Connective Tissue PDF

# Connective Tissue Connective tissue consists of cells that are widely separated by varying amounts of extracellular matrix. The extracellular substance consists of fibers embedded in a ground substance. ## Origin * Mesodermal origin ## Characteristics of connective tissue: 1. Formed of widely...

# Connective Tissue Connective tissue consists of cells that are widely separated by varying amounts of extracellular matrix. The extracellular substance consists of fibers embedded in a ground substance. ## Origin * Mesodermal origin ## Characteristics of connective tissue: 1. Formed of widely separated several types of cells within large amount of matrix. 2. Rich in blood vessels, nerves and lymphatics. ## Functions: * **Mechanical support:** connective tissues connect and hold tissues and organs in place. * **Nutritional support:** provides media through which nutrients are transported from capillaries that lie in the intercellular substance to the cells. It provides a passage for blood vessels and nerves to organs and tissues. * **Immunological protection:** The principal cell types involved in immunological defense are found within connective tissue (as macrophage cells, plasma cells, etc.) and the ground substance that prevents the spread of infection. Connective tissue is the usual site for inflammation and edema. * **Fat:** storage site. ## Connective tissue is composed of: * **Connective tissue cells.** * **Extracellular matrix:** * Ground substance * Fibers ## Types of connective tissue: * **Embryonic connective tissues:** * Mesenchymal connective tissue. * Mucous connective tissue. * **Connective tissue proper:** * Loose (areolar) connective tissue. * Dense connective tissue (regular and irregular). * Reticular tissue. * Adipose tissue. * **Specialized connective tissue:** * Cartilage. * Bone. * Blood. ## Connective Tissue Cells **A. Fixed cells:** Stable and long population cells which include: * Mesenchymal cells * Fibroblasts and fibrocytes * Fixed Macrophages (Histocytes) * Reticular cells * Fat cells * Endothelium * Pericytes **B. Free cells (wandering):** * Free macrophage cells * Plasma cell * Mast cells * Leukocytes ## Undifferentiated Mesenchymal Cells (UMCs) Undifferentiated cells that can differentiate into other types of CT cells and blood cells in the bone marrow. * **Sites:** Mesenchymal tissue in embryo, umbilical cord and small population of these pluripotential cells is found in adult tissues. * **LM:** They are small fusiform or branched cells with dark nuclei * **EM:** few amounts of cytoplasmic organelles, and heterochromatic nucleus. * **Functions:** Differentiate to CT cells, blood cells, smooth muscles and endothelium ## Fibroblasts * **The most abundant CT cell.** * **Origin:** UMCs. * **LM:** It is flattened or fusiform in shape with a centrally placed nucleus and presents numerous processes. Young and active fibroblast possess open faced nucleus with prominent nucleolus and abundant basophilic cytoplasm. * **EM:** show criteria of protein forming cells (abundant rough endoplasmic reticulum, prominent Golgi apparatus and mitochondria.) * **Functions:** * Fibroblasts produce all types of fibers (collagen, elastic and reticular) and the amorphous ground substance. * They help in the healing of wounds by continued proliferation and subsequent formation of connective tissue components. In the process of repair, fibroblasts are capable of contraction (thus, named myofibroblasts) due to the presence of cytoskeletal proteins that are normally found in smooth-muscle cells. These myofibroblasts participate in wound repair by contracting the edges of the wound. ## Fibrocytes * When the fibroblasts become old and inactive, they are converted into fibrocytes. * **LM:** flattened with few processes, oval dark nuclei, and less basophilia of cytoplasm. * **EM:** scanty organelles and heterochromatic nucleus * **Functions:** change to active fibroblast during healing & after wound ## Macrophages (Histiocyte) * **Origin:** monocytes of the blood. * **LM:** Large cell with irregular cell boundary and variable shapes due to pseudopodia. It possesses dark hyperchromatic kidney-shaped (indented) nucleus and acidophilic cytoplasm. It can be stained by vital dyes such as trypan blue. * **EM:** irregular cell outlines with pseudopodia, the cytoplasm contains numerous lysosomes, residual bodies, and many mitochondria. * **Resting macrophages are fixed to the reticular fibers of the connective tissue. When active, they become free, assume ovoid outline, and undergo amoeboid movements.