Connective Tissue PDF

1 Connec&ve Tissue Most diverse and abundant tissue Main classes Connective tissue proper Cartilage Bone tissue Blood Components of connective tissue: Cells (varies according to tissue) Matrix  Fibers (varies according to tissue)  Ground substance (var...

1 Connec&ve Tissue Most diverse and abundant tissue Main classes Connective tissue proper Cartilage Bone tissue Blood Components of connective tissue: Cells (varies according to tissue) Matrix  Fibers (varies according to tissue)  Ground substance (varies according to tissue)  dermatin sulfate, hyaluronic acid, keratin sulfate, chondroitin sulfate… Common embryonic origin – mesenchyme 2 3 Classes of Connec+ve Tissue 4  Func%on Storage of energy Protection of organs Provision of structural framework for the body Connection of body tissues Connection of epithelial tissues to muscle ;ber. Supply of hormones all over the body Nutritional support to epithelium Site of defense reactions Repair of body tissues 5  Connec&ve Tissue Consists of cells separated by varying amounts of extracellular substance. In CT, cells typically account for only a small fraction of the tissue volume. 6 Connec&ve &ssue The extracellular substance (matrix) consists of Fibers Ground substance containing tissue 9uid 7 Connec&ve &ssue Fibers in connective tissue can be divided into three types: Collagen 7bers Reticular 7bers Elastic 7bers. 8 Collagen Fibers The dominant ,ber type in most CT. The primary function : to add strength to the CT. The thickness of the ,bers varies from ~ 1 to 10 µm. Longitudinal striations may be visible in thicker ,bers. These striations reveal that the ,bers are composed of thinner collagen ,brils (0.2 to 0.5 µm in diameter). Each of these ,brils is composed of micro,brils, which are only visible by using electron microscopy. Fibre  Fibrils  Micro,brils 9 10 Collagen )bers Micro&brils are assemblies of tropocollagen, which, in turn, is an spiral-like assembly of three collagen molecules (triple helix). The organization of the tropocollagen within the micro&brils is highly regular. A small gap is found between the subsequent tropocollagens which form the micro&brils. 11 Collagen )bers Coarse collagen ,bers are formed by type I tropocollagen. There are many di9erent tropocollagen types: I to XXI These types di9er in their Content of the amino acids. Amount of carbohydrates attached to the collagen molecules. 12 Collagen )bers The di'erent types of tropocollagen give the 5bers the structural and functional features which are appropriate for the organ in which the 5bers are found. Types I, II and III are the major 5ber-forming tropocollagens. Type IV is an important structural component of the basal lamina. A tensile force of several hundred kg/cm2 is necessary to tear human collagen 5bers. The 5bers stretch by only 15-20%. 13 Re#cular Fibers Very delicate and form 0ne networks instead of thick bundles. Usually not visible in histological sections but can be demonstrated by using special stains e.g. silver stain. 14 Re#cular *bres The main type of tropocollagen found in reticular 4bers, type III, is di9erent from that of the coarse collagen 4bers. Give support to individual cells, for example, in muscle and adipose tissue. 15 Elas%c Fibers Are light yellow - but this colouration is only visible if large amounts of elastic 7bers are present in the tissue, e.g. the ligaments of the vertebral column. Special stains are necessary to show elastic 7bers in tissue sections. 16 Elas%c Fibers LM does not reveal any substructure in the elastic 4bers. EM: elastic 4bers consist of individual micro4brils, which are embedded in an amorphous matrix. The matrix accounts for about 90% of the 4ber and is composed of the protein elastin. Neither the elastin nor the micro4brils are collagens. 17 Elas%c Fibers Elastic )bers can be stretched to about 150% of their original length. They resume their original length if the tensile forces applied to the elastic )bers are relaxed. Elastin molecules are cross-linked to each other by desmosin and isodesmosin links, which are only found between elastin molecules. Tensile forces straighten the cross-linked mesh of elastin coils 18 Ground Substance Is found in all cavities and clefts between the 3bers and cells of connective tissues. The structural constituents are Proteoglycans Water Salts Other low molecular substances 19 Proteoglycans Are responsible for the highly viscous character of the ground substance. Consist of Proteins (~5%) Polysaccharide chains (~95%) The polysaccharide chains belong to one of the @ve types of glycosaminoglycans, which form the bulk of the polysaccharides in the ground substance. 