Connective tissueSeema Zargar

Most diverse and abundant tissue Main classes

◦ Connective tissue proper◦ Cartilage◦ Bone tissue◦ Blood

Characteristics◦ Mesenchyme as their common tissue of origin

(mesenchyme derived from mesoderm)◦ Varying degrees of vascularity◦ Nonliving extracellular matrix, consisting of

ground substance and fibers ◦ Cells are not as abundant nor as tightly packed

together as in epithelium

Connective tissue

Enclose organs as a capsule and separate organs into layers. Areolar

Connect tissues to one another. Tendons and ligaments.

Support and movement. Bones.

Storage. Fat

Insulation. Fat.

Transport. Blood.

Protection. Bone, cells of the immune system.

Functions of connective tissue

Ground substance – unstructured material that fills the space between cells

Fibers – collagen, elastic, or reticular

Cells – fibroblasts, chondroblasts, osteoblasts, hematopoietic stem cells, and others.

Collagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% of the total protein content.

Structural Elements of Connective Tissue

Connective tissue can be classified into three

categories:

proper,

embryonic,

specialized

Classification of Connective tissue

Subclassifications of C.T(proper)

Embryonic: Is further divided into mesenchyme and mucoid.

Specialized: Is further divided into bone, cartilage and blood.

Connective tissue proper: Is further divided into elastic tissue, reticular tissue, adipose tissue, areolar (loose tissue) and dense tissue.

Sub classifications of CT

Elastic fibres

Reticular connective tissue

Embryonic connective tissue

Specialized connective tissue

Collagen fibres

Collagen is rich in proline and glycine, both of which are important in the formation of the triple-stranded helix. Proline facilitates the formation of the helical conformation of each αchain because its ring structure causes “kinks”in the peptide chain. Glycine, the smallest amino acid, is found in every third position of the polypeptide chain. It fits into the restricted spaces where the three chains of the helix come together. The glycine residues are part of a repeating sequence, -Gly–X–Y–, where X is frequently proline and Y is often hydroxyproline(but can be hydroxylysine,).

Most of the α-chain can be regarded as a polytripeptide whose sequence can be represented as (–Gly–Pro–Hyp–)

Structure of collagen

Collagen biosynthesisEndoplasmic Reticulum

mRNA attached to ERprotein synthesized into ER lumencotranslational and post-translational modifications3 proto-a-chains form soluble procollagenmoved to golgi apparatus

Golgi Apparatuspacked into secretion vesiclesfuse with membrane

Outside Cellprocollagen processed by enzymes outside cellassemble into collagen fiberscollagen fibrils form lateral Interactions of triple helices

Collagen biosynthesis

Tropocollagen as the basic structural unit of Collagen Tropocollagen has a mass of about 285 kdal and

consist of three polypeptide chains. Tropocollagen -280 nm long -head & tail region

30% glycine, 30% proline& hydroxyproline re-aggregate -native collagen (64nm)

non Covalent hydrogen bonding between the three αchain is via hydroxyproline.

tropocollagenpolarized in fiber, 1/4 staggered array –period accounted for by gaps fall in dark bands

Staggered array of tropocollagen Molecules

Collagen fibres exhibit crosssstriations every 640A.

The length of the tropocollagenmolecule s 2800A.

There is a gap of 400oA between the end of one tropocollagenand the start of another. This gap play an important role in mineralization process.

Tropocollagen structure

Tropocollagen stabilization

Lysyl oxidase is an extracellular copper enzyme that catalyzes formation of aldehydes from lysine residues in collagen and elastin precursors. These aldehydes are highly reactive, and undergo spontaneous chemical reactions with other lysyl oxidase-derived aldehyde residues, or with unmodified lysine residues. This results in cross-linking collagen and elastin, which is essential for stabilization of collagen fibrils and for the integrity and elasticity of mature elastin.

Complex cross-links are formed in collagen called Pyridinoline which is derived from three lysine residues.

