Connective tissue is a diverse group of supporting tissues that is associated with epithelial
tissue: connective tissue connects the epithelium to the rest of the body.
Other types of connective tissue includes bone, fat, and blood.
Connective tissues vary widely in appearance and func- tion, but they all share three basic
components: (1) specialized cells, (2) extracellular protein fibers, and (3) a fluid known as
ground substance.
The extracellular fibers and the ground substance make up the matrix, which surrounds
the cells.
The matrix typically accounts for most of the volume of connective tissues.
Connective tissues occur throughout the body, but they are never exposed to the outside
environment: many are highly vascular and contain sensory receptors that detect pain,
pressure, temperature, and other stimuli.
The functions of connective tissues include:
- Establishing a structural framework for the body.
- Transporting fluids and dissolved materials.
- Protecting delicate organs.
- Supporting, surrounding, and interconnecting other types of tissue.
- Storing energy, especially in the form of triglycerides.
- Defending the body from invading microorganisms.
We classify connective tissues by their physical properties. The three general categories of
connective tissue are connective tissue proper, fluid connective tissues, and supporting
connective tissues.
- Connective tissue proper includes connective tissue with many types of cells and
extracellular fibers in a viscous (syrupy) ground substance.
This broad category contains a variety of connective tissues that are grouped into
loose connective tissues and dense connective tissues: based on the number of cell
types present, and on the relative properties and proportions of fibers and ground
substance.
Both adipose tissue, or fat (a loose con- nective tissue), and tendons (a
dense connective tissue) are connective tissue proper, but they have very different
struc- tural and functional characteristics.
- Fluid connective tissues have distinctive populations of cells suspended in a watery
matrix that contains dissolved proteins. (blood or lymph)
- Supporting connective tissues differ from connective tissue proper in having a less
diverse cell population and a matrix containing much more densely packed fibers.
Protects soft tissues and supports the weight of part or all of the body.
The two types of supporting connective tissues are cartilage and bone.
The matrix of cartilage is a gel with characteristics that vary with the
predominant type of fiber. The matrix of bone contains mineral deposits, primarily
calcium salts, that provide rigidity.
, Connective tissue proper contains a varied cell population. It also contains a matrix made up
of extracellular fibers and a viscous ground substance.
Some cells in connective tissue proper function in local maintenance, repair, and energy
storage. (includes fibroblasts, fibrocytes, adipocytes, and mesenchymal cells; permanent
residues of the connective tissue)
Other cells defend and repair damaged tissues. These cells are more mobile and include
macrophages, mast cells, lymphocytes, plasma cells, and microphages. (not permanent
residents: they migrate through healthy connective tissues and collect at sites of tissue
injury)
The fixed and migratory cells of connective tissue proper include the following:
- Fibroblasts are the only cells that are always present in connective tissue proper.
Also the most abundant fixed residents of connective tissue proper.
Secrete hyaluronan (a polysaccharide derivative; helps lock epithelial cells
together) and proteins.
- Fibrocytes are the second most abundant fixed cell in connective tissue proper, but
they differentiate from fibroblasts.
Spindle-shaped cells that maintain the connective tissue fibers of connective
tissue proper.
- Adipocytes are also known as fat cells: a typical adipocyte contains a single,
enormous lipid droplet.
The nucleus, other organelles, and cytoplasm are squeezed to one side, making a
sectional view of the cell resemble a class ring.
- Mesenchymal cells are stem cells that are present in many connective tissues:
respond to local injury or infection by dividing to produce daughter cells that
differentiate into fibroblasts, macrophages, or other connective tissue cells.
- Melanocytes synthesize and store the brown pigment melanin, which gives tissues a
dark color.
Melanocytes are common in the epithelium of the skin, where they play a major
role in determining skin color.
- Macrophages are large phagocytic cells scattered throughout the matrix: engulf
damaged cells or pathogens that enter the tissue.
Important in mobilizing the body’s defenses: when stimulated, they release
chemicals that activate the immune system and attract large numbers of additional
macrophages and other cells involved in tissue defense.
Macrophages are either fixed macrophages, which spend long periods in a tissue,
or free macrophages, which migrate rapidly through tissues.
- Mast cells circulate in the blood in an immature form before they migrate to other
vascularized tissues and undergo final maturation.
The cytoplasm of a mast cell is filled with granules containing histamine and
heparin.
Histamine, released after injury or infection, stimulate local inflammation.
Basophils, blood cells that enter damaged tissues and enhance the inflammation
process, also contain granules of histamine and heparin.
Heparin is an anticoagulant that enhances local blood flow during
inflammation and reduces the development of blood clots in areas of slow-moving
blood.
