Cells are organized into groups or aggregates, known as tissues, that perform one or more specific functions. Tissues, in turn, create a stable environment in which the cells live and function. Unlike epithelial, muscle, and nerve tissues which are composed principally of cells, connective tissue contains a matrix of macromolecules in the extracellular space. Known as the extracellular matrix (ECM), these macromolecules are synthesized and then secreted by cells that live within the tissue.
Adhesion is also important for maintaining a stable tissue environment and allows for communication between cells and the ECM. Therefore, structural components within tissues must include mediators of cell-to-cell and cell-to-ECM adhesion. Adhesive properties of cells also facilitate cellular growth and differentiation and allow cells to migrate. The cells and the ECM within a tissue influence each other by altering their adhesive connections, creating a complex feedback mechanism.
A substantial part of tissue volume is an extracellular space largely filled by the intricate network of macromolecules of the ECM. Proteins and polysaccharides of the ECM are secreted by cells of the tissue and are then assembled into an organized meshwork. The ECM is specialized to perform different functions in different tissues. For example, the ECM adds strength to tendons and is involved in filtration in the kidney and attachment in skin.
Cellular and extracellular materials make up different proportions of different tissues (Figure 2.1). While epithelial tissue is principally cellular with a small amount of ECM, connective tissue is principally ECM with fewer cells. The ECM of epithelial tissue is known as the basement membrane or basal lamina and is secreted in the same direction from all epithelial cells in the layer. Therefore, the basement membrane appears beneath the epithelial cells that have secreted it and separates epithelial cells from connective tissue underlying it. The lamina propria is a thin layer of loose connective tissue found beneath the epithelial cells or epithelium. Together the epithelium and the lamina propria constitute the mucosa. “Mucous membrane” or “mucosa” always refers to the epithelium with the lamina propria. Submucosa refers to a layer of tissue beneath a mucous membrane.
FIGURE 2.1.Connective tissue underlying an epithelial cell sheet.
The physical nature of the ECM also varies from tissue to tissue. Blood is fluid, while cartilage has a spongy characteristic owing to the nature of extracellular materials in those tissues. Three categories of extracellular macromolecules make up the ECM: glycosaminoglycans (GAGs) and proteoglycans; fibrous proteins including collagen and elastin and adhesive proteins, including fibronectin and laminin. While two of these categories are proteins, proteoglycans are mostly carbohydrate in their structure.
Proteoglycans are aggregates of GAGs and proteins. GAGs are also known as mucopolysaccharides and are composed of repeating disaccharide chains where one of the sugars is an N-acetylated amino sugar, either N-acetylglucosamine or N-acetylgalactosamine (Figure 2.2), and the other is an acidic sugar. Most GAGs are sulfated. They are organized in long, unbranched chains. GAGs contain multiple negative charges and are extended in solution. The most prevalent GAG is chondroitin sulfate. Other GAGs include hyaluronic acid, keratin sulfate, dermatan sulfate, heparin, and heparan sulfate.
FIGURE 2.2.Repeating disaccharide unit of GAGs.
Characteristics of proteoglycans: Because of their net negative surface charges, GAGs repel each other. In solution, GAGs tend to slide past each other, producing the slippery consistency we associate with mucous secretions. Also, because of the negative charges, water floods into the matrix containing GAGs and creates swelling pressure (turgor). This pressure is balanced by the tension from collagen, a fibrous protein of the ECM, helping the ECM resist opposing forces of tissue compression. Cartilage matrix lining the knee joint has large quantities of GAGs and is also rich in collagen. This cartilage is tough, resilient, and resistant to compression.
The bones of the joint are cushioned by the water balloon–like structure of the hydrated GAGs in the cartilage. When compressive forces are exerted on it, the water is forced out and the GAGs occupy a smaller volume (Figure 2.3). When the force of compression is released, water floods back in, rehydrating the GAGs, much like a dried sponge rapidly soaking up water. This change in hydration in the ECM is referred to as resilience and is seen in cartilage as well as in synovial fluid and the vitreous humor of the eye (see also LIR Biochemistry, Chapter 14).
FIGURE 2.3.Resilience of GAGs.
Structure of proteoglycans: With the exception of hyaluronic acid, GAGs are found covalently attached to protein and form proteoglycan monomers. These monomers consist of a core protein with GAGs extending out from it and remaining separated from each other owing to charge repulsion. In cartilage proteoglycans, the GAGs include chondroitin sulfate and keratin sulfate. The resulting proteoglycan is often described as having a “bottle brush” or “fir tree” appearance (Figure 2.4). The individual GAG chains resemble needles on an evergreen tree and the core protein, a branch. The trunk of the tree is the hyaluronic acid, as individual proteoglycan monomers then associate with this large GAG, to form proteoglycan aggregates. The association occurs primarily through ionic interactions between the core protein and the hyaluronic acid and is stabilized by smaller, link proteins.
