Soft Tissues - Microstructures David Taylor This lecture is an introduction to the composition and structure of the various kinds of soft tissues in the body. Most of the information here can be found in: Skeletal Tissue Mechanics by Martin et al (1998), which also covers bone, and Biomechanics by Fung (1993), and in a chapter which I wrote for Comprehensive Structural Integrity (Publ.Elsevier, Volume 9, Chapter 2).
The Building Blocks All tissues in the body are made up of a relatively small number of materials, combined in various different ways and proportions. The main materials are: 1) Hydroxyapatite: found only in the hard tissues (bone and tooth materials) so not discussed here.
2) Collagen Collagen is the body’s principal polymeric material. Its long-chain protein molecules, made up of various amino acids and glycine, are arranged in a triple helix. Many different forms of collagen have been identified; the main type of collagen in bone which is also found in structural soft tissue (e.g. tendons, and ligament) is known as type I collagen. In many tissues it forms the matrix in which other materials, notably hydroxyapatite, are dispersed. It frequently adopts an oriented, fibrous form (see next slide), so fibre orientation is an important factor. Typical fibres are 0.2 m in diameter, with a very high aspect ratio.
Collagen Fibres Alligned collagen fibrils from a tendon. The width of the picture is 2.5 m (Reproduced from Fung 1993)
3) Water; Proteoglycans Water is present in all tissues, and makes up a major constituent of many soft tissues such as cartilage. Proteogycans are hydrophilic (i.e. water-loving) molecules, which have an important mechanical function because they form strong chemical bonds with water, allowing tissues to hold on to water which might otherwise be squeezed out by compressive loading. 4) Elastin Elastin is a fibrous protein found in many soft tissues including skin, blood vessels and lung tissue. It is a highly elastic material, having almost linear-elastic properties with a low Young’s modulus (around 0.6MPa) and the capacity to endure strains over 50% with good elastic recovery.
5) Keratin Keratin is a hard, strong, extensible material made up of protein molecules, which can align themselves into fibrous crystals. It is the principal component of hair. It has high stiffness (around 10GPa) and is capable of extensions of over 50% before failure, making it an excellent material for absorbing energy. 6) Ground Substance Ground substance is a hydrophilic gel material found in all tissues to varying extents. It fills up the gaps between other components (e.g. cells, fibres etc); it contains fluid, polysaccharides and proteoglycans.
7) Cells Almost all tissues contain living cells. Generally speaking, the material of the tissue itself is not alive – it lies outside the cells (i.e. it is 'extracellular'). The major exception to this is muscle (see below). Cells vary in size and morphology from one type of tissue to another: they are given the general suffix ‘cyte’: thus osetocytes are bone cells, chondrocytes are found in cartilage. A typical cell diameter is 1 m, though this varies considerably. Many cells are equipped with ‘processes’, which are long, thin extensions like arms reaching out from the cell itself. These processes are part of the cell; they make contact with processes from other cells at gap-junctions. Cell processes seem to play an important role in the supply of nutrients and in passing signals from one cell to another.
<ul><li>Cells do not contribute to the mechanical strength of tissues (except muscles) and they have to be continually supplied with nutrient fluid in order to stay alive, so why are they there? </li></ul><ul><li>During growth of tissues (in early life); cells perform the function of making new tissue, and removing it to change the shape of organs etc. </li></ul><ul><li>They monitor the mechanical state of tissues and initiate actions if required. For example if you get a torn ligament, cells detect this and initiate repair. Also they initiate ‘adaptation’ reactions in which organs change their geometry in response to mechanical factors (e.g. blood vessels get thicker if pressure rises – see below). </li></ul>
SoftTissues There are many different soft tissues; here we will discuss the following: (in brackets is the key to their mechanical function) Tendon and Ligament (tension) Cartilage (compression, sliding) Muscle (active tension) Blood Vessels (pressure vessels) Skin (barrier) Hair (protection, warmth)
Tendon and Ligament Ligaments and tendons were described by Kerr (in Vincent 1992) as ‘strings and straps’: an apt description of structures whose primary purpose is to transmit tensile load. Ligaments join one bone to another, and are mostly found reaching across joints. In complex joints such as the knee, ligaments help to ‘stabilise’ the joint (preventing undesired motions) and the forces taken by ligaments help to reduce the stress across the cartilage of the joint itself. Injuries which result in the loss or loosening of ligaments tend to lead to arthritis in later life. Tendons form similar attachments, but in this case between bone and muscle. Muscle (which is distinguished by its ability to create an active force – see below) turns into tendon at the point where it is attached to the bone.