Stability of the collagen helix™ The temperature at which half of the helical structure is lost

is called the melting temperature™.

The Tm of tropocollagenis a criterion of the stability of its helical structure. Tm depends on the body temperature of the source species. collagens from icefishhas the lowest Tm while warm blooded animals have the highest Tm. This difference in thermal stability is correlated with the contents of iminoacid (prolineand hydroxyproline) in the collagen. The higher the iminoacid content , the more stable the helix. Tm of (pro-pro-Gly) is 24C while poly (Pro-Hyp-Gly) is 580C indicating hydroxylation stabilizes triple helix.

The experiments using αα\-bipyridylan iron chelatorwhich inhibit hydroxylation shows that without hydroxylation triple helix formation does not occur.

Collagenase and its types Source: Clostridium histolyticum

I.U.B.: 3.4.24.3 Crude collagenase preparations contain not only several

collagenases but also a sulfhydryl protease, clostripain, a trypsin-like enzyme, and an aminopeptidase. This combination of collagenolytic and proteolytic activities is effective at breaking down intercellular matrices, the essential part of tissue dissociation. One component of the complex is a hydrolytic enzyme which degrades the helical regions in native collagen preferentially at the Y-Gly bond in the sequence Pro-Y-Gly-Pro- where Y is most frequently a neutral amino acid. This cleavage yields products susceptible to further peptidase digestion. Crude collagenase is inhibited by metal chelating agents such as cysteine, EDTA or o-phenanthroline but not DFP. It is also inhibited by α2-macroglobulin, a large plasma glycoprotein. Ca2+ is required for enzyme activity. Particular enzymatic profiles of each collagenase have been correlated with the tissues from which the cells for study were obtained (or with the uses to which the cells are put) and as a result of the correlations several types of crude collagenases have been established by Worthington: Types 1, 2, 3, and 4.

 Type 1 crude collagenase has the original balance of collagenase, caseinase, clostripain and tryptic activities.

Type 2 contains higher relative levels of protease activity particularly clostripain.

Type 3 contains lowest levels of secondary proteases.

Type 4 is designed to be especially low in tryptic activity to limit damage to membrane proteins and receptors.

Elastin composition

Elastin Elastin is a protein in connective

tissue that is elastic and allows many tissues in the body to resume their shape after stretching or contracting. Elastin helps skin to return to its original position when it is poked or pinched. Elastin is also an important load-bearing tissue in the bodies of vertebrates and used in places where mechanical energy is required to be stored. In humans, elastin is encoded by the ELN gene.

Composition of elastin Elastic fiber is composed of the

protein fibrillin and elastin made of simple amino acids such as glycine, valine, alanine, and proline.

Elastin is made by linking many soluble tropoelastin protein molecules, in a reaction catalyzed by lysyl oxidase, to make a massive insoluble, durable cross-linked array. The amino acid responsible for these cross-links is lysine. Tropoelastin is a specialized protein with a molecular weight of 64 to 66 kDa, and an irregular or random coil conformation made up of 830 amino acids.

Desmosine and isodesmosine are types of links for the tropoelastin molecules.

Tissue distribution Elastin serves an important function in arteries as

a medium for pressure wave propagation to help blood flow and is particularly abundant in large elastic blood vessels such as the aorta.

Elastin is also very important in the lungs, elastic ligaments, the skin, and the bladder, elastic cartilage.

It is present in all vertebrates above the jawless

fish.

Biosynthesis of elastin

Structure protein Mr 64 to 66 kDa composed of the amino acids glycine, valine,

alanine, and proline cross-linked tropoelastin monomers first secreted as soluble precursors (tropoelastin) assembly and crosslinking of tropoelastin

monomers form insoluble elastin matrix into functional fibres

◦ lysine residues in the cross-linking domain of secreted tropoelastin rapidly cross-linked (both inter- and intra-molecularly by lysyl oxidase)

◦ hydrophobic segments - elastic properties◦ α-helical segments (alanine- and lysine-rich) - form

cross-links between adjacent molecules