1. Topic 7: Collagen and Collagenous Tissues Structure of collagen fibrils –Biochemistry –Molecular Biology –Morphology Biomechanics of collagenous tissues –1DLigament/Tendon –2DIntestine/Blood Vessels/Pericardium –3D Skin/Heart

2. Collagen: Overview Collagen is the primary structural protein in the body Collagen is the most prevalent protein comprising ~30% of ALL proteins Collagen is highly conserved between species (i.e.not undergone many evolutionary changes)

3. Molecular Structure Triple Helix (Gly-X-Y) N –X=proline –Y=hydroxyproline Triple helix + crosslinks: Structure give rise to a material that is very stiff and stable Crosslinks (covalent bonds) occur between the ends (insert diagrams) of molecules

4. Collagen Molecular Structure Molecules arranged in staggered pattern X-ray diffraction or electron microscopy give rise to a banded pattern Also relatively resistant to enzymatic breakdown

5. Collagen: Molecular Biology >20 different types have been identified characterized by different  -chains each  -chain is coded by a different gene exons are often 54 bp long –3 bp in a codon –18 amino acids – 6 sets of Gly-X-Y HomotrimerHeterotrimer Type III=(  1(III)) 3 Type I=(  1(I)) 2  2(I) Type XI=  1(XI)  2(XI)  3(XI)

6. Collagen Types ClassificationsExamples FibrillarITendon, Skin, Ligament IICartilage IIISkin Vessels, Tendon VFetal Membranes - Assoc w/ Type I VICartilage - Assoc w/ Type II Fibril AssociatedIXCartilage, Cornea XIIEmbryonic Tendon XIVFetal Skin & Tendon Network FormingIVBasement Membrane XHypertrophic Cartilage VIIIDescemets Membrane FilamentousVIVessels, Skin AnchoringVIIAnchoring Filaments Fibrillar Collagen (I (mostly), III) has greatest stiffness

7. Material Properties But that's not enough information to predict behavior in tissues... Tissues are composites Complex organization Complex boundary conditions MaterialStiffnessUTS Collagen1000 MPa100 MPa Steel200 GPa1000 MPa Wood10GPa100MPa Rubber1000-1400 kPa Bone 18000 MPa125 MPa Elastin500-600 kPa100-500 MPa Silk10000 MPa

8. Tissue Structure-Function Structure Function Model Constitutive Law  =f(  ) Architecture Anatomy Tissue Stress-Strain Force-Elongation Material Properties Boundary Value Problems Conservation Laws

9. Ligament/Tendon - 1D: Physiological Functions connect bones together (Ligament) connect bones to muscle (Tendon) Transmit forces Aid in smooth joint motion Absorb impacts/stresses Prevent large displacements such as dislocations Basically uniaxial loading elements

10. Ligament/Tendon: Structure/Biochemistry Loading –Fibers are parallel to load axis Organization –some fascicular organization –Unloaded = crimped –loaded = straight Composition –Collagen 75-80% –Elastin ‹5 % –PG 1-2%

11. Tendon Hierarchy

12. Knee Ligaments: Anatomy

13. Ligament/Tendon Histology Rabbit MCLRabbit ACL

14. Ligament/Tendon: Mechanical Properties

15. Ligament/Tendon: Structure/Histology Unloaded Loaded

16. Intestine - 2D: Physiological (mechanical) Functions Allow distension when digesting food Prevent over-stretching and consequent damage to other internal organs FINITE DEFORMATIONS

17. Intestine: Structure Crimped 2 primary planes doesn’t need to be as complex as skin because deformations are predictable Circumferential axis of intestine

18. Intestine: Structure as tissue gets loaded, the collagen fibers become more aligned with the axis of load

19. Large Intestine: Physiology Anatomy: serosa outer longitudinal muscle inner circular muscle submucosa mucosa Function: Secretory/Absorptive (Goblet cells) Epithelial protection (Epithelial cells) Immune regulation (Lamina propria lymphocytes) Contractility (Smooth muscle)

20. Blood Vessels - 2D Physiological Function Allow distension with increasing blood pressure Prevent damage to endothelial cells and smooth muscle cells

21. Blood Vessels: Structure Typically Blood vessels have more type III collagen (which is more compliant) Also have a lot of elastin collagen fiber diameter ~50nm

22. Blood Vessels: Collagen straightens with distension Measure Extinction Angle with Polarized Light Microscopy Theoretical Predictions with Analysis of Sine wave

