • 2 week samples immunostained with vWF (vessels) • Light microscope, camera, and image analysis program • Count blood vessels manually • Normalize by tissue area Both treatments displayed significant increases in blood vessel density

15. Blood Vessel Density Density doubles compared to control! Most found near periphery PLGA control +VEGF +bioglass +bioglass

16. Top-view Note healing bone doesn’t meet in center Micro-CT Analysis Side-view Initial callus has nearly bridged defect and is thickening +VEGF +bioglass +bioglass

17. Bone Mineral Density ~25% increase Minor increase PLGA control +VEGF +bioglass +bioglass

18. Discussion • Composite materials hybridize properties • Local delivery of inductive factors from osteoconductive scaffolds • Low concentrations of bioglass is angiogenic (500 ug) • Mimic environment of natural healing (indirect) • Upregulation of growth factors in surrounding cells? • VEGF (3 ug) is much more potent (direct, focused) • Relatively similar results in direct comparison

19. Discussion • Localized, prolonged VEGF delivery • Improved bone cell maturation over controls • Increased bone mineral density • Slight increase in bone volume • Similar osteoid, but biomineralization is key • Amount of bone unchanged, bone formation rate increases • VEGF promotes establishment of vascular network • Nutrient transport • Supply progenitor cells to participate in healing

20. Discussion • Lack of in vitro osteogenesis • Low concentrations of bioglass -> angiogenic • Higher concentrations of bioglass -> osteogenic • Orders of magnitude greater • Bioglass surface coating • Limited by dissolution rate (ions) • Inductive component • Dissolution products upregulate important genes in osteoblasts

21. Important contributions • Nutrient diffusion limitation • Poor once tissue mineralizes • Lacks vessels, blood supply • Inner tissue becomes necrotic • Scaffold eventually fails • Inflammatory bone resorption • Promoting angiogenesis is vital for long-term success • Porosity • Growth factors

22. Important Contributions • Strengthened proposed link between bone remodeling and angiogenesis • Bone remodeling process • Could osteoporosis be a vascular disease?

23. Important Points • Statistical significance vs. practical significance • Is VEGF necessary? In vitro, no. In vivo, yes. • Small animal models sometimes don’t scale up well • Greater amounts of growth factors (expensive) • Time of healing is a major consideration • Just a snapshot, time depends on severity of defect • Too long -> bone will resorb due to mechanical disuse

24. Criticisms • No references for BMD of skull • Too dense and bone becomes brittle • Modulus mismatch -> stress concentrations -> fracture • High BMD not necessarily a good thing! • Passage 6 mesenchymal stem cells • Slow phenotypic drift in vitro • Earlier passage (~2-3) may show crisper effect • Why no CT scan at week 2? • Interesting to see early response

25. Main ideas • Materials-based approach can lead to effective tissue engineering strategies (i.e. tissue engineering is more than just stem cell therapy) • Reproducible • Less risk than direct cellular therapy • Strong, fundamental link between angiogenesis and bone formation • Exploit through composite materials such as bioglass and growth factors like VEGF which promote both • Goal: achieve a desired tissue response • ECM degradation components • Inductive factors released from the matrix

26. Thank you for your attention!!!