Like most soft tissues, tendons and ligaments contain a lot of water – about 60% in this case. Of the remaining dry weight, 70-85% is collagen, mostly in the form of fibres oriented longitudinally. A key feature of soft tissues is their heirarchical structure; the next two slides show the structure of a collagen fibre and of a tendon, in which bundles of collagen fibres (fascicles) are grouped together with blood vessels and enclosed in an outer sheath. Thus it is difficult to say what constitutes the ‘material’ and what constitutes the ‘structure’ (e.g. the tendon). In practice the distinction is meaningless because tissue materials always adapt their microstructure to suit the organ that they are part of. Composition and Structure
Structure of a typical collagen fibre in a ligament, showing the size scale of each element and the imaging techniques used. (Reproduced from Martin et al 1998).
The structure of tendon, in this case from a rat’s tail. (Reproduced from Martin et al 1998).
Tendons and ligaments can afford to be highly anisotropic in their structure and properties because normally the only load they experience is uniaxial tension. Exceptions occur when, for example, a tendon passes over a protruberance in a bone, so that the side of the tendon rubs on the bone. At this point the tendon modifies its structure to become more similar to cartilage material to accommodate the local compressive and shear loads. Mechanical Property Notes
Alexander has shown that tendons also have an important function in storing elastic energy during motion (Alexander and Bennet-Clark 1977, Alexander 1988). For example, up to 50% of the energy required for running is stored in the tendons and ligaments of the foot and leg. Energy can be stored and released over a short time period with an efficiency of up to 95%. Mechanical Property Notes
Another mechanical function of ligament is ‘proprioception’: the ability to monitor strain. Studies have shown that ligaments are capable of detecting excessive stretching and of signalling to the adjoining muscles, triggering a force in the muscle which can prevent damage to the nearby joint. Tendons and ligaments are also capable of repair and adaptation. With careful surgical treatment, even a completely severed tendon can be induced to repair itself by collagen fibre deposition and fusion; this frequently occurs in the tendons of the hand, for example. Adaptation is clearly evident in the range of properties found in these tissues in different locations in the body, where they experience different levels of stress and strain. Ker has made an extensive study of the comparative properties of tendons. Advantages of being alive
Cartilage Cartilage is made up of collagen and another biological polymer - proteoglycan - plus about 70% water. It works so well as a bearing surface (resisting huge compressive stresses) because the proteoglycan is hydrophilic and so holds on to the water, preventing it from being squeezed out. This resistance is also aided by the microstructure of cartilage, which consists of a very fine network of collagen, with porosity on the scale of 50Angstroms, in which water molecules become trapped. As the next slide shows, structure varies within a cartilage layer. Near the outer (articulating) surface it’s mostly collagen fibres arranged to withstand tensile stresses (aligned horizontally in this picture). Inside there is more proteoglycan and water, and close to the bone surface it is partly calcified with hydroxyapatite. Composition and Structure
Typical Structure of Cartilage Layer in a Joint C artilage is a material whose microstructure is particularly difficult to separate from its macrostructure, because it is invariably found in relatively thin layers, intimately associated with bone in skeletal joints
Cartilage is a material designed with the purpose of resisting compressive forces and providing low friction and wear at a bearing surface. It has been claimed in the past that cartilage also acts as a shock-absorber, but this idea has largely been discredited in recent years. Though the material is capable of accepting high strains before failure, and therefore of absorbing quite a lot of energy per unit volume, the total volume available is very small in the thin layers of cartilage found in joints, so the total amount of energy that can be absorbed is probably much less than that of the underlying cancellous bone and the surrounding muscles, tendons and ligaments. Mechanical Property Notes
<ul><li>The next slide shows some typical stress/strain curves for cartilage. Its elastic modulus and ultimate strength are considerably lower than those of tendon and ligament, not because the materials are so fundamentally different but because, in a tensile test, a tendon or ligament will be stressed in the direction of preferential fibre orientation (the direction in which it is used in life), whereas a sample of cartilage will most conveniently be loaded perpendicular to its optimal direction, and in any case cartilage is designed to resist compression, not tension. </li></ul>Mechanical Property Notes
<ul><li>There is also a preferential direction in the plane of the articular surface; if a layer of collagen is pricked with a pin it will tend to split in one direction. This direction varies from place to place around the articular surface. The graphs compare data from tests carried out parallel and normal to the split lines, showing a lower elastic modulus in the latter case. The figure also shows results from specimens taken at different depths (depth increases in the order: 1,2,3), showing that material closer to the articular surface is much stiffer.. </li></ul>
Muscle Muscles differ from all other types of tissue in being composed almost entirely of living cells. Most other tissues contain living cells but are not themselves alive; muscles, on the other hand, are made up largely of long, fibrous cells which have the special property of being able to contract if suitably stimulated. There are three types of muscles: (i) smooth muscles which are found in blood vessels, intestines etc, whose stimulation is not under our voluntary control; (ii) skeletal muscles, which are attached to bones (via tendons), causing bones to move or to apply force, and; (iii) heart muscle, which controls the beating of the heart. Here we will consider only skeletal muscle.
