The part of information is from the book "Memmler's The Human Body In Health And Disease" by Barbara Janson Cohen and Jason J. Taylor
The supporting fabric of all parts of the body is connective tissue. This is so extensive and widely distributed that if we were able to dissolve all the tissues except connective tissue, we would still be able to recognize the contours of the entire body. Connective tissue has large amounts of nonliving material between the cells. This intercellular background material or matrix contains varying amounts of water, fibers, and hard minerals. There are several ways of classifying connective tissue. Some is considered more generalized because it occurs throughout the body wherever structure and protection are needed. Others, such as bone and blood, have a more specialized function. Based on the composition of the matrix, the various connective tissues also differ in their degree of hardness. For simplicity, we will categorize them according to these physical properties:
* Liquid connective tissue-blood and lymph (the fluid that circulates in the lymphatic system) are examples of liquid connective tissues (Fig. 1-7). The cells in liquid connective tissue are suspended in a fluid environment.
* Soft connective tissue-loosely held together with semi-liquid material between the cells; includes adipose (fat) tissue and areolar (loose) connective tissue
* Fibrous connective tissue-most connective tissue contains some fibers, but this type is densely packed with them. Cells called fibroblasts produce the fibers in connective tissue. (The word ending -blast refers to a young and active cell). Examples of structures composed of fibrous connective tissue are ligaments, tendons, and the capsules (coverings) around certain organs.
* Hard connective tissue-has a very firm consistency, as in cartilage, or is hardened by minerals in the matrix, as in bone.
Figure 1-7 Liquid and soft connective tissue. (A) Blood smear showing various blood cells in a liquid matrix. (B) Areolar (loose) connective tissue, a mixture of cells and fibers in a jellylike matrix. (C) Adipose tissue showing stored fat. The nuclei are at the edges of the cells.
Soft Connective Tissue
The areolar, or loose, form of connective tissue (see Fig. 1-7) is found in membranes around vessels and organs, between muscles, and under the skin. It is the most common type of connective tissue in the body. It contains cells and fibers in a very loose, jellylike background material. Adipose tissue (see Fig. 1-7) contains cells that are able to store large amounts of fat. The fat in this tissue is used as a reserve energy supply for the body. Adipose tissue also serves as a heat insulator and as protective padding for organs and joints.
Fibrous Connective Tissue
Fibrous connective tissue (Fig. 1-8 A) is very dense and has large numbers of fibers that give it strength and flexibility. The main type of fiber in this and other connective tissues is collagen, a flexible white protein. (see Box, Collagen: The Body’s Scaffolding).
Some fibrous connective tissue contains large amounts of elastic fibers that allow the tissue to stretch and then return to its original length. This type of elastic connective tissue appears in the vocal cords, the passageways of the respiratory tract, and the walls of the large arteries (blood vessels). Fibrous connective tissue makes up the fibrous membranes that cover various organs, as described later in this chapter. Particularly strong forms make up the tough capsules around certain organs, such as the kidneys, the liver, and some glands. If the fibers in the connective tissue are all arranged in the same direction, like the strands of a cable, the tissue can pull in one direction. Examples are the cordlike tendons, which connect muscles to bones, and the ligaments, which connect bones to other bones.
Figure 1-8 Fibrous and hard connective tissue. (A) Fibrous connective tissue. In tendons and ligaments, the fibers are arranged in the same direction. (B) In cartilage, the cells (chondrocytes) are enclosed in a firm matrix. (C) Bone is the hardest connective tissue. The cells (osteocytes) are within the hard matrix.
Collagen: The Body’s Scaffolding
The most abundant protein in the body, making up about 25% of total protein, is collagen. Its name, derived from a Greek word meaning “glue,” reveals its role as the main structural protein in connective tissue. Fibroblasts secrete collagen molecules into the surrounding matrix, where the molecules are then assembled into fibers. These fibers give the matrix its strength and its flexibility. Collagen fibers’ high tensile strength makes them stronger than steel fibers of the same size, and their flexibility confers resilience on the tissues that contain them. For example, collagen in skin, bone, tendons, and ligaments resists pulling forces, whereas collagen found in joint cartilage and between vertebrae resists compression. Based on amino acid structure, there are at least 19 types of collagen, each of which imparts a different property to the connective tissue containing it. The arrangement of collagen fibers in the matrix reveals much about the tissue’s function. In the skin and membranes covering muscles and organs, collagen fibers are arranged irregularly, with fibers running in all directions. The result is a tissue that can resist stretching forces in many different directions. In tendons and ligaments, collagen fibers have a parallel arrangement, forming strong ropelike cords that can resist longitudinal pulling forces. In bone tissue, collagen fibers’ meshlike arrangement promotes deposition of calcium salts into the tissue, which gives bone strength while also providing flexibility. Collagen’s varied properties are also evident in the preparation of a gelatin dessert. Gelatin is a collagen extract made by boiling animal bones and other connective tissue. It is a viscous liquid in hot water but forms a semisolid gel on cooling.
