Connective Tissue I PDF

Connective Tissue I Khalid Said Mohammad, MB.,BCh, MSc, Ph.D. Professor of Anatomy & Cell Biology Histology Course Director Department of Anatomy, Research Director Master Program, Molecular and Cell Biology Track Director [email protected] ...

Connective Tissue I Khalid Said Mohammad, MB.,BCh, MSc, Ph.D. Professor of Anatomy & Cell Biology Histology Course Director Department of Anatomy, Research Director Master Program, Molecular and Cell Biology Track Director [email protected]  Learning Objectives ❑ Describe the main structural criteria of CT ❑ Know the general classifications of CT categories ❑ Describe the different cells of the CT ❑ Know the types of the CT fibers ❑ Describe collagen fibers and the main step in collagen synthesis  Main Structural Criteria of Connective Tissue (CT) The major structural constituents of the body Seemingly diverse, structurally and functionally they possess many shared qualities; therefore, they are considered in a single category Mostly are derived from mesoderm, which form the multipotential mesenchyme from which bone, cartilage, tendons, ligaments, capsules, hematopoietic cells, and lymphoid cells develop Functionally, serve in support, defense, transport, storage, and repair, among others  Main Structural Criteria of CT In general, consists of cells and an extracellular matrix (ECM) Unlike epithelium, which consist mainly of cells, the major constituent of connective tissue is the extracellular matrix (ECM) Extracellular matrices consist of different combinations of protein fibers and ground substance  Classification of CT Classification of CT is based Mesenchymal on the Mucous composition and organization of its cellular and extracellular components and on its functions  Cells of the Connective tissue Resident Cells Wondering Cells o Fibroblasts o Plasma cells o Adipocytes o Lymphocytes o Mast cells o Eosinophils o Macrophages o Neutrophils o Adult stem cells o Basophils o Monocytes  Fibroblasts The most common cells in the CT proper Produce and maintain most of the tissue’s extracellular components They synthesize and secrete: o Fibers; collagen (the most abundant protein of the body) and elastin, which both form large fibers o Ground substance: glycosaminoglycans (GAGs), proteoglycans, and multiadhesive glycoproteins that comprise  ECM Synthesis by Fibroblasts A fibroblast synthesizes and secrets molecules of collagen, elastin, fibrillin, proteoglycans (PG) and others into an extracellular space The structural components of the extracellular matrix such as fibrous structures and proteoglycans are formed from these molecules  Fibroblasts Elongated, fusiform cells possessing pale- staining cytoplasm, which is often difficult to distinguish from collagen when stained with hematoxylin and eosin The most obvious portion of the cell is the darker-stained, large, granular, ovoid nucleus containing a well-defined nucleolus Cells with intense synthetic activity are morphologically different from the quiescent fibroblasts  Fibroblasts The active fibroblast has more abundant and irregularly branched cytoplasm, containing much (RER) and a well-developed Golgi apparatus, with a large, ovoid, euchromatic nucleus and a prominent nucleolus Active fibroblasts often reside in close association with collagen bundles, where they lie parallel to the long axis of the fiber The quiescent cell is smaller than the active fibroblast, more ovoid, with fewer processes, much less RER, and a darker, more heterochromatic nucleus  Adipocytes Large, spherical in shape and ranges in size from 25–250 μm in Diameter Mesenchymally derived cells Normally contain one large droplet of neutral lipid, occupying the whole central part of the cell and surrounded by a thin cytoplasmic limbus whose thickened part contains a nucleus  Adipocytes Specialized for cytoplasmic storage of lipids as neutral fats, or less commonly for the production of heat Involved in the synthesis of a variety of hormones, inflammatory mediators, and growth factors Tissue with a large population of adipocytes, called adipose tissue, serves to cushion and insulate the skin and other organs  Mast Cells Oval or irregularly shaped cells between 7 and 20 μm in diameter, filled with basophilic secretory granules that often obscure the central nucleus These granules are electron dense and of variable size, ranging from 0.3 to 2.0 μm in diameter The granules are poorly preserved by common fixatives, so mast cells may be difficult to identify in routinely prepared slides  Mast Cell Granules Store mediators of inflammation including: o Histamine, which promotes increased vascular permeability and smooth muscle contract o Heparin, acts locally as an anticoagulant o Serine proteases, which activate various mediators of inflammation o Eosinophil and neutrophil chemotactic factors, which attract those leukocytes Release of certain chemical mediators stored in mast cells promotes the allergic reactions known as immediate hypersensitivity reactions  Macrophages Specialized cell population that carries out the function of clearing an organism (phagocytic capability) from naturally degraded tissues and those destroyed under the influence of a destructive factor Originate from hematopoietic stem cells in