FIGURE 2.4.Model of cartilage proteoglycan.
Use of glucosamine as therapy for osteoarthritis
Osteoarthritis is the most common form of joint disease and the leading cause of pain and physical disability in elderly persons. Traditional management of the disease has been with drugs that treat the symptoms but do not improve joint health. Glucosamine and chondroitin have been reported both to relieve pain and to stop progression of osteoarthritis. These compounds are readily available as over-the-counter (OTC) dietary supplements in the United States. Based on several well-controlled clinical studies, it does appear that glucosamine sulfate (but not glucosamine hydrochloride) and chondroitin sulfate may have a small-to-moderate effect in relieving symptoms of osteoarthritis. Additionally, there is evidence that glucosamine sulfate and chondroitin sulfate can help to halt the progression of osteoarthritis.
Fibrous proteins are extended molecules that serve structural functions in tissues. They are composed of specific amino acids that combine into regular, secondary structural elements. In contrast to fibrous proteins, globular proteins have more compact structures resulting from secondary, tertiary, and, sometimes, quaternary protein structure (see LIR Biochemistry, Chapter 4). Collagen and elastin are fibrous proteins in the ECM that are important components of connective tissue as well as of skin and blood vessel walls.
- Collagen: The most abundant protein in the human body, collagen forms tough protein fibers that are resistant to shearing forces. Collagen is the main type of protein in bone, tendon, and skin. Bundled collagen in tendons imparts strength. In bone, collagen fibers are oriented at an angle to other collagen fibers to provide resistance to mechanical shear stress applied from any direction. In the ECM, collagen is dispersed as a gel-like substance and provides support and strength. Collagen is a family of proteins, with 28 distinct types. However, over 90% of collagen in the human body is in collagen types I, II, III, and IV. Together the collagens constitute 25% of total body protein mass. Types I, II, and III are fibrillar collagens whose linear polymers of fibrils reflect the packing together of individual collagen molecules. Type IV (and also type VII) is a network-forming collagen that becomes a three-dimensional mesh rather than distinct fibrils.
a. Structure of collagen: Collagen molecules are composed of three helical polypeptide ? chains of amino acids that wind around one another forming a collagen triple helix (Figure 2.5). The various types of collagen have different ? chains, occurring in distinct combinations (Table 2.1). For example, collagen type I has two type I ?1 chains and one type I ?2chain, while collagen type II has three type II ?1 chains. In the primary amino acid sequence of ? chains, every third amino acid is glycine, whose side chain is simply a hydrogen atom. Collagen is also rich in the amino acid proline. The amino acid sequence of most of the ? chains can be represented as repeating units of -X-Gly-Y- where X is typically a modified form of either proline or lysine (hydroxyproline or hydroxylysine), Gly refers to glycine, and Y is proline. In the collagen triple helix, the small hydrogen side chains of glycine residues (amino acids within proteins) are placed toward the interior of the helix in a space too small for any other amino acid side chain. Three ? chains in this conformation can pack together tightly. Proline also facilitates the formation of the helical conformation of each ? chain because it has a ring structure that causes “kinks” in the peptide chain.
FIGURE 2.5. Triple helical structure of collagen.
TABLE 2.1.Chain Compositions of Collagen Types
|I||[?1(I)]2[?2(I)]||Most abundant collagen
Found in bones, skin, tendons
Present in scar tissue
|II||[?1(II)]3||Found in hyaline cartilage
Present on the ventral ends of the ribs, in the larynx, trachea, and bronchi, and on the articular surface of bone
|III||[?1(III)]3||Collagen of granulation tissue in healing wounds
Produced before tougher type I is made
Forms reticular fibers
Found in artery walls, intestine, uterus
|IV||[?1(IV)2[?2(IV)]||Found in basal lamina and eye lens
Part of the filtration system in glomeruli of nephron in kidneys
b. Synthesis of collagen: Collagen manufacture occurs as individual fibrillar collagen polypeptide chains are translated on membrane-bound ribosomes (Figure 2.6). An unusual modification is then made to certain amino acid residues. In a reaction dependent on vitamin C, selected proline and lysine amino acid residues are hydroxylated. Some of the hydroxylysine residues are then glycosylated (carbohydrate is added to them). In the Golgi complex, three pro-? chains assemble into a helix by a zipper-like folding. A secretory vesicle then buds off from the Golgi complex, joins with the plasma membrane, and releases the newly synthesized collagen triple helices into the extracellular space. Propeptides, small portions of each end (C-terminal and N-terminal) of the newly synthesized chains, are cleaved by proteases (such as procollagen peptidase) to yield tropocollagen. Self-assembly and cross-linking of tropocollagen molecules then occur to form mature collagen fibrils. Packaging of collagen molecules within fibrils leads to a characteristic repeating structure with a banding pattern that can be observed with electron microscopy. The fibrillar array of collagen is acted on by the enzyme, lysyl oxidase, which modifies lysine and hydroxylysine amino acid residues, forming allysine residues and allowing them to form the covalent cross-links seen in mature collagen fibers. This cross-linking is essential to achieve the tensile strength necessary for proper functioning of connective tissue.
