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- 1 Elastic Fibres
- 1.1 Introduction
- 1.2 History
- 1.3 Structure and Components
- 1.4 Assembly of Elastic Fibre
- 1.5 Function
- 1.6 Clinical Significance
- 1.7 Future Directions
- 1.8 References
Elastic fibres are an abundant extracellular matrix macromolecule of connective tissues found within the skin, blood vessels, lungs and ligaments.  It provides resilience and the mechanical recoil of tissues and organs to repetitively deform and reform to its original shape without compromising structural integrity. This deformability is essential to the functioning of tissues. Its recoiling properties are attributed to the elastin polymer that makes up most of the elastic fibre.  The impairment or loss of elastic tissues results in profound pathological implications.
Elastic fibres are complex due to the multitude of components involved in forming the structure of the macromolecule. It involves a hierarchal procedure of assembly with unique biomechanical functions within different tissues. Progress in the research of elastic fibres involves ultrastructural analysis, the study of mouse models and the identification of interacting molecules that will help to increase our understanding of this extracellular matrix component. 
We will be discussing the overall structure, components involved, process of assembly, function and the clinical manifestations of elastic fibres.
Elastin was first recognized in the mid 19th century, but due to its high insolubility, it was an extremely difficult protein to study. Substantial progress in the study of elastin was hindered until the soluble precursor, tropoelastin, was discovered in the early 1970s. This sparked significant interest in the study of elastin and elastic fibres.  In the late 1970s, Helene Sage and William Gray used histological staining techniques and amino acid composition to identify elastin in arteries of vertebrates. This research was significant towards the progress of study of elastic fibres as the results of this study are still valid today. No elastin has been identified in invertebrates.  The Gordon Research Conference on elastin began in 1976, which represented a large international interest group in this area. Due to its success, it has been held every year since, with scientists revealing their studies of the biochemistry, cellular biology, pathology and molecular biology of the protein elastin. 
By 1995, the conference name changed from “Elastin” to “Elastin and Elastic Fibres” which highlights that this area of research had extended from the single elastin protein to the entire elastic fibre matrix. This encompassed the discovery of microfibrils and microfibrillar-associated proteins as scientists continued to describe the elastic fibre structure. With the years to follow, this field has continued to expand to discover many other molecules that influence the function, regulation and assembly of elastic fibres that include lysyl oxidases, fibulins, emilins and proteoglycans. 
Structure and Components
Elastic fibres play a significant role in the extracellular matrix of cells and the organisation of elastic fibres varies in different tissues. In the lungs, skin and ligaments, elastic fibres are organised in a rope-like manner, in blood vessels it is organised as thin concentric sheets and has a large honeycomb structure in elastic cartilage. Elastic fibres are composed of two significant components; 90% of which is an amorphous core of highly cross-linked elastin protein and the remaining 10% is a fibrillar mantle of microfibrils. 
Elastin and Tropoelastin
Elastin is a very insoluble complex structure, being one of the most insoluble proteins of the human body.  The study of elastin was significantly hindered due to its insolubility. With the discovery of the soluble precursor of elastin, tropoelastin, scientists were able to describe chemical properties of elastin. 
Tropoelastin and elastin are made significantly of only four hydrophobic amino acids: glycine, alanine, valine and proline. Mature elastin appears as an amorphous branching structure with extensive cross-linking tropoelastin monomers. An important feature of the composition of pure elastin is the absence of methionine, histidine and tryptophan.  Desmosines and its isomer, isodesmosine are formed from the covalent cross-linking of four lysine residues that are unique to elastic fibres and are responsible for the its hydrophobic properties.  Lysyl oxidase (LOX) is the enzyme that initiates cross-linking within elastin as it modifies lysine residues to form desmosines and isodesmosine. 
Under a light microscope, elastin is typically recognised by its wavy appearance. 
During the assembly of elastic fibres, microfibrils function as a scaffold for the deposition of elastin.  The microfibrillar region is organised with 10-12nm fibrils that have a beaded appearance that are loosely parked into parallel bundles to form a sheath around the amorphous elastin core.  The major structural component of the microfibrils is fibrillins, large rod-like cystine-rich glycoproteins. Three fibrillins have been identified: Fibrillin-1, Fibrillin-2 and Fibrillin-3. Fibrillin-1 has Arg-Gly-Asp sequences that interact with integrins located on the cell surface and is believed to have a role in signalling during elastic fibre assembly. Fibrillin-1 and -2 are responsible for mediating the binding of tropoelastin to the microfibril. It is currently unknown whether Fibrillin-3 is related to elastic fibres. 