Structure <ul><li>As we saw also for tendon and ligament, muscle has essentially a fibrous structure but with a hierarchical arrangement on several scales. Muscle fibres are 10-60 m in diameter and can be several millimetres or even centimetres long, sometimes running the whole length of a muscle. </li></ul>
Microstructure <ul><li>Muscle fibres are cells; they contain many ‘myofibrils’ which consist of molecules of actin and myosin arranged in overlapping parallel bands. </li></ul>
When suitably stimulated (e.g. by nerve impulses or electrical signals) the molecules move over each other to cause contraction: the lengths labelled H and I can contract to zero, but the length of the A band (made up of myosin molecules) remains constant. This sets an upper limit to the amount of contraction.
<ul><li>This figure shows results due to Gordon et al (1966) showing how much force can be developed in a muscle as a function of its length. The normal, resting length of the muscle fibres used, was 2.1 m; the force drops to zero if this length is increased, or decreased, by about a factor of two: at longer extensions, actin and myocin molecules no longer overlap, at shorter lengths adjacent actin fibres begin to overlap each other. </li></ul>
<ul><li>The maximum force which a muscle fibre can develop seems to be fairly constant from one muscle to another, and from one animal to another, so the force that a muscle is capable of delivering depends only on its cross-sectional area, a fact which simplifies calculations of joint biomechanics and helps in understanding the strength of muscles and tendons. </li></ul><ul><li>Muscle fatigue is an important factor in the structural integrity of bone, because one function of muscle action is to reduce bending stresses in bone. As we become tired after repeating the same exercise for some time, the maximum stresses in our bones can increase as our muscles become fatigued. Muscles also help to absorb energy. </li></ul><ul><li>The mechanism of muscle action will be considered in another lecture. </li></ul>Mechanical Property Notes
Blood Vessels The figure shows the composition of typical blood vessels of different sizes. Three concentric layers can be identified: the intima (inner layer), media (middle layer) and adventitia (outer layer). All three layers contain collagen-based materials, and the media layer contains muscle material (muscle cells and elastin).
<ul><li>From a structural point of view there would seem to be nothing simpler than a blood vessel: approximately circular in cross section and required to withstand an internal pressure which fluctuates in a known and regular manner fashion. The choice of material properties and thickness for such a structure would seem to be a very easy problem to solve. </li></ul><ul><li>However, the thickness/diameter ratios of blood vessels are not small enough for them to be considered as classical thin-walled cylinders. </li></ul><ul><li>This means that significant variation in stress through-thickness will occur, with the highest circumferential stress occurring on the inside of the cylinder. </li></ul>Mechanical Property Notes
To overcome this, and thus to optimise the use of material in the vessel wall, blood vessels have an inbuilt pattern of residual stress: compressive on the inside surface and tensile on the outer surface. This can be revealed if the vessel is removed and cut, when it is seen to spring open, as shown here . In addition, the material of the intima and media layers has a higher elastic modulus than that of the outer adventitia, and this modulus varies a lot in different blood vessels.
Other Tissues: Skin and Hair The body contains many other soft tissues which, being collagen-based, have similar properties to those of tendon, ligament etc. See, for example, Fung 1993 Chapter 7 for a description of the elasticity and viscoelasticity of skin. The wrinkling of skin which affects us all in old age has been considered biomechanically. It appears that temporary wrinkles, during normal facial expressions, occur due to buckling, but eventually wrinkles become permanent because creep effects lead to incomplete recovery of strain.
Other Tissues: Skin and Hair Hair is made from the material keratin, which is the general name for the protein products of epidermal cells of vertebrates. It contains fibres in the form of alligned crystals of the protein, in a matrix of the same material in non-alligned form. Essentially the same material is used in wool, silk, horn and hoof. Hair has an excellent combination of properties: the stiffness and strength of the hard tissues, combined with a high strain to failure, typically 50%. Properties are very sensitive to moisture content, which is affected by humidity. The high stiffness may be useful in hair and wool for trapping air for warmth. The high capacity for energy absorption (i.e. the large area under the stress/strain curve) is useful in the spider’s web for absorbing the energy of incoming flies!
Tissue Engineering <ul><li>You will be hearing a lot about tissue engineering in another course – basically it is about growing living tissue in vitro to be implanted into the body. </li></ul><ul><li>Currently most activities in tissue engineering are about growing soft tissues of some kind. </li></ul><ul><li>Hopefully this lecture will have given you an idea of how difficult that is. Soft tissues that are grown inside the body are not just materials but complex structures, difficult to reproduce in a dish. </li></ul><ul><li>What’s clear is that the mechanical environment is a key factor and so must be simulated during tissue engineering. </li></ul>THE END – Thanks for Listening!