Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1228 Chapter 22: Stem Cells and Tissue Renewal
Fibroblasts and Their Transformations:
the Connective-Tissue Cell Family
From epithelia, with their varied patterns of renewal and their enormous variety
of protective, absorptive, secretory, sensory, and biosynthetic functions, we turn
now to connective tissues. Connective tissues typically consist of cells dispersed
in extracellular matrix that they themselves secrete, as discussed in Chapter 19.
They originate from the mesodermal (middle) layer of the early embryo, sandwiched
between ectoderm and endoderm (see Chapter 21, Figure 21–3).
In the adult body, virtually all epithelia are supported by a connective-tissue
bed, or stroma; and specialized types of connective tissue, such as bone, cartilage,
and tendon, form the supporting framework of the body as a whole. No less
important than its mechanical role, connective tissue also contains the blood vessels
that bring the oxygen and nourishment on which all cells depend. Cells of
the immune system roam through connective tissue, passing in and out of blood
vessels and lymphatics, and providing defence against infection; and through the
meshes of connective tissue run peripheral nerves. Also embedded in connective
tissue are the muscles that enable us to move. In these many ways, the cells
that form connective tissue and synthesize its various types of extracellular matrix
contribute to the support and repair of almost every tissue and organ.
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.
bone cell
(osteoblast/osteocyte)
fibroblast
cartilage cell
(chondrocyte)
fat cell
(adipocyte)
smooth muscle cell
Figure 22–11 The family of connectivetissue
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.