Connective tissue classes are recognized by the physical properties of their ground substance. (class – structure – location – function)
- Adipose – adipocytes that store fat (little matrix) – subcutaneous and around eyes and kidneys – stores excess energy and cushions
- Areolar (loose) – fibroblasts and a matrix of tissue fluid, collagen, and elastin fibers – subcutaneous where it connects skin to muscles and in mucous membranesof the digestive, respiratory, urinary, and reproductive tracts
- Blood – plasma (matrix), RBCs, WBCs, and platelets – within the blood vessels – plasma (transports materials), RBCs (carries oxygen), WBCs (destroys pathogens), and platelets (prevents blood loss)
- Bone – osteocytes in a matrix of calcium salts and collagen – bones – supports the body, protects internal organs from mechanical injury, stores excess calcium, and contains and protects red bone marrow
- Cartilage – chondrocytes in a flexible protein matrix – walls of trachea (keeps airway open), on joint surface of bones (smooth to prevent friction), on tip of nose and outer ear (support), and between the vertebrae (absorbs shock)
- Elastic – mostly elastin fibers (matrix) with few fibroblasts – walls of large arteries and around alveoli in lungs – helps maintain blood pressure and promotes normal exhalation
- Fibrous – mostly collagen fibers (matrix) with few fibroblasts – tendons and regular ligaments and irregular dermis – provides strength to inner layer of skin and strength to withstand forces of joint movements
Types of connective tissue fibers (released by subunits secreted by fibroblasts)
- Collagen fibers, the most common fibers in connective tissue proper, are long, straight, and unbranched.
- Elastic fibers contain the protein elastin and are branched and wavy, returning to their original length after being stretched.
- Reticular fibers are the least common of the three, thinner than collagen fibers, forming a branching, interwoven network in various organs.
Connective tissue proper refers to connective tissues having many types of cells and fibers and surrounded by a syrupy ground substance. (Ground substance fills all the spaces between cells and surrounds all the connective tissue fibers. Normally, it is clear and colorless with a consistency similar to maple syrup). Examples are:
- underlying skin tissue,
- fatty tissue,
Connective tissue proper is categorized as loose or dense.
- Loose connective tissue(areolar tissue) is found beneath the dermis of skin:
- in the digestive, respiratory, and urinary tracts
- between muscles
- around blood vessels, nerves, and joints
Loose connective tissue is the least specialized connective tissue in the body. It forms adipose tissue, a layer of fat cells (adipocytes) that separates the skin from underlying muscle, providing both with padding. It also pads the buttocks and breasts as well as in the sockets behind the eyes, around the kidneys, and in the pericardial and peritoneal cavities.
- Dense connective tissue, or fibrous tissue, consists mainly of collagen fibers and found:
- between the skeletal muscles and the tendons
- between bones and ligaments
- covering skeletal muscles and visceral organs.
Tendons are cords of dense connective tissue that attach skeletal muscles to bones. Ligaments are bundles of fibers containing elastic and collagen fibers that connect one bone to another.
Connective tissue proper cell types
- Fibroblasts are the most abundant cells responsible for the production and maintenance of the connective tissue fibers and the ground substance.
- Macrophages are scattered among the fibers which engulf (phagocytize) pathogens or damaged cells encountered in the tissue; responsible for chemicals released that mobilize the immune system into drawing more macrophages into an area.
- Fat cells are also called adipocytes. A fat cell contains such a large droplet of lipid that the nucleus and other organelles are pushed to one side. The number of cells varies from one tissue type to another, from one region of the body to another, and from one individual to another.
- Mast cells are mobile connective tissue cells often found near blood vessels. The cytoplasm is packed with vesicles filled with chemicals that are released to start the body’s defense system when the need arises.
- Phagocytic and antibody-producing WBCs may move throughout the connective tissue increasing their numbers during an injury.
Fluid connective tissues are simply blood and lymph. Their connective tissues contain a distinctive collection of cells in a liquid matrix. In the blood, most of the volume is composed of red blood cells and white blood cells in a matrix of plasma. Lymph is made up largely of lymphocytes in a special fluid and is largely responsible for the immune system.
Supporting connective tissues are cartilage and bone that provide a strong supportive framework for the body. In these connective tissues, the matrix contains numerous fibers and, in some cases, deposits of insoluble calcium salts. There are three major types of cartilage:
- Hyaline cartilage is the most common type, connecting ribs to the sternum, supporting the conducting pathways of the respiratory tract, and covering the surfaces of bones within joints.
- Elastic cartlage contains numerous elastic fibers, making it extremely resilient and flexible. Elastic cartilage supports the pinna (external flap of the ear), the epiglottis, and the tip of the nose.
- Fibrocartilage is dominated by collagen fibers, making it extremely durable resisting compression, absorbing shocks, and preventing damage from bone to bone contact. It also lies between the bones of the pelvis and around, or within, some joints and tendons. Damaged tissue, especially in such joints as the knee, can interfere with movement and tends to heal poorly.