In loose connective tissue, fibers are loosely organized, leaving large spaces in between. It is found between many organs where it acts both to absorb shock and binds tissues together. There are three types of loose connective tissue, adipose, areolar, and reticular tissue.

Adipose tissue consist mostly of fat storage cells (adipocytes), with little extracellular matrix (Figure 4.9). White adipose tissue is most abundant. It contributes mainly to lipid storage and can serve as insulation from cold temperature and mechanical injury. For example, it protects the kidneys and cushions the back of the eye. Brown adipose tissue is mainly found in infants, hence the term “baby fat”. In adults it is mainly found in the neck and shoulder (clavicular) region. Brown fat tissue has many mitochondria and is very efficient in metabolizing stored fat. In contrast to white fat, when brown fat is broken down metabolic heat is released and not adenosine triphoshate (ATP), a key molecule used in metabolism. Thus, brown fat is mainly used to maintain the body temperature. Areolar tissue contains all the cells and fibers previously described and is distributed in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. It underlies most epithelia and represents the connective tissue component of epithelial membranes. It is a reservoir of water and salts for surrounding tissues.

Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver (Figure 4.10). It derives its name from the Latin reticulus, which means “little net.”

Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are two major categories of dense connective tissue: regular and irregular (Table 4.1). In dense regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fiber orientations. Ligaments and tendons are made of dense regular connective tissue, but in ligaments not all fibers are parallel. Dense regular elastic tissue contains elastin fibers in addition to collagen fibers, which allows the ligament to return to its original length after stretching. The ligaments in the vocal folds and between the vertebrae in the vertebral column are elastic. In dense irregular connective tissue, the direction of fibers is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. In some tissues, fibers crisscross and form a mesh. In other tissues, stretching in several directions is achieved by alternating layers where fibers run in the same orientation in each layer, and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibers. Dense irregular elastic tissues give arterial walls the strength and the ability to regain original shape after stretching (Figure 4.11).

Supportive Connective Tissues

Two major forms of supportive connective tissue, cartilage and bone, allow the body to maintain its posture and protect internal organs.
Cartilage has a distinctive appearance due to polysaccharides called chondroitin sulfates, which bind with ground substance proteins to form proteoglycans. Embedded within the cartilage matrix are chondrocytes, or cartilage cells, and the space they occupy are called lacunae (singular = lacuna). A layer of dense irregular connective tissue the perichondrium, encapsulates the cartilage. Cartilaginous tissue is avascular (i.e., without blood vessels), thus all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues.

The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage (Figure 4.12). Hyaline cartilage, the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints. It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. Menisci (singular = meniscus) in the knee joint and the intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity. Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage.

Bone is the hardest connective tissue. It provides protection to internal organs and supports the body. Bone’s rigid extracellular matrix contains mostly collagen fibers embedded in a mineralized ground substance containing hydroxyapatite, a form of calcium phosphate. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. Osteocytes, bone cells like chondrocytes, are located within lacunae. The histology of transverse tissue from long bone shows a typical arrangement of osteocytes in concentric circles around a central canal. The central canal contains nerves, lymphatic and blood vessels. Thus, bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time.

There are two types of bone tissue, cancellous or spongy, and compact bone tissue. Most bones have both tissues. Spongy (cancellous) bones looks like a sponge under the microscope and contain spaces between so-called trabeculae (singular = trabecula) (or arches of bone proper). The spaces are often filled with red bone marrow. The trabeculae form a network and contain the osteocytes in lacunae. Spongy bone is lighter than compact bone. Compact bone has a greater structural strength. In compact bones the osteocytes are in lacunae within concentric rings of calcified matrix called lamellae (singular: lamella). (Figure 4.13).