Microfibrillar Associated Proteins
The outer mantle of fibulin-rich microfibrils has many distinct proteins associated with it. Microfibril associated glycoproteins-1 (MAGP-1) and MAGP-2 and are small glycoproteins proteins that are important for the structural integrity of elastic fibres.  MAGP-1 is able to bind with fibrillin-1, tropoelastin and is believed to be involved in elastic fibre assembly as it bridges the two molecules together.  MAGP-2 interacts with and binds to fibrillin-1 and fibrillin-2 and is believed to be functionally involved in cell signalling in the assembly of microfibrils.
Fibulins are a family of five different acidic glycoproteins, but fubulin-1, fibulin-2 and fibulin-5 are the three that are most implicated in the biology of elastic fibres. Fibulin-1 is not associated with individual microfibrils but is associated with the elastin core of elastic fibres. Fibulin-5 localises to the interface of elastin and microfibrils. Fibulin-2 is able to strongly bind with tropoelastin and fibrillin-1. Fibrilin-4 has some but relatively weaker affinity to tropoelastin. 
Elastin microfibril Interface Located Protein (EMILIN)-1 is another microfibril protein localised to the elastin-microfibril interface. EMILIN-1 is able to regulate the deposition of tropoelastin on microfibrils and bind to elastic fibres on the surface of cells. 
Assembly of Elastic Fibre
The formation of elastic fibers is a complex process which is comprised of multiple interactions between different proteins, enyzmes and binding sites. Elastic fibers are mostly comprised of elastin and microfibrils which makes up most of the structure and the processes of formation are closely regulated and carried out by the smaller protein components; tropoelastin, lysyl oxidases, fibrilin-4 / fibrilin-5 and integrins.
Initially, tropoelastin molecules are secreted from elastogenic cells  (e.g. dermal fibroblast, vascular smooth muscle cells and lung alveolar cells) as a soluble precursor  that has a mass of approximately 70 kDa . These tropoelastin molecules undergo the process of coacervation  where the molecules rapidly self-associate, driven by the hydrophobic interactions to form protein rich aggregates. These aggregates of tropoelastin are organised in a cross-linked manner and are guided by a 67-kDa elastin-binding protein to the cell surface  with the assistance of glycosaminoglycans as well as one or more proteins from the LOX family .
Within the aggregate tropoelastin molecules, fibulin-4 and/ or fibulin-5 extracellular matrix proteins (ECMP), work to facilitate and assist in the cross-linking of individual tropoelastin molecules as well as maintaining and limiting the size of the aggregates  on the cell surface. Fibulin-4 and fibulin-5 ECMP’s are crucial molecules in the assembly of elastic fibres and also play a non-redundant role during elastic fibre formation.  Newly generated tropoelastin molecules from elastogens continuously attach to the growing aggregates or proceed to form their own aggregates with the assistance of LOX.
Coacervated tropoelastin molecules from the cell surface are then deposited to the onto microfibrils, which acts as a scaffold allowing for the formation of elastic fibres, held near the cell surface by integrins. Integrins are the transmembrane receptors are the bridges for cell-cell interactions.  Upon transportation and attachment to the microfibril structures, coalesced tropoelastin molecules form larger aggregates on the whole surface of the fibres with the assistance and regulation of more LOX and fibulin proteins leading to the complete formation of elastic fibres when enough aggregates have been attached.
Step by Step Procedure of Elastic Fibre Formation (refer to image 'Process of Elastic Fibre Assembly)
|1||Initially Tropoelastin is secreted by cells and then organised into cross-linked aggregates on the cell surface with the assistance with cell surface glycosaminoglycan’s and one or more LOX|
|2||Fibulin-4 and/or Fibulin-5 closely interact with the aggregated tropoelastin molecules on the cell surface to facilitate cross-linking or to limit the aggregate size|
|3||Newly generated Tropoelastin molecules are constantly formed and attached to elastin aggregates that already exist on the cell surface. These larger aggregates are then transferred onto pre-existing Microfibrils : [Microfibrils are made primarily of Fibrilin-1 and/or Fibrilin-2, possibly containing other microfibril associated proteins. These Microfibrils bind to the cell surface via Integrins which are transmembrane receptors that bridge cell-cell interactions]|
|4||Fibulin-4 and Fibulin-5 both bind to Fibrilin-1 and may help transfer the elastin aggregates to the microfibril|
|5||Elastin aggregates on the microfibril coalesce into larger structures and are further cross-linked by LOX to form the complete and functional elastic fibre, during this stage, Fibulin-4 and/or Fibulin-5 may assist in facilitating cross-linking|
Structure serves as function
If you think about a rubber band, it can be stretched when a force is applied to it. When the force stretching the rubber band is released, the rubber band resumes back to its original shape. Similarly, with age and repetitive stretching of the skin, the elastic fibres that produce this elongation become less "elastic" resulting in reduced resilience, recoiling and adaptability.
