CONNECTIVE TISSUE

Functions of Connective TissueStructural support (capsules, bone, cartilage)NutritionDefense (non-specific and immune)Cell growth and differentiationCell migrationInsulation

Connective Tissues: Special CharacteristicsCommon embryological origin (from mesoderm)

Innervated and Vascular (direct blood supply)Cartilage is the one exception with no capillary beds

Extracellular Matrixground substance (gelatinous glycoproteins)structural fibers (fibrous proteins, e.g., collagen, elastin, reticulin)

Types of Connective TissuesConnective Tissue Properareolar (loose fibrous) connective tissueadipose tissuereticular connective tissuedense (fibrous) regular connective tissuedense (fibrous) irregular connective tissueCartilagehyaline cartilageelastic cartilagefibrocartilageBoneBlood

Types of Connective TissueClassified by the characteristics of the matrixgelatinousmineralizedliquid

Connective Tissue ElementsGround substance supports cells, binds them togethermay be solid, fluid or gelComponets:Interstitial Fluid Cell Adhesion Proteins-Fibronectin, laminin and othersServe as connective tissue glue

Connective Tissue ElementsProteoglycans - large polysaccharide molecules bound to a protein core (like a bottle brush)Glycosaminoglycans (GAGs) are attached to proteoglycansThey trap water. As GAGs increase, so does viscosityhyaluronic acid gelatinous, separates cells, traps extracellular fluid; lubricates joints; gives shape to eyeballs; fills body spaceschondroitin sulfate capable of being mineralized; cartilage, bones, skin, blood vesselsdermatin sulfate harder; skin, tendons, blood vessels, heart valveskeratin sulfate - still harder; bone, cartilage, cornea of the eyes

Connective Tissue ElementsFibersProteins that are embedded in the ground substanceProvide structural support, adhesion, connect cellsCollagentough; provides high tensile strengthAlso called white fibershighly polymerized, gigantic moleculestough, moderate flexibilitybone, cartilage, tendons, ligamentselastic fibers = elastinbranched; smaller, thinner fibers than collagenvery flexible and elastic but also strongcan be stretched to 150% of its original lengthAlso called yellow fibers

Connective Tissue Elementsreticular fibersbranched collagenous fibers that form delicate networks thin, less polymerized collagen fibers

elastic & reticular fibers require special stains to be seen in the light microscope

Cells fewer, rarely touching, surrounded by a matriximmature forms (-blasts) secrete the matrix and can still divideonce the matrix is secreted, the cells mature into -cytes which have decreased cell divisions and secrete less matrix materialchondro- cartilage, osteo- bone, fibro connective, etc. Fibroblasts connective tissue properChondroblasts cartilage Osteoblasts boneHematopoietic stem cells bloodWhite blood cells, plasma cells, macrophages, and mast cellsConnective Tissue Elements

Connective Tissue Structure

Connective tissueFibers

FibersLong, rope-like protein extracellular polymersPresent in variable proportions in the different types of connective tissuesThree types: collagen, reticular and elastic fibers. Collagen and reticular fibers are composed of various types of collagen, elastic fibers are composed mainly of elastin

Collagen fibersCollagens are the most abundant proteins in the body. There are many types of collagen that differ in their origin, chemical composition, functions, distribution and pathology

Collagen biosynthesis

Collagen biosynthesis

Collagen biosynthesis

Collagen fibrils1,5 nm TRIPLE HELIX2 x alpha1 + 1x alpha2

Collagen fibrils

Collagen fibrils, TEM

Collagen typesFibril-forming collagen: types I, II, III, V and XIFibril-associated collagen: types IX and XIINetwork-forming collagen: type IVAnchoring collagen: type VII

Collagen typesCollagen I2 x a1 + a2 or 3 x a1Forms fibrils, the most resistant to mechanical tensionIn: skin, bone, tendons, connective tissue capsulesCollagen II3 x a(II)1Forms fibrilsIn hyaline and elastic cartilageCollagen IV3 x a(IV)1 or 3 x a(IV)2Forms a network in the basal laminae

Elastic fibersIsolated, thin fibers or arranged in networksLocalised in lung, urinary bladder, skin, aorta and elastic cartilageSpecial staining : orcein

Elastic fibersElastin molecules are joined by covalent bonds to generate an extensive cross-linked network. Because each elastin molecule in the network can expand and contract like a random coil, the entire network can stretch and recoil like a rubber band. )