FIGURE 2.6. Synthesis of collagen.
Collagen and aging
Because collagen plays a key role in the support structure of the skin, if collagen production slows or its structure is changed, the appearance of the skin will also change. As skin ages, collagen production slows down. Additionally, over time, collagen fibers become rigid. Theoretically, this damage is caused by free radicals that attach to collagen, causing collagen strands to bind together and become thicker and more unyielding to movement. This process is the basis for wrinkle formation and the loss of firmness in mature skin. Collagen injections are sometimes given to restore volume and reduce the appearance of wrinkles. Antiaging products commonly contain antioxidants that may inhibit free radicals and slow the damage to collagen.
Some other OTC topical products contain plant or animal collagen, in the hope that it will be absorbed by the skin and replenish the natural collagen that has been lost due to aging. More promising topical products may contain retinoids that inhibit synthesis of collagenases, the enzymes that break down collagen. Retinoids, available as prescription creams, can also stimulate the production of new collagen fibers in the skin. While hundreds of OTC antiaging products are available, they contain either much lower concentrations or different active ingredients than creams available by prescription. The efficacy of OTC antiaging products is not proven.
2. Elastin: The other major fibrous protein in the ECM is elastin. Elastic fibers formed by elastin enable skin, arteries, and lungs to stretch and recoil without tearing. Elastin is rich in the amino acids glycine, alanine, proline, and lysine. Similar to collagen, elastin contains hydroxyproline, although only a small amount. No carbohydrate is found within the structure of elastin and therefore it is not a glycoprotein.
a. Synthesis of elastin: Cells secrete the elastin precursor, tropoelastin, into the extracellular space. Tropoelastin then interacts with glycoprotein microfibrils including fibrillin, which serve as a scaffolding onto which tropoelastin is deposited. Side chains of some of the lysine amino acid residues within tropoelastin polypeptides are modified to form allysine residues. In the next step, the side chains of three allysine residues and the side chain of one unaltered lysine residue from the same or neighboring tropoelastin polypeptide are joined covalently to form a desmosine cross-link (Figure 2.7). Thus, four individual polypeptide chains are covalently linked together.
FIGURE 2.7.Desmosine cross-link in elastin.
b. Characteristics of elastin: The structure of elastin is that of an interconnected rubbery network that can impart stretchiness to the tissue that contains it. This structure resembles a collection of rubber bands that have been knotted together, with the knots being the desmosine cross-links. Elastin monomers appear to lack an orderly secondary protein structure because elastin can adopt different conformations both when relaxed and when stretched (Figure 2.8).
FIGURE 2.8.Elastin in relaxed and stretched conformations.
Fibrous proteins and disease
Because collagen and elastin play important structural roles in tissues, impaired production of these fibrous proteins can result in disease states. Both acquired and inherited defects can result in abnormal fibrous proteins that change the physical properties of the tissue, sometimes with serious consequences. In other situations, the fibrous proteins are synthesized properly but are then degraded inappropriately, also impacting the normal functional characteristics of the tissue. The disease states described below serve to highlight the importance of normal fibrous proteins to the health of tissues and of the individual.
1. Scurvy: Dietary deficiency in vitamin C causes scurvy, a disorder that results from aberrant collagen production. In the absence of vitamin C, hydroxylation of proline and lysine amino acid residues cannot occur, resulting in defective pro-? chains that cannot form a stable triple helix. These abnormal collagen pro-? chains are degraded within the lysosomes of the cell. As a consequence, there is less normal, functional collagen available to provide strength and stability to tissues. Blood vessels become fragile, bruising occurs, wound-healing is slowed, and gingival hemorrhage and tooth loss occur (Figure 2.9A).
FIGURE 2.9.Signs of conditions with abnormal fibrous proteins.
A. Bleeding gums of a patient with scurvy.B.Fractured bone of a patient with osteogenesis imperfecta.C. Stretchy skin of an individual with Ehlers-Danlos syndrome.
