THE GROUND TISSUES
CONNECTIVE AND SUPPORTIVE TISSUES
HISTOLOGY 1.5.:CONNECTIVE AND SUPPORTIVE TISSUESA diverse group of tissues that share a common origin from the mesenchyme of the embryo.They provide structural and metabolic support for other tissues andorgans throughout the body, bind the tissues to form organs, and theorgans to form organ systems and organism. They contain blood vessels and mediate the exchange of nutrients, metabolites and waste products between tissues and the circulatory system.
Origin: from the mesodermBlood supply: densely vascularizedNerve supply: densely innervated
Tissue components of the connective and supportive tissues:
Cells of great variety (fixed and free cell types, see later) Extracellular matrix (ECM)- ground substance- fibers (collagen, elastic, reticular)
Extracellular matrix (ECM)I. THE GROUND SUBSTANCEA highly hydrated gel in which the cells and fibers are embeddedMolecular 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 (arteries, 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 reservoir of the organism.
It is a part of the connective and supportive tissues not visible in histological preparates, unless special staining methods are applied: Toluidine-blue PAS staining Alcian-blue staining
Extracellular matrix (ECM)II. CONNECTIVE TISSUE FIBERSCOLLAGEN fibersThe most abundant protein in the body.H.E-stained light micrographScanning electron micrographTwo isolated fibersartificially colouredin the computer(SEM)Cross-striationobserved in thetransmissionelectron micro-scope
Synthesis of collagen fibers: Takes place in the rER of fibroblasts: pro-a-chains form triple helices These procollagen molecules are transferred to the Golgi complex Tropocollagen molecules are released via exocytosis from Golgi-vesicles Tropocollagen molecules are assembled to form collagen fibers in ECM
Properties of collagen fibers Non-branching bundles of changing thickness High tensile strength Poor sheer strength They can be stretched to approximately 5 % of their initial length
SYNTHESIS OF COLLAGEN FIBERS BY FIBROBLASTS
Specific staining for the visualization of collagen fibers:Azan staining stains the collagen fibers selectively in blue colourother tissue componenst are stained red
TYPES OF COLLAGEN FIBERSType I.: 2 a1 chains + 1 a2 chainthe most common (bone, skin,tendon, ligaments)
Type II.: 3 a1 chainspredominates in cartilage
Type III.: 3 a1 chains in embryonic CT and skin
Type IV.: still unknown compositionin the basal laminae of adults
Type V.: still unknown composition mainly in the embryo
2. ELASTIC FIBERSThey are present in organs whose normal function requires elasticity in addition to tensile strengthWall of the aorta stained with H.E.andresorcin fuchsin,specific staining for elastic fibres
SYNTHESIS OF ELASTIC FIBERS:
Takes place in the rER of fibroblasts (and smooth muscle cells):tropoelastin molecules Transformed to elastin, cross-linked and assembled in the extracellular space Microfibrils secreted prior to elastin provide scaffolding on which elastin forms fibers and sheets
PROPERTIES OF ELASTIC FIBERS
Occur in the form of individual, branching and anastomosing fibers
They can be stretched 2.5 times of their original length to which they return when released
SYNTHESIS OF ELASTIC FIBRES BY FIBROBLASTS
3. RETICULAR FIBERSIndividual collagen fibers coated by proteoglycans and glycoproteins.This coating causes their affinity for silver salts.They form delicate flexible networks around capillaries, muscle fibers,nerves, adipose cells and hepatocytes.Serve as scaffolding in endocrine, lymphatic and blood cell-forming organs.Reticular fibers around hepatocytes and around kidney tubules (as part of basement membrane)
SYNTHESIS OF RETICULAR FIBRES BY FIBROBLASTS
CELL TYPES OF THE CONNECTIVE TISSUES
Fixed cells:fibrocytesfibroblastsmesenchymal cellsadipose cells
II. Free cells:macrophagesmast cellsplasma cellsmelanocytesblood-derived cells(lymphocytes, monocytes, eosinophils,neutrophils)
FIXED CELLSI.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 and nucleus, less cellorganelles (green arrows)
FIXED CELLSI.2. Mesenchymal cellsPluripotent undifferentiated cells around blood vessels: Serve as a reservoir of cells that can differentiate into other connective tissue cell types.
I. FIXED CELLS:I.3. Adipose cells:LMTEMSEMAdipose cells, single or in groups are normal components of the connective tissue (they mayform a separate tissue: adipose tissue).Spherical or polyhedral cells with a large lipid droplet surrounded by a thin cytoplasmic rim.
II. FREE CELLS:II.1. MACROPHAGES (histiocytes): in non-reactive CT are fixed, upon stimulation they become motile. Phagocytotic cell type, rich in hydrolytic enzymes.LMTEMSEM
THE MONONUCLEAR PHAGOCYTE SYSTEM
Connective tissue:histiocytesLiver:Kupffer-cellLung:alveolar macrophageLymph nodes:free and fixed macrophagesSpleen:free and fixed macrophagesBone marrow:fixed macrophagesSerous cavities:pleural and peritoneal macrophagesBone:osteoclastsCentral nervous system:microglial cellsSkin: histiocytesSynovia:type A cells
II.FREE CELLS:II.2. Mast cells: around blood vessels in skin and intestine.TEMLMLMLarge metachromatic granules: histamine, heparin, serotonin
II. FREE CELLS:II.3. Plasma cells:In the loose CT of gastrointestinal tract,respiratory system and female reproductive system, in the lymphatictissue.They develop from B-lymphocytes, produce antibodies.LMTEM
II. FREE CELLSII.4. MelanocytesLarge pigmented cells of neural crest-origin. They occur in the dermis of skin, meninges, choroid and iris.
II. FREE CELLS:II.5. Other free cells temporarily occurring in the CTLymphocytes, monocytes, granulocytes (in details see the chapter Blood later)