bone marrow that circulate as monocytes and migrate across blood vessel walls to enter CT Present in the CT of most organs and sometimes referred to as “histiocytes” Especially abundant at sites of inflammation  Macrophages Size and shape vary considerably, corresponding to their state of functional activity. The cells can be flattened, circular, elongated or irregular. The most characteristic macrophage morphologic peculiarity is the presence of various cytoplasm projections through which these cells grasp foreign particles. The number of processes significantly increases in activated cells Measures 10-30 μm in diameter and has an eccentrically located, oval or kidney-shaped nucleus  Plasma Cells Lymphocyte-derived, antibody-producing cells The nucleus is spherical, eccentrically placed Nuclei contain compact, peripheral regions of heterochromatin alternating with lighter areas of euchromatin “clock face appearance” Cytoplasm rich in RER and a large Golgi apparatus near the nucleus that may appear pale in routine histologic preparation The cells leave their sites of origin in lymphoid tissues, move to CT, and produce antibodies that mediate immunity  Other White Blood Cells (Leukocytes) Other white blood cells, or leukocytes, besides macrophages and plasma cells normally comprise a population of wandering cells in the CT Derived from circulating blood cells, they leave blood by migrating between the endothelial cells of venules to enter connective tissue. This process increases greatly during inflammation Most leukocytes function in CT only for a few hours or days and then undergo apoptosis  Connective Tissue Fibers The fibrous components of connective tissue are elongated structures formed from proteins that polymerize after secretion from fibroblasts The three main types of fibers include: o Collagen fibers o Reticular fibers o Elastic fibers Collagen and reticular fibers are both formed by proteins of the collagen family, and elastic fibers are composed mainly of the protein elastin These fibers are distributed unequally among the different types of CT  Collagen Present only in extracellular matrices Found throughout the animal kingdom and are noted for their high tensile strength, that is, their resistance to pulling forces It is estimated that a collagen fiber 1 mm in diameter is capable of suspending a weight of 10 kg (22 lb) without breaking Is the single most abundant protein in the human body—constituting more than 25% of all protein  Collagen Fibers The most abundant type of CT fibers and are the most abundant structural components of the CT Each collagen type is restricted to particular locations within the body, but two or more different types are often present together in the same ECM The most common collagens are: o Type I (dermis, bone, capsules of organs, fibrocartilage, dentin, and cementum) o Type II (hyaline and elastic cartilages) o Type III (reticular fibers) o Type IV (Basal lamina) o Type V (placenta) o Type VII (anchoring fibrils of the basal lamina)  Collagen Fibers A major product of fibroblasts, but also secreted by several other cell types (smooth muscle, epithelium) A family of at least 35 collagens These 35 or so types of collagen are grouped into four different classes: 1. Fibril-forming 2. Network-forming 3. Fibril-associated 4. Transmembrane collagens (collagen like proteins)  Collagen Fibers In the light microscope, collagen fibers typically appear as wavy structures of variable width and indeterminate length They stain readily with eosin and other acidic dyes When examined with the TEM, collagen fibers appear as bundles of fine, threadlike subunits, these subunits are collagen fibrils Within an individual fiber, the collagen CC fibrils are relatively uniform in diameter CL  Collagen Fibril The threadlike collagen fibrils are aggregated in some areas to form relatively thick bundles; in other areas, the fibrils are more dispersed A closely spaced transverse bands that repeat every 68 nm along the length of the fibril  Categories of Collagen Fibers Fibrillar collagens Fibril associated ▪ Collagen type I, the Network or sheet- collagens most abundant and forming collagens ▪ Type XII: widely distributed Combines with ▪ forms large, ▪ Type IV: Afford type I eosinophilic bundles support and act ▪ In tendons etc., usually called collagen as a filter fibers ▪ In basement membrane Transmembrane ▪ Densely fill the CT, collagens forming structures such as tendons, organ ▪ Type XVIII: Enzymatic capsules, and dermis cleavage ▪ Reticular lamina  Stages of Collagen Synthesis ▪ 3, α chains form a triple helix to form procollagen ▪ Procollagen peptidase produces Tropocollagen ▪ Tropocollagens aggregate to form cross- striated collagen fibrils ▪ The fibrils assemble to form collagen fibers ▪ Fibers assemble to form collagen bundles  Summary ❑ The main structural criteria of CT and how to differentiate between CT and epithelium ❑ The general classifications of CT categories ❑ know the resident and wondering cells and the functional and structural details of the different cells of CT ❑ The types of the CT fibers ❑ Collagen fibers, it structure and its different categories and main step in collagen synthesis Thank you! 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