Often the structure inside a biological system will serve well its main function. This can be clearly seen in cross sections of the branches of the aorta, stemming from the heart. What can be seen in histological cross sections are walls that are thick, muscular and contain high amounts of elastic fibres. The reason for this is that blood exiting the heart via the aorta needs to supply all the organs of the human body. Therefore, understandably, it is under enormous pressure and carries large volumes of blood. The structure of the artery needs to be able to carry out function. Elastic fibres of this type can stretch up to 1.5 times their length and resume back to their original length in repetitions with every heart beat.
The human body without and elastic fibres would not be able to maintain many functions. Elastin can be found in various places in the human body. Some of these places and the functional component of elastin are found in this section. Common places where elastin usually resides are:
• Blood vessels
• Elastic Cartilage
• Connective Tissue
Tropoelastin - Elastin's bioactive assembly module 
Branches of the heart that stem off the aorta can come in various shapes and sizes, such as the thickness of their wall, their length and type of artery (e.g. muscular, elastic)  .If we take a closer look at the cross sections of these vessels, there are three distinguished layers or tunics to the division of the walls. From the lumen of the artery to its most superficial layer they are Tunica; Intima, Media, and Adventitia respectively.
The tunica intima envelops the lumen of the blood vessel. Lying closely to the lumen is the simple squamous endothelial cells  which are situated above its associated connective tissue. Dividing the Tunica intima from the Tunica media, is the internal elastic lamina .
The internal elastic lamina is the first elastic layer found in the blood vessel from the lumen and serves the role of allowing the tunica intima to move independently from the other tunicas as the elastic fibres expand with increased systolic blood pressure.  Systolic blood pressure refers to the maximum blood pressure the that will be experienced by the blood vessel under ventricular contraction. Thus, it is understandable that the tunica intima of elastic arties is significantly different to that of other arteries due to the nature of serving out its purpose.
As depicted in the histological diagram below, the tunica media consists of:
• fine elastic fibres that are dispersed across this layer and are less visible than those found in the tunica intima and adventitia 
• circular smooth muscle fibres with little to no connective tissue when compared to other types of arteries.
• Collagen fibres are also found between the elastic fibres .
There are concentric fenestrated elastic fibres found in the tunica media function  as a pressure reservoir as blood passes through these blood channel and springs back to its original state as the blood pumping process continues and then begins again.
Some scientists will include external elastic lamina, as part of the tunica media and others as part of the Tunica Adventita. However, the underlying fact it that it the external elastic lamina divides the media from the adventitia. The external elastic lamina provides added allowance for stretching to occur and has the same function as the internal elastic lamina.
Under the elastic lamina lies the smooth muscle which help support the movement of blood along its course  . The adventitia is the most thinnest tunica of the elastic wall. It also contains collagen fibres and small amounts of connective tissue.
Elastic fibres are ubiquitous in the lung, for the need to expand and stretch with every breath that is inhaled and recoil back to its original resting state with air is exhaled. Elastic fibres are mainly found in the terminal branches of the respiratory tree, their ubiquity is increased, and the structure of the fibres take on a circular or helical arrangement as you move down the respiratory tree. In order for maximum contact to occur between the terminal branches of the respiratory tree, the alveoli, and the oxygen breathed in, a cyclic (expansion and shortening) exchange surface area is provided for by elastic fibres. Research has suggested that elastin and collagen fibres from alveoli, bronchi, interlobular septae, and the pleura all appear to have associations with the fibres of the pulmonary arteries.
As with the heart, elastic fibres in the lung are arranged in circular and longitudinal arrangements. . AS the fibres continue distally they take on a circular or helical arrangement . Elastic fibres in the lung can stretch up to 140% of their length before they break o the elastic fibres contained within the elastin become more and more .
