8.18:
Connective Tissue Fibers and Ground Substance
The extracellular matrix, or ECM present abundantly in connective tissues is mainly made of protein fibers and an amorphous ground substance.
There are three types of fibers— collagen, elastic and reticular, whose composition and arrangement vary in different body parts.
In bones and cartilage, unbranched bundles of type I collagen associate with proteoglycans imparting tensile strength to these tissues.
In contrast, blood vessels contain elastic fibers with two long thin stretchable proteins, tropoelastin and fibrillin, that form branched networks to provide elasticity.
Smaller blood vessels have a similar network of reticular fibers made of short type III collagen proteins.
The ground substance includes the interstitial fluids that allow nutrients to diffuse across tissues.
It also contains cell adhesion proteins like fibronectin that connect ECM fibers to the cells.
The viscous gel-like appearance of ground substance comes from proteoglycans— protein cores with long glycosaminoglycans or GAG chains that help trap water.
8.18:
Connective Tissue Fibers and Ground Substance
One of the significant functions of connective tissue is connecting tissues and organs. Unlike epithelial tissue that is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a matrix. The matrix usually includes a large amount of extracellular material produced by the connective tissue cells that are embedded within it. It plays a significant role in the functioning of this tissue. The major component of the matrix is a ground substance, often interspersed with protein fibers. Ground substance is usually a fluid but can also be mineralized and solid, as in bones. Connective tissues have a wide variety of forms, yet they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and protein fibers. The amount and structure of each of these components correlate with the function of the tissue. For example, while the rigid ground substance in bones supports the body, the fluid ground substance in blood helps to transport oxygen, nutrients, and wastes throughout the body.
Fibroblasts secrete three main types of fibers—collagen fibers, elastic fibers, and reticular fibers—all of which are embedded in the ground substance. Collagen fibers are made from fibrous protein subunits linked together to form a long, straight fiber. Even though they are flexible, these fibers have significant tensile strength and can resist stretching.Collagen fibers give ligaments and tendons their characteristic resilience and strength. These fibers hold connective tissues together, even during the body's movement.
Elastic fibers mainly contain elastin protein with some other proteins and glycoproteins. The main property of elastin is to return to its original shape after being stretched or compressed. Elastic fibers are prominent in elastic tissues, such as the skin and ligaments.
Reticular fibers are formed from the same protein subunits as collagen fibers; however, these fibers remain narrow and are arrayed in a branching network. They are found throughout the body but are most abundant in the reticular tissue of soft organs, such as the liver and spleen, where they anchor and provide structural support to the parenchyma.
In addition to the fibers, fibroblasts also secrete the components of the ground substance, such as hyaluronic acid and proteins, which combine to form proteoglycan Proteoglycan molecules can attract and trap water molecules, giving ground substance a clear, viscous, and colorless characteristic.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 4.3: Connective Tissue Fibers and Ground Substance