Scurvy: Past and present
In centuries past, scurvy was a ravaging illness among seamen. They began to carry limes onboard ships to provide a source of vitamin C during long voyages. While much less common in the 21st century, scurvy does still occur today, even in industrialized societies. It is seen predominantly among elderly, indigent persons who live alone and prepare their own food; persons with poor dentition; persons with alcoholism; and those who follow fad diets. Other individuals may avoid fruits and vegetables, dietary sources of vitamin C, because of perceived food allergies or intolerances. Scurvy is less common in the pediatric population. Signs of scurvy in children may mimic child abuse, while scurvy may actually occur as a result of parental neglect in not providing proper foods to the children. Because vitamin C deficiency today is typically accompanied by deficiencies in other essential nutrients, diagnosis may be delayed.
2. Osteogenesis imperfecta: In contrast to the acquired collagen deficiency of scurvy, inherited collagen defects may impact individuals throughout their life span. A family of inherited collagen disorders, osteogenesis imperfecta has been called “brittle bone disease,” because many affected individuals have weak bones that fracture easily (Figure 2.9B). The disorder is caused by any one of several inherited mutations in a collagen gene, resulting in weak bones.
A mutation may result in decreased production of collagen or in abnormal collagen. Eight forms of osteogenesis imperfecta are currently known. Some forms have more severe signs and symptoms than others (Table 2.2). The most common is osteogenesis imperfecta type I, in which most affected persons have mild signs and symptoms including bones that fracture easily, especially prior to puberty. They may also have spinal curvature and hearing loss. By contrast, type II is lethal prior to or shortly after birth. Most types have autosomal dominant inheritance patterns and affected persons inherit a mutant gene from one parent who is also affected.
TABLE 2.2.Osteogenesis Imperfecta Types and Characteristics
|I||Autosomal dominant||Normal structure
|Bones fracture easily, most before puberty
Loose joints, weak muscles
Bone deformity absent
Brittle teeth possible
Hearing loss possible in 20s and 30s
|II||Autosomal dominant||Abnormal structure
Collagen I mutation
|Lethal at or shortly after birth
Severe bone deformity
|III||Autosomal dominant||Abnormal structure
Collagen I mutation
|Bones fracture easily
Fractures often present at birth
Loose joints and poor muscle development
Barrel-shaped rib cage
Hearing loss possible
|IV||Autosomal dominant||Abnormal structure
Collagen I mutation
|Bones fracture easily, most before puberty
Sclera normal in color
Mild-to-moderate bone deformity
Tendency toward spinal curvature
Brittle teeth possible
Hearing loss possible
|V||Autosomal dominant||Abnormal structure
Normal collagen I
|Clinically similar to type IV
Histologically, bone is “mesh-like”
Eight reported cases
Normal collagen I
|Clinically similar to type IV
Histologically, bone has “fish-scale” appearance
CRTAP gene mutation
|Resembles type IV or lethal type II|
LEPRE1 gene mutation
|Resembles lethal type II or type III
Severe growth deficiency
Extreme skeletal undermineralization
3. Ehlers-Danlos syndrome: A group of relatively uncommon disorders that result from inherited defects in the structure, production, or processing of fibrillar collagen is known as Ehlers-Danlos syndrome. There are six major types of Ehlers-Danlos syndrome, categorized based on signs and symptoms. Most are inherited as autosomal dominant traits. All six types include joint involvement. Most also affect skin. Some of the more prominent signs and symptoms include joints that extend beyond the normal range of movement and skin that is especially stretchy or fragile (Figure 2.9C).
4. Marfan syndrome: Another condition inherited as an autosomal dominant trait is Marfan syndrome. In this disorder, a mutation occurs in the gene that codes for the fibrillin-1 protein essential for maintenance of elastin fibers. Because elastin is found throughout the body and is particularly abundant in the aorta, the ligaments, and in portions of the eye, these sites are most affected in individuals with Marfan syndrome. Many affected individuals have ocular abnormalities and myopia (near-sightedness) and abnormalities in their aorta. They also have long limbs and long digits, tall stature, scoliosis (side-to-side or front-to-back spinal curvature) or kyphosis (curvature of the upper spine), abnormal joint mobility, and hyperextensibility of hands, feet, elbows, and knees.
5. ?1-Antitrypsin deficiency Also related to elastin is ?1-antitrypsin deficiency. In this condition, inappropriate destruction of the lung elastin is catalyzed by the protease elastase. Affected individuals are predisposed to emphysema. In the lungs of all individuals, the alveoli are chronically exposed to low levels of neutrophil elastase, released from activated neutrophils. This destructive enzyme is, however, normally inhibited by ?1-antitrypsin, also called ?1-protease inhibitor and ?1-antiprotease, the most important physiological inhibitor of neutrophil elastase. Individuals deficient in ?1-antitrypsin have reduced ability to inhibit elastase in the lung. Because the lung tissue cannot regenerate, the destruction of the connective tissues of alveolar walls is not repaired and disease results (see also LIR Biochemistry, Chapter 4).