Connective Tissue

Home Up

    Connective tissue is found throughout our body.  There are many types of connective tissues depending on what it has to do.  For example:

bulletthe connective tissue directly under our skin, in the hypodermis, contains a lot of fat-- that helps to maintain our body temperatures
bulleta very loose type of connective tissue, areolar connective tissue, is found connecting our hypodermis to other organs (like muscles) and is found surrounding many organs-- this tissue allows for some movement of the attached structures relative to one another
bullettendons (that attach muscle to bone) are made up of dense regular connective tissue; they need the toughness of this material to withstand the high forces the muscles apply to them
bulleteven bone is considered a type of connective tissue, and it is very hard material

    In order to understand connective tissue, you must understand its general properties, the extracellular matrix material in which it lies as well as the cell types it contains.  You can follow the links in the last sentence, or just keep reading, in order to spend more times on these concepts.  If you understand them already, jump down below those sections to read more about some of the specific types of connective tissue.

    Click here to link to another website that describes connective tissue... it is a beautiful site with lots of images (clicking on an "E" button will show you an Electronmicrograph while clicking on a "L" button will show you a Light micrograph).  Just keep in mind that it goes into even more detail than I require, OK?

General Properties of Connective Tissue

    It is difficult to describe general properties of CT in the same way as I did for epithelial tissue.  That's because CT is so much more diverse.  So, keep in mind that some of the characteristics that I am writing may not fit all of the CT types perfectly-- but they will fit most of them.

bulletconnective tissues are typically well-vascularized
bulletthey can usually reproduce well (to recover from injury)  Exception:   They need a good number of cells to help with this, and dense connective tissue has only sparse numbers of cells.
bulletthey have a lot of noncellular material, called extracellular matrix material (or just matrix).   Exception:  adipose tissue.
bulletthere is no single function that can be described for this tissue, since there are so many.

Extracellular Matrix

    The space between cells can be called the extracellular space/material or the intercellular space/material.  extracell.jpg (7603 bytes)"Extra-" means outside of, while "inter-" means between.  I prefer to use the term extracellular space to prevent other confusion... that's because intercellular is easily confused with another term we will be using:  intracellular.  "Intra-" means within, and we will use intracellular to discuss what is inside the cell.

    In the drawing to the right, the extracellular space is all in pink.   This space is not a vacuum, but it is filled with material.  If the material is only liquid, the tissue as a whole will be loose.  An example of that is in blood.   If the material in the extracellular space has some tough strands (called fibers) of protein in it, that gives the entire tissue a stronger consistency (because the cells are now sitting in a mesh of fibers).

    This diagram shows a connective tissue with a lot of extracellular space.  There are some connective tissues with less (and even more) extracellular space.  If there's a lot of extracellular space (with little material in it), the tissue could be quite loose, while if there is little extracellular space the material will tend to hold together well.

    You should now understand that it is important to know how much and of what consistency the extracellular material is for any one type of connective tissue.   Please note that I will refer to it from now on as extracellular matrix material, or even just as matrix, because it is a complicated enough material to be called a matrix.

Ground Substance:

    This is the liquid portion of the extracellular matrix.  It is never entirely watery, but more gel-like.  A thin ground substance is seen in blood.   The ground substance is not just water, but it is also filled with many dissolved solute particles.

Extracellular Matrix Fibers:

    The number, properties, and alignment of fibers in the extracellular matrix will help determine the properties of the connective tissue.  There are three main types of connective tissue fibers.  Two are made out of a protein called collagen, while the third is made out of a protein called elastin.  Collagen is a protein that forms a long strand.  If many of these strands are put together the large resultant bundle can be quite strong.  If only a few of these strands are intertwined, the small resultant bundle is only somewhat strong.  Either way, collagen is has greater properties of strength than of elasticity.  Elastin, on the other hand, is not so strong, but has elastic properties.

bulletCollagenous fibers:  Very thick bundles of collagen.  The picture at the far right is an image (from RasMol) of a large collagen bundle, containing 12 collagen fibers all intertwined; this is representative of a collagenous fiber.
bulletReticular fibers:  Thin bundles of collagen.  The picture immediately to the right shows a small bundle of collagen fibers, only 3; this is more representative of a reticular fiber.
bulletElastic fibers:  stretchy branching bundles of elastin.  Also called yellow fibers, because they tend to look yellower than collagen bundles do.  (I have not included a photo).

