Connective tissue

thyroid, adrenal and pituitary glands. Their secretions diffuse directly into the blood vessels.

Exocrine glands remain connected to their ducts, and secrete their products through the ducts to the epithelial surface. Exocrine glands can be both internal and external, and are found in the pancreas and liver, as well as sweat and oil glands.

Connective tissue

Connective tissue connects body parts. As such, it is the most common and widely distributed of all body tissues. Connective tissues perform multiple functions, but are primarily concerned with protecting, supporting and binding together body tissues.

Characteristics of connective tissue

Connective tissues have the following characteristics:

They have variations in blood supply. Although most connective tissues have good blood supply, meaning they are well vascularised, there are exceptions. Tendons and ligaments have poor blood supply, while cartilage is avascular.

Connective tissues are made of many different types of cells, as well as non-living substances found outside of cells, called the extracellular matrix. The extracellular matrix is what makes connective tissue different from other tissues. The matrix is produced by the connective tissue cells and secreted to the exterior of the cell. Depending on the connective tissue type, the matrix may be liquid, semisolid or very hard. The extracellular matrix is what allows connective tissue to be able to bear weight and withstand stretching.

The matrix also consists of various types and amounts of fibers. These include collagen fibers known as white fibers, elastic fibers known as yellow fibers, and reticular, or fine collagen, fibers. The fibers are made and secreted by the connective tissue cells. Connective tissue types are largely determined by the amount and type of fibers in the matrix.

Bone

Bone, or osseous tissue, is composed of bone cells sitting in cavities called lacunae. These cells are surrounded by a very hard matrix that contains calcium salts and large amounts of collagen fibers. Figure 5 describes what the bony tissue looks like when cells are removed:

Figure 5

Cartilage

Cartilage is less hard and more flexible than bone, and is found in fewer locations throughout the body.

The most common cartilage is hyaline cartilage, which has an abundance of collagen fibers hidden by a rubbery matrix. Hyaline cartilage forms the supporting structures of the larynx, attaches the ribs to sternum, and forms joints at the end of bones.

Elastic cartilage is found where a structure with elasticity is needed, such as the ear. Fibrocartilage, which is highly compressible, forms the cushion-like discs between the vertebrae.

Dense connective tissue

Dense connective tissue is also known as dense fibrous tissue, has collagen fibers as its main matrix element. These fibers are produced by fibroblasts, which are fiber-forming cells found between the collagen fibers. Dense connective tissue forms strong, elongated structures such as tendons and ligaments, as well as the lower layers of the skin.

Loose connective tissue

Loose connective tissues are softer and have more cells and fewer fibers than most dense connective tissue. Two types of loose connective tissue exist: areolar tissue and adipose tissue.

Areolar tissue is the most common connective tissue. It is the soft tissue that surrounds body organs. It functions as a packing tissue as

holds internal organs together and in position. A thin layer of areolar tissue underlies all mucous membranes.

Areolar tissue has a loose, fluid matrix that contains all types of fibers. These fibers, however, are very loosely connected, giving the impression that the majority of them matrix is just empty space. Areolar tissue provides a storage facility of water and salts for surrounding tissues, and all body tissues obtain nutrients and release wastes into the areolar tissue.

Figure 6 indicates what areolar tissue looks like:

Figure 6

Adipose tissue is commonly referred to as fat. It is essentially areolar tissue in which an abundance of fat cells are found. Adipose tissue forms the subcutaneous tissue beneath the skin, and is responsible for insulation of the body from both hot and cold temperatures. Adipose tissue surrounds and cushions some of the body’s organs, such as the kidneys and eyeballs.

Blood

Blood, or vascular tissue, is considered to be a connective tissue as it consists of blood cells surrounded by a non-living fluid matrix called blood plasma. The fibers of blood are soluble proteins that only become visible during blood clotting.