The different types of connective tissues are specified by the relative content of three distinguishing types of extracellular fibers: collagenous fibers, elastic fibers, and reticular fibers.
The molecules of collagen fibers are oriented to form a lattice in many regions. However in some tissues (tendon, ligaments), the fibers are oriented in dense parallel arrays to provide strength in one direction. These fibers are inelastic, but have great tensile strength. Thus they can be bent without breaking.
While the fibers themselves do not stretch, their lattice-like arrangement can allow tissues containing the fibers to stretch to some extent. Thus, collagen fibers impart both strength and flexibility to tissue.
Collagen is synthesized from the aggregation of precursor molecules, primarily procollagen. Collagen molecules aggregate to larger chains of molecules called fibrils. Fibrils are further organized to create fibers which are visible on light microscopy. Fibers often organized in parallel array forming a bundle.
There are 5 generally accepted types of collagen:
- Type I: most common, found in every connective tissue.
- Type II: found in hyaline and elastic cartilage and in vitreous body of eye.
- Type III: found in reticular fibers, healing wounds, smooth muscle, and fetal skin.
- Type IV: found in basal laminae of epithelia
- Type V: found in placental basal laminae, tendon, and muscle sheaths.
Elastic fibers are composed primarily of the protein elastin. These fibers are often organized into lamellar (parallel) sheets, as in the walls of arteries. Dense, regular, elastic tissue is an important component in many ligaments and tendons.
Elastic fibers are stretchable because they are normally disorganized – stretching these fibers makes them take on an organized structure.
Reticular fibers are composed of type III collagen. Unlike the thick and coarse collagenous fibers, reticular fibers form a thin reticular network. Such networks are widespread among different tissues and form supporting frameworks in the liver, lymphoid organs, capillary endothelia, and muscle fibers.