1.5. Epithelial Tissues

1.5. Epithelial tissues Connective & supportive tissues A diverse group of tissues that share a common origin from the mesenchyme of the embryo The provide structural & metabolic support for other tissues &organs throughout the body, bind the tissues to form organs, & the organs to form organ systems & organism. They contain blood vessels &mediate the exchange of nutrients, metabolites & waste products b/w tissues & the circulatory system. Thus Origin: from the mesoderm Blood supply: densely vascularized Nerve supply: densely innervated Tissue components of the connective & Supportive tissues: - Cells of great variety (fixed & free cell types, see later) - Extracellular matrix (ECM) - ground substance - fibers (collagen, elastic, reticular) Extracellular matrix (ECM) 1. The ground substance A highly hydrated gel in which the cells & fibers are embedded Molecular components: Proteoglycan consisting of dimeric glycosaminoglycans (GAGs) bound to a protein core Chondroitin sulphate (cartilage, arteries, skin, cornea) Keratane sulphate (cartilage, bone, cornea) Heparane sulphate (arterie, lung) Dermatan sulphate (skin, tendon, ligaments sclera, lung) Hyaluronic acid (vitreous humor of eye, synovial fluid, loose CT, skin, cartilage, umbilical cord) Hydrophilic side chains of GAGs bind a considerable amount of Extracellular fluid - water reser voir of the organisms It is part of the connective and supportive tissues not visible in histological preparates, unless special staining are applied: - Toluidine-blue - PAS staining - Alcaine- blue staining 2. Connective tissue fibers -> collagen fiber The most abundant protein in the body ← H.E stained micrograph Scanning electron → micrograph 2 isolated fibers artificially coloured in the computer (SEM) Cross-striation obs. in the transmission electron microscope -> Synthesis of collagen fibers 1. Takes place in the rER of fibroblasts: Pre-a-chains from triple helices 2. These procollagen molecules are transferred to the Golgi complex 3. Tropocollagen molecules are released via exocoytosis from Golgi-vesicles 4. Tropocollagen molecules are assembled to form collagen fibers in ECM. Properties of collagen fibers 1. Non-branching bundles of changing thickness 2. High tensile strength 3. Poor sheer strength 4. They can be stretched to approx. 5% of their initial length -> Synthesis of collagen fibers by fibroblasts Specific staining for the vizualization of collagen fibers: Azan staining stains the collagen fibers selectively in blue colour other tissue componenst are stained red Types of collagen fibers Type 1: 2 a1 chains + 1 a2 The most common (bone, skin, tendon, ligaments) Type 2: 3 a1 chains Prédominantes in cartilage Type 3: 3 a1 chains In embryonic CT & skin Type 4: still unknown composition in the basal laminate of adults Type 5: still unknown composition mainly in the embryo Elastic fibers They are present in organs whose normal function requires elasticity in addition to tensile strength Wall of the aorta stained with H.E & Resorcin fuchsin, specific staining for elastic fibers -> Synthesis of elastic fibers: - takes place in the rER of fibroblasts (& smooth muscle cells): tropoelastin molecules - transformer to elastin, crosslinked & assembled in the Extracellular space - microfibrils secreted prior to elastin provide scaffolding which elastin forms fibers & sheets -> Properties of elastic fibers - occur in the form of individual, branching & anastomosing fibers - they can be stretched 2.5 times of their original length to which they return when released -> Synthesis of elastic fibers by fibroblasts 3. Reticular fibers Individual collagen fibers coated by proteoglycans & glycoproteins. This coating causes their affinity for silver salts. They from delicate flexible net works around capillaries, muscle fibers, nerves, adipose cells & hepatocytes Serve as scaffolding in endocrine, lymphatic & blood cell-forming organs < Reticular fibers around hepatocytes & Around kidney tubules (as part of basement membrane > -> Synthesis of reticular fibres by fibroblasts Cell types of the connective tissues I. Fixed cells : Fibrocytes Fibroblasts Mesenchymal cells Adipose cells II. Free cells : Macrophages Mast cells Plasma cells Melanocytes Blood- derived cells (Lymphocytes, monocytes, eosinophils, neutrophils) I. Fixed cells: 1. Fibrocytes, fibroblasts: The most common fixed cell types. Fibroblast: metabolically more active (see before: synthesis of ECM components purple arrows) Fibrocyte: metabolically less active, more slender cell & nucleus, less cell organelles (green arrows) 2. Mesenchymal cells Pluripotent undifferentiated cells around blood vessels: Ser ve as a reser voir of cells that can differentiate into other connective tissue cell types. 3. Adipose cells: Adipose cells, single or in groups are normal components of the connective tissue (they may from a separate tissue: adipose tissue). Spherical or polyhedral cells w/ a large lipid droplets surrounded by thin cytoplasmic rim. II. Free cells: 1.Macrophages (histiocytes) In non-reactive CT are fixed, upon stimulation they become motile. Phagocytotic cell type, rich in hydrolytic enzyme. The mononuclear phagocyte system Connective tissue: Liver: Lung: Lymph nodes: Spleen: Bone marrow: Serous cavities: Bone: Central ner vous system: Skin: Synovia: Histiocytes Kupffer-cell Alveolar macrophage Free & fixed macrophages Free & fixed macrophages Fixed macrophages Pleural & peritoneal macrophages Osteoclasts Microglial cells Histiocytes Type A cells 2. Mast cells: around blood vessels in skin & intestine Large metachromatic granules: histamine, herparin, Serotonin 3. Plasma cells: In the loose CT of gastrointestinal tract, respiratory system, in the lymphatic tissue. They dev. From B-lymphocytes, produce antibodies 4. Melanocytes Large pigmented cells of neural crest-origin. They occur in the dermis of skin, meninges, choroid & iris 5. Other free cells temporarily occurring in the CT Lymphocytes Monocytes Granulocytes