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Connective Tissues

In contrast to epithelial tissues where cells are tightly adherent to one another, connective tissues consist of dispersed cells that typically lack intercellular contact.  Also, most connective tissues are vascularized with the single exception being cartilage.  Extracellular spaces in connective tissues are therefore more abundant and contain vessels.  In connective tissues, the extracellular space is termed the extracellular matrix because products of specialized cells accumulate here.  These products include protein fibers and ground substances(mixes of  various chemical substances).

Connective tissues are broadly classified into three large groups:

Connective Tissue Category	Tissue Types
1.  Fluid Connective Tissues	Blood and Lymph
2.  Connective Tissue Proper	Loose and Dense Connective Tissues
3.  Supportive Connective Tissues	Cartilage and Bone

Fluid connective tissue types, of which there are only two, are important in transport and body defense.  Most connective tissues are variants of the second group, connective tissue proper where cells are interspersed among protein fibers in a fluid-filled matrix.  Supportive connective tissues include cartilage and bone, more durable connective tissue types due to the semisolid or solid ground substance that accumulates in the matrix.  Since the matrix of supportive connective tissues is either a semisolid or solid, cells of these tissues occupy small spaces or lacunae.

Grab and pull the skin on the back of your hand.  Now try this on the palm.  You cannot pull and stretch the skin of your palm like you can on the back of the hand.  The skin is obviously attached to underlying tissues differently in these two locales!  Now grab the tip of your nose or your ear lobe and stretch or wiggle it.  Then attempt this for the bridge of your nose.  It should become immediately apparent the connective and supportive tissues in these locales vary in their ability to accommodate movement or stretch.  As you proceed in your study of connective tissues, remember the following statement!  Whether connective, supportive, or other, the functions of any connective tissue type are products of a specific mix of protein fibers and molecular complexes in the extracellular matrix.

Take a quick look at a connective tissue to see fibers and abundant extracellular space!

Remember, connective tissues typically possess blood vessels with cartilage an exception to this.  If you are looking at a tissue and are not sure if it is a connective tissue, ask these two questions!

1. Are cells separated by space?  If so, it probably is a connective tissue!
2. Are there blood vessels present?  If so, it is not an epithelium!

Take a look at a connective tissue with blood vessels(capillaries) next to cartilage!

Functions of connective tissues vary and are diverse but the following is a good summary.  Connective tissues:

1. form the structural framework for the body(bone, cartilage)
2. transport nutrients and wastes(blood)
3. provide protection for vital organs(bone, cartilage, adipose)
4. provide support and interconnection(tendons, ligaments, capsules)
5. insulate, store energy and produce important cells of blood and lymphatic tissues(adipose tissue, bone marrow)
6. contribute to body defense and repair(blood, lymph, dense c.t.)

Remember, connective tissues do have diverse functions, but all possess two common attributes, extracellular matrix and specialized cells.

Extracellular Matrix

The extracellular matrix includes all materials surrounding cells.  The matrix consists of protein fibers and ground substance(both products of specialized cells).

• the ground substance is a mix of mucopolysaccharides, glycoproteins, minerals, and other substances.  Physically it is a fluid in connective tissues proper, blood, and lymph.  In cartilages, ground substances are a durable but flexible gelatinous solid.  In bone, the ground substance is a crystalline solid.  Key ingredients of these ground substances include:
  1. hyaluronic acid - forms the viscous fluid of connective tissues proper.
  2. chondroitin sulfate - forms the gelatinous material of cartilage.
  3. hydroxyapatite - forms the solid, crystalline matrix of bone.
• protein fibers embedded in the ground substance of the various connective tissues provide strength, support, and elasticity.  There are three types:
  1. collagen - very strong, flexible, inelastic fibers made from the protein collagen.  These typically appear as large, pink fibers in stained slides.
  2. elastic - strong but stretchable fibers made of the protein elastin.  These are typically thinner and darker staining in most stained slides.
  3. reticular - small branching collagen fibers that form a reticulum or network to support cells.  These are visible in silver stained slides.

Look at collagen and elastic fibers in loose c.t.!

Compare collagen and elastic to reticular fibers in a silver stain of a lymph node!

Remember names of the protein fibers because these are part of the classification or descriptive scheme used for connective tissues.  Fibrous or white connective tissues contain a preponderance of collagen fibers(these appear white when unstained).  Elastic or yellow connective tissues contain many elastic fibers(yellow when unstained).  There are fibrous and elastic tendons.  In addition, one type of cartilage is called elastic.  So you see, fibers of a connective tissues can determine the name, at least in part!

Specialized cells

These vary depending on tissue type but are usually named with key prefix and suffix word roots(pay attention to these!):

Prefixes:

• fibro - cells of connective tissue proper(loose, dense, regular, irregular)
• chondro - cells of cartilage
• osteo - cells of bone
• hemo or erythro - cells of blood

Suffixes:

• blasts - immature but metabolically active cells.  In most connective tissues, these form the extracellular matrix and protein fibers.
• cytes - less active cells that typically occupy the matrix of mature connective tissues.  Think of these as maintaining a completed tissue rather than forming one.
• clasts - a type of cell involved in remodeling or breakdown of bone matrix.

Put these together to form names of representative cells  for connective tissues:

• osteoblasts, osteocytes, osteoclasts-cells found in bone
• fibroblasts, fibrocytes-cells of loose and dense connective tissues proper
• chondroblasts, chondrocytes-cells found in cartilage
• hemocytoblasts, erythroblasts, erythrocytes-cells of blood or bone marrow

Watch these prefixes and suffixes and know their meanings!  Some connective tissues are converted into others(i.e. bone can form within cartilage).  Knowing names of these cells and what they do helps make descriptions of developmental processes more understandable.  For example, fibroblasts can form collagen fibers that later become surrounded by the semisolid ground substance secreted by chondroblasts.  Mature cells of the cartilage that forms are called chondrocytes.  When bone replaces the cartilage, osteoblasts form new bone within spaces preoccupied by cartilage.  Osteoblasts eventually are surrounded by the ground substance of bone to become osteocytes, the mature cells of bone.

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