Connective Tissue

Connective Tissue Proper

Connective tissue proper consists of loose connective tissue (including reticular connective tissue and adipose tissue) and dense connective tissue (subdivided into dense regular and dense irregular connective tissues.) Loose and dense connective tissue are distinguished by the ratio of ground substance to fibrous tissue. Loose connective tissue has much more ground substance and a relative lack of fibrous tissue, while the reverse is true of dense connective tissue. Dense regular connective tissue, found in structures such as tendons and ligaments, is characterized by collagen fibers arranged in an orderly parallel fashion, giving it tensile strength in one direction. Dense irregular connective tissue provides strength in multiple directions by its dense bundles of fibers arranged in all directions.

Special Connective Tissue

Special connective tissue consists of cartilage, bone, blood and lymph. Other kinds of connective tissues include fibrous, elastic, and lymphoid connective tissues. Fibroareolar tissue is a mix of fibrous and areolar tissue. Fibromuscular tissue is made up of fibrous tissue and muscular tissue. New vascularised connective tissue that forms in the process of wound healing is termed granulation tissue. All of the special connective tissue types have been included as a subset of fascia in the fascial system, with blood and lymph classed as liquid fascia.

Bone and cartilage can be further classified as supportive connective tissue. Blood and lymph can also be categorized as fluid connective tissue, and liquid fascia.

Membranes

Membranes can be either of connective tissue or epithelial tissue. Connective tissue membranes include the meninges (the three membranes covering the brain and spinal cord) and synovial membranes that line joint cavities. Mucous membranes and serous membranes are epithelial with an underlying layer of loose connective tissue.

Fibrous Types

Fiber types found in the extracellular matrix are collagen fibers, elastic fibers, and reticular fibers. Ground substance is a clear, colorless, and viscous fluid containing glycosaminoglycans and proteoglycans allowing fixation of collagen fibers in intercellular spaces.

Examples of non-fibrous connective tissue include adipose tissue (fat) and blood. Adipose tissue gives mechanical cushioning to the body, among other functions. Although there is no dense collagen network in adipose tissue, groups of adipose cells are kept together by collagen fibers and collagen sheets in order to keep fat tissue under compression in place (for example, the sole of the foot). Both the ground substance and proteins (fibers) create the matrix for connective tissue.

Type I collagen is present in many forms of connective tissue, and makes up about 25% of the total protein content of the mammalian body.

Types of connective tissue

They include the following:

• Loose/areolar connective tissue has a loose arrangement of fibers in a matrix with a thick fluid consistency. A variety of cells are able to move through the matrix. It is found, for example, in the middle layer of the skin (dermis) and between the serous layers of the mesenteries.
• Dense regular connective tissue has mostly dense bundles of collagen (protein) fibers that run parallel to each other. Fiber-making cells (fibroblasts) are occasionally interspersed between fibers. The cells in this tissue are not able to move (immobile). This arrangement of fibers gives strength and resistance to pulling forces for the tendons and ligaments composed of this tissue.
• Dense irregular connective tissue has an interwoven pattern to its many composing fibers. It can be found supporting the skin's middle layer, and the weave of its fibers is much denser than that of loose/areolar connective tissue.
• Adipose connective tissue is composed of lipid-storing fat cells. These cells are so full of lipids that the nucleus and other organelles seem to be pushed aside to make room for the lipid droplet they contain. They are active cells that convert carbohydrates to fats. Adipose tissue can be found in the deepest layer of the skin, where it serves as insulation; in the breast; around organs; and in the greater omentum.
• Blood connective tissue is composed of red and white blood cells and platelets in a very fluid matrix called plasma.

• Cartilage connective tissue is of three types: hyaline, elastic, and fibrocartilage. The fibers involved determine their type. All three types of cartilage have cells surrounded by a very durable gel-like matrix.

  • Hyaline cartilage connective tissue (HIGH-ah-lin) has a very smooth, glassy appearance. Its collagen fibers are so fine that they are virtually invisible. This cartilage is found at the ends of long bones, the larynx, the nose, bronchi, and the cartilages between the ribs and sternum.
  • Elastic cartilage connective tissue has elastic fibers running in all directions. These fibers allow this cartilage to snap back to shape if bent. Elastic cartilage can be found in the ear and the epiglottis.
  • Fibrocartilage connective tissue has dense bundles of collagen fibers all running in the same direction. These fibers allow this cartilage to function as a shock absorber. Fibrocartilage connective tissue can be found in the disks between vertebrae and in the meniscus of the knee.
• Bone connective tissue has bone cells isolated by a dense, concretelike matrix that makes bone very hard. Collagen fibers in the matrix allow a little bit of flex so that the bone is not brittle.