Elastic fibers

RETICULAR FIBERS

Thin fibers, forming networksDistribution : liver, spleen, lymph nodes, haematopoietic organsSpecial staining : silver impregnation

Connective tissueElements:

Ground substanceFibersCellsExtracellular matrix

Ground substanceComposed of glycoproteins and proteoglycansParticipates to binding cells to fibersColorless and transparent in usual stainsViscous

Ground substanceFibronectin (homodimer) binds cells, collagen and GAG Laminin (heterotrimer) mediates attachment of epithelial cells to basal laminaeCells have membrane receptors, integrins, that bind collagen, fibrinectin, laminin and other extracellular structural componentsIntegrins are also attached to the cytoskeleton (actin fibers)

Ground substanceProteoglycans (PG) = proteic core + glycosaminoglycans (GAG)GAG are linear polysaccharides composed of repetitive disaccharide unitsDisaccharide units = uronic acid + hexosamineGlucuronic ac.Iduronic ac.GlucosamineGalactosamine

Ground Substance: Proteoglycan Structure

Ground substanceExcept hyaluronic acid, GAG are part of PGPG are intensly hydrophilic polyanionsThey bind cations (Na+) thus attracting water they regulate consistency of connective tissueGAG examples: dermatan sulfate, chondroitin sulfate, keratan sulfate, heparan sulfate

Ground substanceStructural glycoproteins (GP) are glycosilated proteins branched oligosaccharide moietiesGP mediate adhesion of cells to extracellular matrix components

Ground substanceFibronectin (homodimer) binds cells, collagen and GAG Laminin (heterotrimer) mediates attachment of epithelial cells to basal laminaeCells have membrane receptors, integrins, that bind collagen, fibrinectin, laminin and other extracellular structural componentsIntegrins are also attached to the cytoskeleton (actin fibers)

The molecular structure of proteoglycans and glycoproteins.

A: Proteoglycans contain a core of protein (vertical rod in drawing) to which molecules of glycosaminoglycans (GAGs) are covalently bound. A GAG is an unbranched polysaccharide made up of repeating disaccharides; one component is an amino sugar, and the other is uronic acid. Proteoglycans contain a greater amount of carbohydrate than do glycoproteins. B: Glycoproteins are globular protein molecules to which branched chains of monosaccharides are covalently attached.

Schematic diagram of cell-surface synedcan proteoglycan. The core protein spans the plasma membrane through the cytoplasmic domain. The syndecan proteoglycans possess 3 heparan sulfate chains and sometimes chondroitin sulfate.

A: The structure of fibronectin. Fibronectin is a dimer bound by SS groups, formed by serially disposed coiled sites, that bind to type I collagen, heparan sulfate, other proteoglycans, and cell membrane receptors. B: The structure of laminin, which is formed by 3 intertwined polypeptides in the shape of a cross. The figure shows sites on the molecule with a high affinity for cell membrane receptors and type IV collagen and heparan sulfate, which are components of basal laminae. Laminin thus promotes adhesion of cells to basal laminae.

Integrin cell-surface matrix receptor. By binding to a matrix protein and to the actin cytoskeleton (via alpha-actinin) inside the cell, the integrin serves as a transmembrane link. The molecule is a heterodimer, with alpha and beta chains. The head portion may protrude some 20 nm from the surface of the cell membrane into the extracellular matrix.

Connective TissueCells

Connective tissue cells classificationProper to CT (fixed cells)- fibroblast - fibrocyte (condro-, osteo-)- adipocyte (uni-, multilocular)- reticular cellsMigrated (mobile cells)- granulocytes- B and T lymphocytes- macrophages- mastocyte- melanocyte

Connective tissue cells classificationCells that produce/degrade the extracellular matrixfibroblasts, osteoblasts, condroblasts, macrophages

Metabolic cellsadipocytes

Defense (specific/non-specific)Lymphocytes, macrophages, neutrophils

FibroblastThe most frequent cellFibers (collagen, reticulin & elastic) and ECM components synthesisElongated cells, 20 mm, branched processes, basophilic cytoplasm, oval , euchromatic nucleus, 1 or 2 nucleoli

Fibroblasts, fibrocytes

FibroblastProduces:Elements of the extracellular matrix: procollagen, proelastin, fibrillin, GAG, PG and GP;Enzymes: matrix metalloproteinases - collagenase (degrades collagen at neutral pH), elastase;Growth factors

