Connective tissue provides a framework and support for a large variety of structures, including organs, blood vessel walls, as well as the better known functions of connecting muscle to bone and bone to bone. Chondrocytes are responsible for the production of connective tissue components, including the principal protein, collagen. The three types of connective tissues offer a wide variety of functions to fulfill the many roles needed by the body. A multitude of diseases result from abnormalities, deficiencies, or overproduction of connective tissues or their components.
Bones provide a mechanical structure for the human body and, in that role, also allow effective muscle contraction and, therefore, movement. Bones offer protection for the body’s internal organs, especially brain, heart, lungs, liver, stomach, and spleen. In the ear, bones transduct sound waves from the ear drum to the inner ear. Osteoblasts, osteocytes, and osteoclasts produce, break down, remodel, and repair both the organic and inorganic matrix that make up bones. In doing so, these cells, as well as several regulatory molecules, help to regulate calcium and phosphate metabolisms and levels in the body. Bones also have a synthetic function. Within their marrow, bones produce red and white blood cells as well as growth factors; and store fatty acids as yellow marrow. Finally, bones provide for the storage of certain minerals, including calcium and phosphorus and, to a lesser extent, zinc, copper, and sodium. In an analogous role, bone can temporarily absorb and store toxic heavy metals to reduce their effects on the body.
Connective tissue is a fibrous tissue made mainly of collagen (Chapter 1) and proteoglycans (Chapter 2) that forms, supports, and/or connects various organs in the body, attaches muscles to bones (e.g., tendons) and bones to bones (e.g., ligaments), forms the supportive matrix during bone formation (see below), and makes up various structures such as parts of blood vessels and intestinal walls. One major example of connective tissue is collagen, which is found in various forms throughout the body (Figure 13-1A–D).
A–D. Distribution of Cartilage in Adults. (A) There are three types of adult cartilage distributed in many areas of the skeleton, particularly in joints and where pliable support is useful, as in the ribs, ears, and nose. Cartilage support of other tissues throughout the respiratory system is also prominent. The photomicrographs show the main features of (B) hyaline cartilage, (C) fibrocartilage, and (D) elastic cartilage. [Reproduced with permission from Mescher AL: Junqueira’s Basic Histology Text and Atlas, 12th edition, McGraw-Hill, 2010.]
Formation of connective tissue relies on chondrocytes, which both produce and maintain the collagen matrix. Chondrocytes differentiate from osteochondrogenic cells, which can alternatively develop into osteoblasts (see below). Although the different structures vary ...