23. 1D and 2D Collagenous Tissues Collagen is organized in tissue in such a way that it allows increased deformations in the tissue without actual stretching/straining collagen very much The organization is usually such that the axes of the fibers are oriented with the axis of maximal forces

24. Skin – 3D: Physiological Functions Protect body from invasion Withstand repeated in-plane stresses (knee-elbow) Transmit impacts into plane stresses Problem: not a well defined direction Solution: have collagen oriented in random direction

25. Skin: Structure Collagen 65 - 70 % (more type III than ligament) Elastin 5 - 10 % PG 1.5 - 2 %

26. Skin: Mechanical Properties More compliant than ligament or tendon; needs to be for its functions. orientation of coiled fibers change with load collagen is stiffer that elastin but has greater hysteresis (absorbs more energy)

27. Skin: Collagen and Aging collagen crimp decreases with age; stiffness increases elastin crimp increases with age; decreasing recoil Is this a mechanical explanation for wrinkles? YoungAdultOld

28. Skin: Aging YoungAdult Old Elastin

29. Heart - 3D collagen ECM: Physiological Functions Pump Blood Allow myocytes to stretch, but prevent overstretch More complicated because heart is pumping Keep blood vessels open Transmit contractile forces to chamber Elastic recoil Lateral slipping - shearing deformation

30. Heart: ECM Structure Composite –only 5% of HW –«1% elastin Hierarchical –Endomysial (MESH, STRUTS) –Perimysial (NETWORK, CPF) –Epimysial

31. Heart: Endomysial Collagen Link adjacent myocytes at Z-line of sarcomere Maintain patency of capillaries

32. Heart: Perimysial Network Organize myocardium into laminar sheet architecture

33. Heart: Coiled Perimysial Fibers Protect myocytes from overstretching Major contributor to passive stiffness

34. Heart: Pathology Explanted human heart tissue from transplant recipient Infarction: Myocytes die Collagen increases in density Coiled structure looks more 2- dimensional Looks like a ligament or tendon?

35. Cardiac Biomechanics if collagen is 5% of HW; –E COLL = 1000 MPa if collagen is all parallel then, –E MYO = E COLL *A F = 1000 *.05 = 50 MPa if randomly oriented

36. Tortuosity T=Fiber length/midline length

37. Tortuosity vs. Pressure Collagen fibers gradually straighten with increasing pressure Pressure (mmHg)

38. Sarcomere Length vs. Pressure Collagen fibers reach near maximal straightening coincident with maximal sarcomere length

39. Cardiac Collagen Biomechanics: Elastica Theory Assumptions: Myocytes and collagen act in parallel Collagen fibers are inextensible Have bending and torsional rigidity Forces act at end of fibers (no torsion) Collagen fibers are circular cylinders Collagen fibers run parallel to myocytes Collagen fibers are helices R P = 4z 0  F z F z

40. Helical Spring Model Stress-Strain relation:

41. Cardiac Collagen: Biomechanics

42. Conclusions Collagen is a key structural protein in the body By organizing this stiff material in different ways, the body achieves may different functions –1Dligament; very stiff; less crimp –2Dskin - versatile organization intestine - less versatile vessel - less versatile –3Dheart - complex organization, large deformations

43. Collagenous Tissues: Summary of Key Points Collagen is a ubiquitous structural protein with many types all having a triple helix structure that is cross-linked in a staggered array.triple helix structure Some of the most common collagen types are fibrillar and the collagen can be organized in 1-D, 2-D or 3-D in different tissues to confer different material properties. fibrillar material properties The 1-D hierarchical arrangement of stiff collagen fibers in ligaments and tendons gives these tissues very high tensile stiffness1-D hierarchical arrangement The 2-D arrangement of collagen fibers in tissues such as blood vessels and intestine is often quite wavy or disordered to permit higher strainsblood vessels intestine

44. Collagenous Tissues: Key Points (continued) Crimping, coiling and waviness of collagen matrix gives the tissue nonlinear properties in tension.Crimping, coiling and waviness Collagen structure in tissues changes with disease and ageing.ageing The hierarchical cardiac collagen matrix organizes cardiac muscle fibers in three- dimensions. Interstitial fibrillar collagen in the heart wall contributes to tissue stiffness during filling.hierarchical cardiac collagen matrixtissue stiffness