** * **Functions:** * The macrophages phagocytose and digest particulate organic materials, foreign bodies or invading micro-organisms and bacteria thereby eliminate them from the body. * In spleen: they phagocytose worn out RBCs. * They are antigen presenting cells to lymphocytes. * In inflammation, they phagocytose cellular debris. * They fuse → multinucleated giant cell to face large foreign body or physiologically present as in osteoclasts of bone. * **NB: Melanophore cells:** macrophage cells engulf melanin pigment from disintegrating or aging melanocytes ## Reticular Cells * Reticular cells are a type of fibroblast that synthesizes collagen type-III to produce reticular fibers. * **LM:** branched flattened cells with poorly staining nuclei and cytoplasm. * **Functions:** * They produce reticular fibers to maintain thin networks of fibers that are a framework for most organs providing structural support. * Phagocytic: can ingest and remove the bacteria. * They act as stem cells. ## Fat Cells * **Origin:** UMCs * **LM:** Each cell is spherical or polygonal, consists of peripheral rim of cytoplasm with an eccentric nucleus and contains a large central lobule of fat. * By H & E, the cell is 'signet-ring' in appearance as fat is dissolved by alcohol used during tissue preparation. * By frozen sections: * Sudan III stain →stained with orange color * Sudan black stain →black color. * **EM:** Few ribosomes, rER, sER, Golgi, mitochondria, and large fat droplet. * **Functions:** * Storage of fat and giving body contour. * Support organs as in perinephric fat. * Heat insulation. * Secretion of hormones as: Leptin which inhibits appetite and Adiponectin which increases insulin sensitivity of skeletal muscle cells. ## Endothelial Cells * **Origin:** UMCs. * **LM:** They are the simple squamous epithelial cells that line blood vessels. * **Functions:** they produce special type of collagen (type IV). They also produce their own basal lamina. So endothelial cells are considered as one of the connective tissue cells. ## Pericytes: * **Origin:** UMCs. * **Site:** found around blood capillaries and venules. * **LM:** flattened and branched cells with many processes with oval and pale nucleus * **EM:** Has few organelles & numerous gap junctions and seen between the endothelial cells and their basal laminae (basement membrane). * **Functions:** * After injury it can differentiate to: * Fibroblast * Endothelium * Smooth muscle cells. * It can contract (contains actin and myosin) &cause vasoconstriction ## Plasma Cells * **Origin:** Plasma cells are activated B-lymphocytes (when the latter are exposed to antigens). Plasma cells form a small population in bone marrow. * **LM:** rounded cells without any process, with intense basophilic cytoplasm showing - ve Golgi area and acidophilic granules called Russel bodies (Ig antibodies). * **EM:** The cytoplasm is rich in RER, numerous ribosomes, mitochondria and prominent supranuclear Golgi complex. The nucleus is eccentric in position and typically presents clumps of chromatins in a radiating manner, resembling 'cart-wheel' or clock face in appearance. * **Functions:** * The plasma cells liberate humoral antibodies to counter-act the action of antigens and help in the defense mechanism of the body. * N.B. The plasma cells are not present at birth but appear in the postnatal life. Therefore, the antibody formation of the newborn is minimum. ## Mast Cells * **Origin:** UMCs * **Site:** Mast cells are present in the CT especially in fibrous capsule of the liver, along the blood vessels, beneath the mucosa of alimentary and respiratory tracts, and in other parts of the body. * **LM:** Mast cell is rounded in shape and presents a central nucleus. The cytoplasm is closely packed with large membrane-bound basophilic granules, which stain metachromatically with toluidine blue. Granules become purple colored when treated with toluidine blue. Histochemically, the granules contain heparin, histamine, eosinophil chemotactic factors and neutrophil chemotactic factors. * **EM:** ribosomes, RER, large Golgi, mitochondria, and larger membrane bound granules. * **Functions:** * The mast cells liberate heparin which is anticoagulant that prevents fibrinogen from clotting into fibrin. This may explain the presence of mast cells near blood vessels. * The mast cells produce histamine, which promotes capillary leakage, edema, and contraction of smooth muscles. Sometimes the antibodies (IgE) are adsorbed on the surface of the mast cells and fail to move. When the specific antigen enters the body, antigen-antibody reaction may take place on the surface of the mast cell. Eventually the mast cell releases the histamine which produces anaphylactic or allergic reactions. * Secretion of eosinophil and neutrophil chemotactic factors which attract leukocytes. ## Leukocytes * Neutrophils, basophils, eosinophils, lymphocytes and monocytes are wandering cells that may migrate from the blood stream to the connective tissue. These cells protect the tissues against invasion from microorganisms.