FibroblastProperties:Ability to switch its fenotype fibroblast fibrocyteCan change shapeMobileInduces differentiation of surrounding cells

Fibrocyte

Less active than fibroblasts: smaller, lesser cytoplasm, a few short unbranched processesEosinophilic cytoplasmElongated and heterochromatic nucleus

Unilocular (white) adipocyte

Round (when isolated) or polygonal in groupsOne large lipid droplet (inclusion)A thin rim of cytoplasm at the periphery that contains a flattened, heterochromatic nucleus (signet ring)

Multilocular (brown) adipocyte

Smaller cellsMany smaller lipid droplets in the cytoplasm foamy lookRound, central nucleusMostly found before birth and in neonatesRole in thermogenesis

Reticular cells Variable functions but similar morpholgy; some contribute to forming the stroma of lymphoid and hematopoietic organsStar-shaped cells with long and thin processes that establish anchoring junctions with neighboring cells; round, central, pale nucleus, larger than nuclei surrounding it

CytoreticuleCytoreticule =

Reticular cells + Reticular fibers

MelanocytesCell of ectodermal originConsequently migrates to dermis, epidermis, iris, hair rootSnowflake-shaped cell, with many branched processes; 30 mmMelanin granules in the cytoplasm, dark-brown;Round, central, small nucleus

MelanocytesMelanosomes visible in EM:Primary melanosomes are Golgi vesicles that accumulate thyrosin (the melanin precursor) and thyrosinase, located at the base of cell processes Secondary melanosomes are heterogenous vesicles (EM) that accumulate melaninTretiary melanosomes are found at the tips of the cell processes; they are released from melanocytes and engulfed by surrounding cells (keratinocytes)

MacrophagesDerived from peripheral blood monocytes, Involved in phagocytosis and inflamatory responseA family of cells with various shapes, localisations and names:Histiocytes: connective tissueKupfer cells: liverAlveolary macrophages: lungOsteoclasts: boneMicroglia: central nervous system

MacrophagesGrouped as the mononuclear phagocytic systemMacrophages of the connective tissue: about 30 mm, ruffled membrane, acidophilic lysosomes in the cytoplasm, can have various heterogenous inclusions ingested materialRound, oval or kidney-shaped nucleus, excentrical, can have nucleoli

MacrophageMain function: phagocytosisTriggered by a specific interaction between membrane receptors and ligands. Consequences:Cell movement towards target particlePseudopodae formation engulfmentRespiratory burstSecretion: cytokines, interferons, complement & coagulation factorsProduction of matrix metalloproteinases

Macrophage, TEM

Mast cellsLocalized in most of the loose connective tissue areas, along blood vesselsOval cell, 20-30 mmCytoplasm has numerous basophilic, metachromatic granules, 0,1-1 mm. Pseudopodae in EM.Round, small and central nucleus

Mast cellsGranules contain heparin or chondroitin sulfate, histamin, Eosinophil Chemotactic Factor, etc.The content can be released out of the cell - degranulation. The process is triggered by chemical, physical stimuli, or through binding of antigen-IgE complexes by specialized receeptorsDegranulation is mediated by cAMP and leads also to leukotriene synthesis

Mast cell degranulation

Mast cells, Toluidin blue stain

Mast cell, TEM

Plasma cellsFound in lymphoid organs (lymph nodes, spleen, bine marrow) and connective tissues associated to the respiratory and digestive mucosaeOriginate in B lymphocytes, that are terminaly differentiated as a response to antigen challengeSecrete immunoglobulins (antibodies): IgM, IgG, IgA, IgE

Plasma cellsOval cell, 20 mmBasophilic cytoplasm (due to abundant RER), with a perinuclear pale area (Golgi apparatus); can contain acidophilic Russel bodies (secretory granules)Excentric nucleus, with hetero- and euchromatin in a characteristic pattern: spokes and barrel, clockface. Visible nucleolus

Plasma cell

Plasma cell, TEM

Plasma cells, TB stain

Lymphocytes

T and B subpopulationsCentral role in the immune response, migrated from the blood stream Small round cells (10 mm), with a round and dark-staining nucleus and a few basophilic cytoplasm

Neutrophils

Migrated from the blood stream, role in phagocytosis (microphage)Eosinophilic cytoplasm with small granulesCharacteristic nucleus: heterochromatic, 2-5 lobes