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

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)

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

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

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)

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

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 Rubber kPa Bone MPa125 MPa Elastin kPa MPa Silk10000 MPa

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

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

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%

Tendon Hierarchy

Knee Ligaments: Anatomy

Ligament/Tendon Histology Rabbit MCLRabbit ACL

Ligament/Tendon: Mechanical Properties

Ligament/Tendon: Structure/Histology Unloaded Loaded

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

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

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

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)

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

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

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

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

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

Skin: Structure Collagen % (more type III than ligament) Elastin % PG %

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)

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

Skin: Aging YoungAdult Old Elastin

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

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

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

Heart: Perimysial Network Organize myocardium into laminar sheet architecture

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

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?

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

Tortuosity T=Fiber length/midline length

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

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

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

Helical Spring Model Stress-Strain relation:

Cardiac Collagen: Biomechanics

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

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

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