    All of these types of extracellular matrix fibers can run together in different ways:

bulletas a mixture of fiber types
bulletmainly of one fiber type

                                    OR

bulletloosely piled, with no one orientation
bulletdensely piled, with no one orientation
bulletdensely piled, all having the same orientation
3collagen.jpg (7547 bytes) 12collagen.jpg (30501 bytes)

Connective Tissue Cell Types

    There are three main cell types in connective tissue.  These three cell types may appear in most of the types of connective tissue.  There are also cell types that are specific for certain connective tissues (and are only found there).

The three main cell types are:

  1. fibroblasts-- these important cells are the ones that lay down the extracellular matrix fibers!  They tend to be elongate in appearance.
  2. macrophages-- these cells are large and are derived from blood cells.   A certain white blood cell can leave the blood and enter tissue, and is then called a macrophage.  This cell is a scavenger in our connective tissues.  It chews up foreign particles in the tissue by phagocytosis, protecting and cleaning out our bodies.
  3. mast cells-- these cells communicate chemically with our blood.   They signal our blood by releasing heparin and histamine, telling our blood when it should clot or allow inflammation of certain tissues.  That means that these cells help begin a repair process, when needed, in tissue.

Other cells that you may find in specific connective tissues are:

  1. osteocytes-- only found in bone
  2. chondrocytes-- only found in cartilage (or developing bone)
  3. adipocytes-- only found in adipose tissue for storing fat
  4. blood cells-- found only in blood (unless you are injured) and there are many types of blood cells.  You will get to know each and every blood cell type in A&P2, but for now know at least red blood cells.
  5. reticulocytes-- found only in reticular connective tissue.  Our textbook just calls them fibroblasts, but our lab manual gives them the different name to match their tissue.

    If a connective tissue has plenty of cells within it, it is better at recovering after injury.  For example, if the skin is cut, and the dermis is thus cut, the mast cells will, of course, help get blood in the area to fill the hole left by the cut and then will also get the blood to begin clotting.  After that, however, we need to replace the clot with more dermis.  This is possible because the fibroblasts in the remaining dermis begin dividing and secreting more fibers for the matrix.  As the fibroblasts make more dermal connective tissue, the macrophages start removing the clot.  And, voila!  The repair is done.

    If, however, a connective tissue has few cells (and/or blood supply is limited), it is more difficult to repair that connective tissue.  An example of this is in tendons and ligaments.  You probably also have heard that it is difficult to repair tendons and ligaments after injury-- the healing time is much longer than for a broken bone.

Some Specific Types of Connective Tissue

Not all of the types are discussed here, since we are also doing them in lab.
I am just highlighting some important features in connective tissues.

Loose connective tissue

    "It binds the skin to the underlying organs and fills spaces between muscles."  This is built from a scattered array of a mixture of collagenous and elastic fibers within a gel-like ground substance.  Fibroblasts, macrophages, and mast cells can be found within it.

Dense connective tissue

    This tissue is made up of A LOT of fibers.  If it is regular dense connective tissue, it is mainly made up of parallel collagenous fibers.   Figure 5.21 in your book shows you this tissue.  In fact, there are so many fibers that there is little room for the fibroblasts to remain embedded within it.   There is also very little room for vascularization.  This is the type of tissue that makes up tendons and ligaments.

    If it is irregular dense connective tissue, it is found making up the dermis of your skin.  It has a lot of collagenous and elastic fibers, but these are not oriented in parallel bundles; the fibers are randomly arranged in orientation.

Cartilage

    This tissue contains chondrocytes, and the extracellular matrix material was secreted by them.  They secrete a dense matrix, so dense that after they secrete it they end up stuck inside of it.  There are different types of cartilage, each having its own appearance and elastic qualities.

Bone

    This tissue contains osteocytes, which are mature bone cells.  These cells also end up stuck inside the dense extracellular matrix that we think of as bone.  Because bone extracellular matrix is so dense, much more so than others, diffusion of nutrients through it is very difficult.  Therefore, osteocytes do not use diffusion to get their nutrients.  Instead, they extend tiny little processes to communicate with each other and with the blood; the development of these processes makes tiny little holes in the matrix, and these holes are called canaliculi.

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.