As noted, elastic fibers are produced by fibroblasts or smooth muscle cells within the walls of the vessels. Elastin synthesis parallels collagen production; in fact, both processes can occur simultaneously in a cell. The orderly modification and assembly of procollagen and proelastin, as well as the synthesis of other connective tissue components, are controlled by signal sequences that are incorporated into the beginning of the polypeptide chains of each of the molecules.
Signal sequences can be compared to airline tags on luggage. Just as the tags ensure that baggage moves correctly from one aircraft to another at airports, so signal peptides ensure that the components of procollagen and proelastin remain separate and properly identified as they pass through the organelles of the cell. During this transit, a series of synthetic events and posttranslational modifications occur before the polypeptides ultimately arrive at their proper destination.
The extracellular matrix (ECM) is a complex and intricate structural network that surrounds and supports cells within the connective tissue. As previously mentioned, it contains a variety of fibers such as collagen and elastic fbers that are formed from different types of structural proteins. In addition, the ECM contains a variety of proteoglycans (e.g., aggrecan, syndecan); multiadhesive glycoproteins (such as fibronectin and laminin); and glycosaminoglycans (e.g., dermatan sulfate, keratan sulfate, hyaluronan). The last three groups of molecules constitute the ground substance.
FIGURE 6.15 • a. Electron micrograph of an elastic fber. The elastin (E) of the fiber has a relatively amorphous appearance. The fibrillin microfibrils (arrows) are present at the periphery and within the substance of the fiber. A number of collagen fibrils (C) are also present in this electron micrograph. 40,000. b. Scanning electron micrograph of an elastic fber. This scanning electron micrograph of the human dens irregular connective tissue from the dermis shows structure of elastic fiber (E) and illustrate it’s relative size in comparison to surrounding collagen fibrils (C). Note the presence of small fibrillin microfibrils (arrows) at the surface of elastic fiber 40,000. (Courtesy of Douglas
R. Keene)
All molecules found in the ECM share common domains, and the function of the ECM relies largely on the interactions between these molecules. Each connective tissue cell secretes a different ratio of ECM molecules that contribute to the formation of many different architectural arrangements; therefore, the ECM possesses specific mechanical and biochemical properties characteristic for the tissue in which it is present. For instance, the properties of the ECM in loose connective tissue are different from those of the ECM in cartilage or bone.
The extracellular matrix not only provides mechanical and structural support for tissue but also influences extracellular communication.
The ECM provides mechanical and structural support as well as tensile strength for the tissue. It also functions as a biochemical barrier and plays a role in regulating metabolic functions of the cells surrounded by the matrix. The ECM anchors cells within tissues through cell-to-ECM adhesion molecules and provides pathways for cell migration (e.g., during wound repair). Recent studies indicate that the ECM exerts a regulatory effect on embryonic development and cell differentiation. The matrix is capable of binding and retaining growth factors, which in turn modulate cell growth. With the aid of cell adhesion molecules, the ECM also influences the transmission of information across the plasma membrane of the connective tissue cells. Thus, the current view of ECM components (fibers and ground substance molecules) is that they form a dynamic and interactive system that informs cells about the biochemical and mechanical changes in their extracellular environment.
Ground substance is the part of the extracellular matrix that occupies the spaces between the cells and fibers; it consists of glycosaminoglycans (GAGs), proteoglycans, and multiadhesive glycoprotein.