Emphysema is a disease that affects all smokers. The elastic walls of the small airways are degraded and the alveoli of the lungs are unable to expand and resume back when breathing occurs . Elastic fibres, in a normal and healthy lung can stretch up to 140% of their length before they break . The percentage of which elastic fibres can stretch in smoker is significantly smaller and the elastin is damaged by continued and repeated inhalation over the years. Thus, often smokers will have a difficult time to take deep breaths, even during activities that not considered rigorous such as climbing stairs. Understanding the critical role for elastin in lung development and disease will facilitate the discovery of therapies aimed at promoting pulmonary regeneration through alveolation and lung growth in an effort to improve the outcomes of patients afflicted with .
The importance of elastic fibres in skin is noticeably evident in the older age of men and woman. Thereby through the aging process elastic fires become stiff and less resilient to stress. Recent research has focused on the generation of collagen elastin in three dimensional imaging of the human dermis to further understand the micro-structures of the skin, which is believed to be helpful in the fabrication of bionic engineered skin. . The elasticity of skin has also gained the interest of the cosmetic industry.
Skin is the largest organ of the human body and is made up of 3 highly distinguishable layers. From superficial to deep, the layers of the skin are; the epidermis, dermis, hypodermis.
The epidermis of the skin is the most outermost layer and therefore subject to the harsh conditions of the environment such as UV rays from the sun and wind. Within this layer there consists of five other layers however, elastic fibres are not found within the epidermal layer. The elasticity of skin is provided by the dermis which is separated from the epidermis by the basement membrane .
The main role of the dermis is to protect it from mechanical stress, tension, provide elasticity and thermoregulation.  The dermis layer of skin is divided into two layers; a papillary layer and a reticular layer. In both of these layers, elastic fibres can be found. Together, these make up approximately 1-2mm in thickness.
The dermis consists of loose areola (papillary layer) and dense connective tissue (reticular layer), which differs in the thickness along the length of the skin. This provides a cushion for mechanical stress such as bumping into objects. This layer also consists of elastic fibres that stretch when skin is stretched or pulled. However, as time progresses and with age, the resilience of which elastic fibres are able to stretch and recoil is lost in skin.
Wrinkling of skin
Elastic fibre degeneration due to aging and lack of collagen production are factors that give rise to the wrinkling of skin . Unlike many other components of the extracellular matrix, elastin has a slower rate of repair and regenerative capacity. Thus, the aging process of skin is usually advanced by continuous exposure to UV radiation from the sun <pubmed>PMC11978565</pubmed></ref>. Heavy smokers can add years to their skin by the extent of which smoking can damage the elastic fibres at such a high decline. With the advancement of technology, the advent of tissue engineering has ignited an interest in tissue engineering and elastic fiber assembly within synthetic matrices, as attempts are made to confer the correct physical properties inherent in blood vessels or skin <pubmed>PMC12784064</pubmed></ref>.
Comparison between human fetal and adult skin
Elastin is important in adults for restoring backing the normal tissue architecture example pinching of skin. According to this research, extracellular components such as elastin are important in the scarless healing process that takes places in on early fetal gestation. The role that elastin or another name, tropoelastin is investigated. Elastin is not found in fetal skin up till week 22. Although it is not a primary extracellular component for scarless healing in fetal wounds, it is still plays a role in skin regeneration. A comparison between fetal skin and adult skin is looked at. 
Ebola virus is an aggressive pathogen that causes a highly lethal hemorrhagic fever syndrome in humans with mortality rates varying between 50 to 90%. An important manifestation of Ebola virus infection is Disseminated Intravascular Coagulation, which is a syndrome characterized by systemic intravascular activation of coagulation and generalized deposition of fibrin in the circulation. As clinical manifestations, the Ebola virus disease can be composed by hypotension, generalized fluid distribution balance, lymphopenia, coagulopathy, hemorrhage and high fever.
Based on previous studies, it was suggested that the Ebola virus glycoprotein is the main determinant of vascular cell injury and thus it was proposed that the direct Ebola virus replication-induced structural damage of endothelial cells triggers the hemorrhagic diathesis. This study suggested, by using the ISM technique (information spectrum method), that the EMILINs (Elastin Microfibril Interface Located Proteins), which are expressed mostly in the vasculature extracellular matrix and distributed throughout the blood vessel walls, play a relevant role in interaction between Ebola virus glycoprotein and the endothelial extracellular matrix, contributing to the infection and the pathogenesis of Ebola virus by damage of the extracellular matrix and vascular homeostasis. While EMILINs are well known to play a role in coagulation, as a component of the vessel wall and a component of the thrombus, these proteins could also be involved in Disseminated Intravascular Coagulation by interaction with Ebola virus glycoprotein. 
