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Connective Tissue
Characteristics of Connective Tissue
Key word is variability. The distribution type changes depending on the organ. For
example the skin has a lot but there is very little in the brain. This is the tissue that binds
us together.
4 major classes
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Connective Tissue
o Fat
o Fibrous
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Cartilage
Bone
Blood
Although the types are diverse they are classified together because they carry out the
following functions:
 Support
 Protection
 Insulation
 Transport
Just as their functions are diverse, their characteristics are as well:
 Share a common origin from the embryonic mesoderm (mesenchyme)
 Vasculature varies from avascular to vascular
 Make a nonliving extracellular matrix that can bear weight and abuse.
Despite their diversity, all connective tissue is made up of the following elements. Their
exact make up varies depending on the tissue’s functions.
 Ground substance
 Fibers
 Cells
Together, the ground substance and fibers make up the matrix. This material varies from
soft delicate material to tough tissue such as tendons and ligaments. The extracellular
matrix (ECM) fills the voids between cells.
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Components of the Matrix
1) Ground Substance.
This is a featureless substance that is made up of water and proteins. This transparent
fluid is found to one degree or another in all of the body's connective tissues. This
viscous liquid, much like raw egg whites in appearance and consistency, surrounds all the
cells in the body. It is made up of the following three components:
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Interstitial fluid
Cell adhesion proteins
Proteoglycans
Interstitial Fluid
The interstitial fluid (or tissue fluid) is a solution that bathes and surrounds the cells of
multicellular animals. It is the main component of the extracellular which also includes
plasma and transcellular fluid. On average, a person has about 11 liters (2.4 imperial
gallons) of interstitial fluid, providing the cells of the body with nutrients and a means of
waste removal.
Interstitial fluid consists of a water solvent containing amino acids, sugars, fatty acids,
coenzymes, hormones, neurotransmitters, salts, as well as waste products from the cells.
Cell Adhesion Proteins (CAP)
Cell adhesion proteins (CAP) are involved in cell-cell and cell-extracellular matrix
binding. The major classes of cell adhesion molecules are the integrins, cadherins,
selectins and the immunoglobulin. These are the glue that holds the cell and tissue
together. They become continuous with the basement membrane and the various types of
adhesion proteins such as desmosomes.
Proteoglycans
Are a complex group of proteins modified with sugar groups that control how viscous the
ground substance is.
Proteoglycans are heavily glycosylated proteins. They have a core protein with one or
more attached glycosaminoglycan (GAG) chain(s). The chains are long, linear
carbohydrate polymers that are negatively charged under physiological conditions, due to
the occurrence of sulfate and uronic acid groups.
Proteoglycans are a major component of the animal extracellular matrix, the “filler”
substance existing between cells in an organism. Here they form large complexes, both to
other proteoglycans and to fibrous matrix proteins (such as collagen). They are also
involved in binding cations, and also regulating the movement of molecules through the
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matrix. Evidence also shows they can affect the activity and stability of proteins and
signalling molecules within the matrix. Examples of these include chondrotin sulfate and
hyaluronic acid. The picture below shows how these proteins weave with each other to
form a net work of anchoring structures for cells. The nature of this matrix depends on
the tissue type.
2) Fibers
There are three types of protein fibers which make up the matrix. These protein fibers
intermingle with the proteins of the ground substance. These proteins are:
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Collagen fibers
Elastic fibers
Reticular fibers
As with other components of the connective tissue network, their distribution depends on
the function of the organ.
Collagen
This is a heavily crossed linked protein and as a result is tough and has a high tensile
strength. Grossly collagen appears white. It is the main component of connective tissue,
and is the most abundant protein in mammals, making up about 25% to 35% of the
whole-body protein content. Collagen, in the form of elongated fibrils, is mostly found in
fibrous tissues such as tendon, ligament and skin, and is also abundant in cornea,
cartilage, bone, blood vessels, the gut, and intervertebral disc.
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Elastic Fibers
Unlike collagen, elastic tissue as the name implies can stretch like a rubber band. It is
made of the protein elastin which can stretch and recoil. It is found in the skin and lungs.
If large amounts are present the tissue can appear yellow. Elastin tends to deplete as
people age, resulting in wrinkled or stretched out skin. One might note the “pregnancy
pouch” many women have many years after having a baby. In part, the leftover skin is a
result of inadequate elastin, and also overstretching of the skin covering the abdomen
during pregnancy.
Reticular Fibers
Reticular fibers or reticulin is a histological term used to describe a type of fiber in
connective tissue composed of type III collagen. Reticular fibers crosslink to form a fine
meshwork (reticulum). This network acts as a supporting mesh in soft tissues such as
liver, bone marrow, and the tissues and organs of the lymphatic system.
3) Cell Component
Within the connective tissue framework are “resident cells” These cells can become
activated and form the characteristic fibers of their matrix. The suffix “blast” is used to
describe these cells. Common cells include the:
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Fibroblast- lays down the connective tissue matrix
Chondroblast- found in cartilage
Osteoblasts- found in bone
Blood cells
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Other cell types found in the connective tissue are often associated with the immune
system and healing these include:
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Fat cells
White blood cells (neutrophils, eosinophils and lymphocytes)
Macrophages
Mast Cells
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