Hard Connective Tissue
The hard connective tissues, cartilage and bone, are more solid than the other groups.
Cartilage Because of its strength and flexibility, cartilage is used as a structural material and as reinforcement. It is also used as a shock absorber and as a bearing surface that reduces friction between moving parts, as at joints. A common form of cartilage known as hyaline cartilage forms the tough, translucent material, popularly called gristle, seen over the ends of the long bones (see Fig. 1-8 B). Hyaline cartilage is also found at the tip of the nose and in parts of the larynx (“voicebox”) and the trachea (“windpipe”). Another form of cartilage, fibrocartilage, is found between segments of the spine, at the anterior joint between the pubic bones of the hip, and in the knee joint. Elastic cartilage can spring back into shape after it is bent. An easy place to observe the properties of elastic cartilage is in the outer portion of the ear. It is also located in the larynx. The cells that produce cartilage are chondrocytes, a name derived from the word root chondro, meaning “cartilage” and the root cyto, meaning “cell.”
Bone The tissue of which bones are made, called osseous tissue, is much like cartilage in its cellular structure (see Fig. 1-8 C). In fact, the skeleton of the fetus in the early stages of development is made almost entirely of cartilage. This tissue gradually becomes impregnated with salts of calcium and phosphorus that make bone characteristically solid and hard. The cells that form bone are called osteoblasts, a name that combines the root for bone (osteo) with a root (blast) that means an immature cell. As these cells mature, they are referred to as osteocytes. Within the osseous tissue are nerves and blood vessels. Enclosed within bones is a specialized type of tissue, the bone marrow. The red bone marrow contained in certain bone regions produces blood cells.
Dense Regular Connective Tissue
Dense regular connective tissue is characterized by large amounts of densely packed collagenous fibers that run parallel to the direction of force placed on the tissue during body movement. Because this tissue is silvery white in appearance, it is sometimes called white fibrous connective tissue. Dense regular connective tissue occurs where strong, flexible support is needed (fig. 1-9). Tendons, which attach muscles to bones and transfer the forces of muscle contractions, and ligaments, which connect bone to bone across articulations, are composed of this type of tissue.
Dense Irregular Connective Tissue
Dense irregular connective tissue is characterized by large amounts of densely packed collagenous fibers that are interwoven to provide tensile strength in any direction. This tissue is found in the dermis of the skin and the submucosa of the GI tract. It also forms the fibrous capsules of organs and joints (fig. 1-10).
Elastic Connective Tissue
Elastic connective tissue is composed primarily of elastic fibers that are irregularly arranged and yellowish in color (fig. 1-11). They can be stretched to one and a half times their original lengths and will snap back to their former size. Elastic connective tissue is found in the walls of large arteries, in portions of the larynx, and in the trachea and bronchial tubes of the lungs. It is also present between the arches of the vertebrae that make up the vertebral column.
Reticular Connective Tissue
Reticular connective tissue is characterized by a network of reticular fibers woven through a jellylike matrix (fig. 1-12). Certain specialized cells within reticular tissue are phagocytic (macrophages) and therefore can ingest foreign materials. The liver, spleen, lymph nodes, and bone marrow contain reticular connective tissue.
FIGURE 1-9 Dense regular connective tissue forms the strong and highly flexible tendons (a) and ligaments. (b) A photomicrograph of the
tissue and (c) a labeled diagram.
FIGURE 1-10 Dense irregular connective tissue forms joint capsules (a) that contain synovial fluid for lubricating movable joints. (b) A photomicrograph
of the tissue and (c) a labeled diagram.
FIGURE 1-11 Elastic connective tissue permits stretching of a large artery (a) as blood flows through. (b) A photomicrograph of the tissue and (c) a labeled diagram.
FIGURE 1-12 Reticular connective tissue forms the stroma, or framework, of such organs as the spleen (a), liver, thymus, and lymph nodes. (b) A photomicrograph of this tissue and (c) a labeled diagram.