Lesson 5 - Connective Tissue Basics

Student Performance Objectives
1. State the 3 fundamental properties of connective tissues.
2. Explain the relationship between the matrix and the cells found in connective tissues.
3. List 4 examples of common body structures that are classified as connective tissues.
4. Describe the structural arrangement of a generalized connective tissue including the 4
terms: cells, ground substance, fibers and matrix.
5. Define: fibroblasts, fixed macrophages (tissue histiocytes), and mast cells.
6. Define: collagen fibers, elastin fibers and reticular fibers.
7. Explain the functions of the following two major types of molecules found in connective
tissue ground substance: glycosaminoglycans (GAG's) and proteoglycans (PG's).

Lesson Outline
A. In general
    1. To bind, to unite and to protect: these functions underlie all connective tissues.
    2. Cells and matrix are the two main parts of any connective tissue. Matrix is intercellular space containing substances secreted by the connective tissue cells that give the specific connective tissue type its structural and functional properties. E.g., the matrix of bone is very hard and that of loose connective tissue (areolar) is comparatively soft.
    3. The connective tissues support each organ internally, provide external, protective wrappings, and visibly provide separation of each body part from other parts.
    4. The origin of connective tissue is from embryonic mesoderm which produces the  mesenchymal cell. Groups of mesenchymal cells form the embryonic tissue called  mesenchyme. As the embryo grows, mesenchymal cells separate from each other and the spaces between them fill with fibers and a viscous, syrupy fluid rich in glycoproteins. This is the body's first connective tissue. Some mesenchymal cells remain in all adult tissues as adult stem cells capable of differentiating into any of the connective tissue cell types as required. Wharton's jelly of the umbilicus and the eye's vitreous humor can be thought of as examples of the body's original connective tissue. 

    5. The connective tissues include many common tissues including adipose, tendon, bone, and cartilage.
    6. A good way to picture a generalized connective tissue is to imagine making a bowl of jello and placing fruit cocktail and strings of different sizes into the jello as it hardens. The jello is the ground substance and the strings are the different fiber types found in the ground substance. The jello and the strings compose the tissue's  matrix. The fruit pieces are cells embedded in the matrix that actually produced the matrix.
B. Basics of Connective Tissue Structure
    1. Cell Types
        a. Cells embedded in a matrix are fundamental to all connective tissues.
        b. The cell types and their numbers and the composition of the matrix vary with each connective tissue type: structure and function always complement each other.
        c. The most basic, specific cell type of most connective tissues is the fibroblast that produces the ground substance and fibers composing the matrix.
d. Another common cell type of most connective tissues is the fixed macrophage (also called a tissue histiocyte), a type of leukocyte that can ingest and process foreign matter, including bacteria. 
  e. A third common cell type in most connective tissues is the mast cell that responds to injury and microbial invasion by releasing chemicals (e.g., histamine) that initiate the inflammatory reaction.
        f. All cell types may either migrate into and remain in the tissue, or differentiate from stem cells (mesenchymal cells) already residing in the tissue.
    2. Matrix
        a. Fiber Types
            (1) Collagen fibers (also called collagenous fibers). These strong fibers are found in abundance when strength and firm support are needed.
            (2) Elastin fibers. These branching fibers are found often in organs that change shape and require an elastic quality to perform their function.
            (3) Reticular fibers. These most delicate of all the connective tissue fibers branch extensively and are found where groups of cells must be bound together in organs like the liver, spleen and lymph nodes.
            (4) All fibers are synthesized in fibroblasts as smaller subunits that are secreted into the matrix where they polymerize in fibers.
        b. Ground Substance - this viscous liquid fills the spaces between cells and fibers. Viscosity is due to several types of large molecules colloidally dispersed in water of the ground substance.  Ground substance viscosity helps to give tissues their characteristic texture and to slow down the movement of bacteria making it easier for macrophages to find and ingest them. The ground substance molecules also bind the cells of the tissue in place in the matrix.
            Examples of molecules found in and composing ground substance are:
            (1) Glycosaminoglycans (GAG's). These are polysaccharides composed of special amino sugars and uronic acid. Their negative charge attracts and holds Na+, K+ and H2O in the matrix.

                 (a) Chondroitin sulfate is a specific, long thin GAG molecule that is found abundantly in cartilage, bones and blood vessel walls.
                 (b) Hyaluronic acid is another GAG that is slippery and very long(15-20 microns - as long as a typical cell) and acts as a lubricant in joints and as a major chemical constituent of the eye's vitreous humor.
            (2) Proteoglycans (PG's). These are large, complexes of long, thick protein rods to which GAGs like chondroitin sulfate are attached at right angles to the axis of the proteins. This makes the molecule look like a bottle-cleaning bristle brush with the bristles being the GAGs and the metal core, to which the bristles are attached, being the protein rod.
 (3) Some PGs of the ground substance embed in the cell membranes of the cells composing the tissue, pass through the membrane and attach to cytoskeletal molecules of the cytoplasm thus binding the cell to the ground substance.
            (4) PGs so attached to cells then may bind to large GAGs like Hyaluronic acid in the ground substance thus firmly anchoring the cell in the matrix.