Dense connective tissue
Connective tissue is mainly a supporting tissue that binds and supports organs and the body as a whole. It is comprised of cells and extracellular matrix, including fibers and ground substance. Dense connective tissue, is one of the types of connective tissue also referred to as dense fibrous tissue due to relative abundance of the collagen fibers. It also contains fewer cells and less ground substance in comparison with the other type, loose connective tissue.
Dense connective tissue is often further divided into two main categories; dense irregular connective tissue and dense regular connective tissue. Dense regular connective tissue comprises structures such as ligaments and tendons, whilst dense irregular tissue is more widely distributed throughout the body.
Fibroblasts are usually the most numerous cells in both forms of connective tissue. They are responsible for the synthesis of collagen and extracellular matrix. They are present close to the collagen fibers and appear as flattened, fusiform or spindle shaped cells. They have large euchromatic nuclei with prominent nucleoli. Cytoplasm of young and active cells show basophilia 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 in circulation but have been shown to increase in quantity due to any pathology such as tissue damage, causing them to become activated and to display the properties of both fibroblasts and macrophages.
In general, fibroblasts become very active during wound repair and healing process, when they proliferate and form fibrous matrix. Specialised 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 smooth muscle cells.
Collagen fibers are tough, thick fibrous proteins found in dense connective tissues. It has many types but in dense connective tissue mainly type I collagen is present. Three stranded collagen molecules are packed together to form collagen fibrils and their alignment yield characteristic ultrastructural striations. Fibrils assemble to form collagen fibers.
Dense Irregular Connective Tissue
The type I collagen fibers of the dense irregular connective tissue are surrounded by small amounts of ground substance. Among the collagen fibers there is a network of elastic fibers as well, to limit the distensibility of the tissue. Ground substance is amorphous and gel-like in texture that surrounds cells. It contains molecules such as proteoglycans and adhesive glycoproteins, and like collagen fibers are produced by fibroblasts. Collagen fibers have good tensile strength and are therefore important in support. The collagen fibers in dense irregular connective tissue are orientated randomly in a compact interwoven 3-dimensional web. The 3-dimensional nature of dense irregular connective tissue gives it a great deal of strength.
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. Its 3-dimensional structure allows it to withstand force from different directions. It can be found in a number of locations within the body, including the dermis of the skin, in glandular tissue, in the walls of organs, and in the whites of the eyes.
Dense Regular Connective Tissue
Dense regular connective tissue is mainly made up of type I collagen fibers. It is found in areas of the body where large amounts of tensile strength are required, like in ligaments, tendons and aponeurosis. 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. Tendons and ligaments attach to bones, and the role of dense regular connective tissue is to transfer forces to bones. A small amount of ground substance is present around dense regular connective tissue fibers and cells.
Tendons are generally resistant to extension, but flexible, meaning that they can be easily distorted around bones and joints. 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 cartilage to mineralised cartilage to 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.