20 Hyaluronic Acid Is the dominant glycosaminoglycan in CT. Has high molecular weight. Serves as a "backbone" for the assembly of other glycosaminoglycans in connective and skeletal tissue, which results in even larger molecule complexes Is also a major component of the synovial Buid, which Clls joint cavities, and the vitreous body of the eye. 21 The remaining four major glycosaminoglycans are Chondroitin sulfate Dermatan sulfate Keratan sulfate Heparan sulfate. These glycosaminoglycans attach via proteins to a backbone formed by the hyaluronic acid. 22 The coiled arrangement of the hyaluronic acid and other attached glucosaminoglycans 4lls a roughly spherical space. This space is called a domain. Neighbouring domains overlap and form a more or less continuous three-dimensional molecular sieve in the interstitial spaces of the CTs. 23 Ground substance The large carbohydrates of the glycosaminoglycans bind large amounts of water and cations. The bound water in the domains forms a medium for the di7usion of substances of low molecular weight such as gases, ions and small molecules, which can take the shortest route, e.g. from capillaries to CT cells. Large molecules are excluded from the domains and have to ?nd their way through the spaces between domains. 24 Ground substance The restricted motility of larger molecules in the extracellular cellular (EC) space inhibits the spread of microorganisms through the EC space. A typical bacterium is essentially immobilized in the meshwork formed by the domains. The pathogenicity of a bacterium is to some extent determined by its ability to @nd its way through the mesh, and some of the more invasive types produce the enzyme hyaluronidase, which depolymerises hyaluronic acid. 25 Connec&ve Tissue Cells CT cells are usually divided into two groups based on their ability to move within the CT. Fixed cells:  Fibrocytes (or =broblasts)  Fat cells Wandering cells:  Macrophages, monocytes, lymphocytes, plasma cells, eosinophils and mast cells 26 Fibrocytes Fibrocytes are the most common cell type in CT. They are the "true" CT cells. Only their oval, sometimes :attened nuclei are visible in LM sections. The cytoplasm of a resting (i.e. inactive) Bbrocyte does not contain many organelles. 27 Fibrocytes When the 'brocytes are stimulated e.g. by damage to the surrounding tissue: They are transformed into !broblasts (active cells), which contain large amounts of the organelles which are necessary for the synthesis and excretion of proteins needed to repair the tissue damage. Fibrocytes do not usually leave the CT. They are, however, able to perform amoeboid movement. 28 Re#cular cells Are usually larger than an average /brocytes. They are the "/brocytes" of reticular connective tissue Form a network of reticular /bers, e.g in the lymphoid organs. Their nuclei are typically large and lightly stained (H&E) The cytoplasm may be visible amongst the cells which are housed within the network of reticular /bers. 29 Adipocytes (Fat Cells) Are %xed cells in loose connective tissue. Their main function is the storage of lipids. If "well fed" the cytoplasm only forms a very narrow rim around a large central lipid droplet. 30 Adipocytes (Fat Cells) The %attened nucleus may be found in a slightly thickened part of this cytoplasmic rim - if it is present in the section, which may not be the case since the diameter of an adipocyte (up to 100 µm) is considerable larger than the thickness of typical histological sections. A "starving" adipocyte may contain multiple small lipid droplets and gradually comes to resemble a Dbrocyte. 31 Adipocytes (Fat Cells) Lipid storage/mobilisation is under nervous (sympathetic) and hormonal (insulin) control. Adipocytes also have an endocrine function - they secrete the protein leptin which provides brain centers which regulate appetite with feedback about the bodies fat reserves. Leptin de?ciency in experimental animals results in obesity. Adipocytes are very long-lived cells. Their number is determined by the number of preadipocytes (or lipoblast) generated during foetal and early postnatal development. 32 Macrophages Macrophages arise from precursor cells called monocytes. Monocytes originate in the bone marrow from where they are released into the blood stream. Monocytes are actively mobile and leave the blood stream to enter connective tissues, where they di:erentiate into macrophages. 33 Macrophages Macrophages change their appearance depending on the demand for phagocytotic activity. Resting macrophages may be as numerous as 8brocytes. Resting macrophages are di9icult to distinguish from 8brocytes in H&E stained sections. 34