Dense connective tissue

Collagen

The most abundant and important fibers of connective tissue are composed of collagen.
Collagen is a protein that is made by specialized cells called fibroblasts. There are different types of collagen that can be found throughout the body, but notably in connective tissue is collagen type I. Structurally, this type of collagen is composed of three stranded collagen molecules that are packed together to form so called collagen fibrils whose alignment yields characteristic ultrastructural striations.

These collagen fibers then fill the extracellular matrix giving the white appearance of dense connective tissue e.g. sclera of the eye. Ground substance is also found in the extracellular matrix. This is a highly hydrated complex mixture of macromolecules filling the space between fibers in the connective tissue.
Ground substance acts both as a lubricant and a barrier to penetration by foreign objects.

Fibroblasts

Fibroblasts are responsible for the synthesis of collagen and extracellular matrix. They appear as flattened, fusiform or spindle shaped cells. They have large euchromatic nuclei with prominent nucleoli. The cytoplasm of young and active cells are basophilic due to the presence of high concentration of rough endoplasmic reticulum, mitochondria and many sets of Golgi apparatus.

The old and inactive fibroblasts, also known as fibrocytes, have reduced cytoplasm with few cell organelles and flattened heterochromatic nuclei. Fibrocytes are fewer in number but have been shown to increase in quantity secondary to pathology such as tissue damage. This causes them to become activated and to therefore perform the function of both fibroblasts and macrophages.

In general, fibroblasts become very active during wound repair and healing process, when they proliferate and form fibrous matrices. A specialized form of fibroblasts known as myofibroblasts also appear during wound contraction. The myofibroblasts are characterized by the presence of actin filaments and dense bodies. Overall, these cells exhibit the properties of fibroblasts as well as that of smooth muscle cells.

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Structure

Dense irregular connective tissue is a 3-dimensional structure that can withstand significant force from different directions.
The strength of dense connective tissue comes from the high proportion of randomly organized type I collagen fibers in the tissue that are oriented randomly in a compact interwoven 3-dimensional web.

Collagen fibers have good tensile strength meaning that they are resistant to breaking under tension. This therefore makes collagen important in the supportive functions of this tissue.
They are also surrounded by small amounts of ground substance. Ground substance is amorphous and gel-like in texture and contains molecules such as proteoglycans and adhesive glycoproteins.
Among the collagen fibers there is also a network of elastic fibers to limit the distensibility of the tissue.

Location

Dense irregular connective tissue is found in high concentrations in body parts where support is needed to prevent the effects of forces that pull in multiple directions.
For example, it can be found within hollow organs, arranged to form a distinct layer known as the submucosa. In this layer, the fiber bundles lie in varying planes which allow the excessive distention of these organs.

Similarly, the skin also has a thick layer of dense connective tissue. This layer is called the reticular layer of the dermis and is essential in resisting any stretching forces that may be applied to the skin.

Structure

Dense regular connective tissue is mainly made up of type I collagen fibers. The collagen fibers are densely packed together and arranged in parallel to each other. This arrangement allows the fibers to have a good resistance to forces pulling along a single axis, but also gives some ability to stretch. A small amount of ground substance is present around dense regular connective tissue fibers and cells.

Location 

It is found in areas of the body where large amounts of tensile strength are required, like in ligaments, tendons and aponeuroses.

Tendons and ligaments attach to bones, and the role of dense regular connective tissue is to dissipate forces to bones.

Tendons are generally resistant to extension, but they are flexible. A tendon comprises fascicles of collagen fibers, which run parallel along their axis. In a cross section, a tendon will usually appear round or oval in shape. Where a tendon attaches to a bone, there is a transition from pure dense fibrous connective tissue, unmineralized fibrocartilage, mineralized fibrocartilage and bone.

Meanwhile, aponeuroses are flat sheets of collagen fibers that normally comprise numerous layers. Aponeuroses are important in attaching bone to muscle. In some muscles, the bone attachment is only partially aponeurotic, and partially tendinous, but in some cases, the bone attachment is entirely aponeurotic.

Ligaments form from thick bundles of connective tissue, mainly forming from collagen. Despite their density, ligaments can still be fairly elastic. Ligaments need to be strong as they connect bones together, and hold joints in place.

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