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Connective Tissues and Cells

Connective tissues have the most varied structures and functions of any of the four tissue types. In general, they support and connect the other three tissue types, and may play a role in fat storage, the immune response, and the transportation of materials throughout the body. The organization of connective tissue differs from that of epithelial tissue in that connective tissue cells are almost always separated from each other and exist surrounded by an extracellular material known as the matrix. The matrix, which is secreted by the cells of the connective tissue, consists of ground material that may contain embedded fibers. Connective tissues are usually categorized according to the nature of the matrix in which the cells exist.
Many specific cell types may be found in different connective tissues:
Fibroblasts are probably the most common type of connective tissue cell. As their name suggests, they secrete proteinaceous fibers into the surrounding matrix. Fibroblasts can produce two basic types of fibers, which will largely determine the character of the matrix surrounding them. Collagenous or connecting fibers are composed of the protein collagen arranged in long bundles. They are very strong and resist forces applied to them, and thus are abundant in connective tissues such as bone, cartilage, tendons, ligaments, and the lower layers of skin. Elastic fibers are largely made up of the protein elastin. As their name implies, they are “elastic” (stretchable without being harmed). These fibers can be found in the walls of arteries and the air passages of the respiratory system.
Mast cells are also abundant, but are usually located near blood vessels. They secrete substances into the blood, such as heparin (which prevents blood clotting) and histamine (often responsible for allergic reactions).
Macrophages are important connective tissue cells that are able to actively move around the body, and are specialized for the process of phagocytosis. Sometimes called scavenger cells, they can engulf and destroy foreign particles, including pathogens, and thus are important defenses against infection. Since they are mobile, they may be found moving between different connective tissue types, often residing temporarily in the lymphatic system and in tissue fluid.
With the abundance of cells and fibers involved, many types of connective tissues are recognized. Each has a particular role to play in the body. We will list and explore the highlights of each major type.
  • Loose connective tissue: Composed mainly of fibroblasts that secrete both collagenous and elastin fibers, the matrix takes on a gel-like consistency. Loose connective tissue is often found below epithelium; it attaches the skin to underlying organs, binds organs together, and fills the spaces between muscles and bones. Adipose tissue is a specialized form of loose connective tissue that stores fat; fat provides energy reserves and insulation, and cushions sensitive body parts.
  • Dense connective tissue: Also consisting mainly of fibroblasts, dense connective tissue has fewer cells and a matrix thickly permeated with collagenous fibers. With its great strength, dense connective tissue largely comprises tendons (which attach muscles to bones) and ligaments (which bind bones to other bones).
  • Cartilage: Cartilage is a very rigid connective tissue. It is composed of cells called chondrocytes  surrounded by a matrix abundant in collagenous fibers embedded in a gel-like ground substance. Due to its rigidity, cartilage plays a role in supporting many body structures. It can be found in rings as the major supporting structure of the trachea, and in the ears and nose. It associates with many bones, including the vertebrae and the knees.
  • Bone: Bone is even more rigid than cartilage, and provides the major structural framework for the entire body. Bone cells, or osteocytes, exist in cavities separated by a matrix rich in collagen. Bone gains most of its amazing strength, however, from the presence of large amounts of mineral salts, mainly calcium phosphate, in the matrix. In addition to providing major structural support for the body as a whole, bones function to anchor muscles and protect vital organs (as with the sternum and cranium). Bones also contain marrow that produces blood cells.
  • Blood and Lymph: Blood and lymph are connective tissues in which the ground material of the matrix is a liquid called plasma. A variety of specialized cells including red blood cells (erythrocytes) and white blood cells (leukocytes) circulate through the body in these tissues by way of the blood and lymphatic vessels. These tissues are involved in the transport of substances throughout the body and in defense against infection.

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