The lung is a very complex and sophisticated matrix structure on which lung epithelium and endothelium reside. The typical destructive lung disease is pulmonary emphysema, a destructive process encountered mostly in smokers and a disease component of chronic obstructive pulmonary disease (COPD). COPD is characterized by persistent airflow limitation that is usually progressive and related to an enhanced chronic inflammation response in the airways and the lungs to noxious particles or gases. Inhaled cigarette smoke and other noxious particles such as smoke from biomass fuels cause lung inflammation, a normal response that appears to be modified in patients who develop COPD. This chronic inflammatory response may induce parenchymal tissue destruction, resulting in emphysema, and disrupt normal repair and defense mechanisms, resulting in small airway fibrosis. These pathological changes lead to air trapping and progressive airflow limitation, causing breathlessness and other characteristic symptoms of COPD. 
The study aimed to understand the roles matrix metalloproteinases play in the development of pulmonary emphysema. Matrix metalloproteinases contribute substantially to lung matrix degradation during the evolution of cigarette smoke induced emphysema by destruction of both elastin and collagen matrix proteins. Elastin and collagen are very important proteins for the maintenance of the lung architecture and structure, and therefore degradation of both elastin and collagen is required for the development of pulmonary emphysema, though evidence remains strongest for elastin, since adults are effectively incapable of elastic fiber repair. 
The clinical manifestations of arterial and venous thrombosis represent the leading causes of death in the developed world. While arterial and venous thrombosis have important pathobiological differences, both are complex and influenced by multiple genetic and environmental factors. Acute thrombosis at the site of a plaque is thought to be a precipitating event in the transition from a stable or subclinical atherosclerotic disease to acute myocardial infarction, ischemic stroke or peripheral arterial occlusion, all important conditions that might lead the patient to death. For individuals undergoing surgery, pulmonary thromboembolism and venous thrombosis are common. 
The EMILIN proteins are a family of extracellular matrix glycoproteins that play important role not only in elastogenesis and vascular architecture, but also in hemostasis and thrombosis. EMILIN2, Elastin Microfibril Interface Protein2, was identified as a gene expressed during cardiovascular development, on cardiac stem cells, and in heart tissue in animal models of heart disease. In humans, EMILIN2 gene is located on the short arm of chromosome 18, and patients with partial and complete deletion of this chromosome region have cardiac malformations. The results presented by the study indicated that EMILIN2 is necessary for platelet aggregation, clot retraction and thrombus formation, showing that it has multiple influences in pathophysiology of thrombosis and suggested that its role as a prothrombotic risk factor may arise from its effects on platelet aggregation. 
Vitamin A deficiency and alterations in the extracellular matrix
Vitamin A or retinol can be considered the most multifunctional vitamin in mammals. It has a lot of biological roles and it is known to modulate the synthesis of extracellular matrix proteins, including elastin. For this reason, the structure and composition of this extracellular compartment is deeply altered as a result of vitamin A deficiency. Its deficiency, along with protein malnutrition, is currently the most common and serious nutritional disorder worldwide. Biologically, vitamin A is important for normal embryonic development and organogenesis, and exert major effects on postnatal tissue homeostasis, vision, reproduction, immune function, growth, cellular differentiation, proliferation and apoptosis. In agreement with the multiple functions of retinoids, deficiency of vitamin A leads to a spectrum of clinical manifestations, known as vitamin A deficiency disorders. These include mild to severe xerophtalmia, squamous metaplasia of transitional and glandular epithelia, growth disturbances, anemia, susceptibility to infections and increased mortality.
The study aimed to show that altered extracellular matrix will potentially compromise organ function and lead to diseases, like liver, pulmonary and renal fibrosis, which are common pathological states associated with alterations in the extracellular matrix, giving to vitamin A a very important role in the maintenance of an adequate extracellular matrix and thus organ function. 
Elastic fibres are very large, complex structures that are still quite a poorly understood component of the extracellular matrix. Challenges for the future include describing how cells are able to regulate elastic fibre assembly by establishing the hierarchy of molecular interactions and the molecular composition. Research continues to better describe the biomechanical properties of microfibrils and microfibrillar-associated proteins.  At the organism or phenotypic level, there is very little understanding about how elastic fibres influence the behaviours of cells. A priority of future research includes aiming to identify cellular receptors, signalling mechanisms and growth factors. With continued progress in the area, we will be able to grasp a greater understanding of elastic fibres and develop methods for elastic fibre regeneration or repair to manage the effects of ageing and diseases. 
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