Connective-tissue cells belong to a family of cell types that are related by origin, and they are often remarkably interconvertible. The family includes fibroblasts, cartilage cells, and bone cells, all of which are specialized for the secretion of collagenous extracellular matrix and are jointly responsible for the architectural framework of the body. The connective-tissue family also includes fat cells (adipocytes) and smooth muscle cells. Figure 22–11 illustrates these cell types and the interconversions that are thought to occur between them. The adaptability of the differentiated character of connective-tissue cells is an important feature of responses to many types of damage.
Fibroblasts Change their Character in Response to Chemical and physical Signals
Fibroblasts seem to be the least specialized cells in the connective-tissue family. They are dispersed in connective tissue throughout the body, where they secrete a nonrigid extracellular matrix that is rich in type I or type III collagen, or both, as discussed in Chapter 19. When a tissue is injured, the fibroblasts nearby proliferate, migrate into the wound (Movie 22.1), and produce large amounts of collagenous matrix that helps to isolate and repair the damaged tissue. Their ability to thrive in the face of injury, together with their solitary lifestyle, may explain why fibroblasts are the easiest of cells to grow in culture—a feature that has made them a favorite subject for cell biological studies.
Figure 22–11 The family of connective-tissue cells. arrows show the interconversions that are thought to occur within the family. For simplicity, the fibroblast is shown as a single cell type, but it is uncertain how many types of fibroblasts exist and whether the differentiation potential of different types is restricted in different ways.
A class of connective-tissue cells in the bone marrow, called bone marrow stromal cells, provides an example of radical connective-tissue versatility. These cells, which can be regarded as a kind of fibroblast, can be isolated from the bone marrow and propagated in culture. Large clones of progeny can be generated in this way from single ancestral stromal cells. Depending on the culture conditions, the members of such a clone either can continue proliferating to produce more cells of the same type, or can differentiate as fat cells, cartilage cells, or bone cells. The fate of the cells depends on physical as well as chemical signals: embedded in a stiff, unyielding matrix, they tend to turn into bone cells, whereas in a softer, more elastic matrix, they tend to turn into fat cells. This effect is mediated by an intracellular pathway that responds to tension in actin–myosin bundles and relays a signal to specific transcription regulators in the nucleus (Figure 22–12). Because of their self-renewing, multipotent character, the bone marrow stromal cells, and other cells with similar properties, are referred to as mesenchymal stem cells.
Cartilage and bone are tissues of very different character; but they are closely related in origin, and the formation of the skeleton depends on an intimate partnership between them.