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

1232 Chapter 22: Stem Cells and Tissue Renewal
Osteoclasts Are Controlled by Signals From Osteoblasts
The osteoblasts that make the matrix also produce the signals that recruit and
activate the osteoclasts to degrade it. Disturbance of the balance can lead to osteoporosis,
where there is excessive erosion of the bone matrix and weakening of the
bone, or to the opposite condition, osteopetrosis, where the bone becomes excessively
thick and dense. Hormonal signals have powerful effects on this balance.
Chronic use of corticosteroid drugs, for example, can cause osteoporosis as a side
effect; but this can be treated by other drugs that redress the balance, including
agents that block the factors that osteoblasts secrete to recruit osteoclasts.
Local controls allow bone to be deposited in one place while it is resorbed
in another. Through such controls over the process of remodeling, bones are
endowed with a remarkable ability to adjust their structure in response to longterm
variations in the load imposed on them. It is this that makes orthodontics
possible, for example: a steady force applied to a tooth with a brace will cause it to
move gradually, over many months, through the bone of the jaw, by remodeling of
the bone tissue ahead of it and behind it.
Bone can also undergo much more rapid and dramatic reconstruction when
the need arises. Some cells capable of forming new cartilage persist in the connective
tissue that surrounds a bone. If the bone is broken, the cells in the neighborhood
of the fracture repair it by a process that resembles the way bones develop
in the embryo: cartilage is first laid down to bridge the gap and is then replaced
by bone. The capacity for self-repair, so strikingly illustrated by the tissues of the
skeleton, is a property of living structures that has no parallel among present-day
man-made objects.
Summary
The family of connective-tissue cells includes fibroblasts, cartilage cells, bone cells,
fat cells, and smooth muscle cells. Some classes of fibroblasts, such as the mesenchymal
stem cells of bone marrow, seem to be able to transform into any of the other
members of the family. These transformations of connective-tissue cell type are regulated
by the composition of the surrounding extracellular matrix, by cell shape,
and by hormones and growth factors. Cartilage and bone both consist of cells and
solid matrix that the cells secrete around themselves—chondrocytes in cartilage,
osteoblasts in bone (osteocytes being osteoblasts that have become trapped within
the bone matrix). The matrix of cartilage is deformable so that the tissue can grow
by swelling, whereas bone is rigid and can grow only by apposition. While osteoblasts
secrete bone matrix, they also produce signals that recruit monocytes from
the circulation to become osteoclasts, which degrade bone matrix. Through the
activities of these antagonistic classes of cells, bone undergoes a perpetual remodeling
through which it can adapt to the load it bears and alter its density in response
to hormonal signals. Moreover, adult bone retains an ability to repair itself if fractured,
by reactivation of the mechanisms that governed its embryonic development:
cells in the neighborhood of the break convert into cartilage, which is later replaced
by bone.
Genesis and Regeneration of Skeletal
Muscle
The term “muscle” includes many cell types, all specialized for contraction but
in other respects dissimilar. As noted in Chapter 16, all eukaryotic cells possess a
contractile system involving actin and myosin, but muscle cells have developed
this apparatus to a high degree. Mammals possess four main categories of cells
specialized for contraction: skeletal muscle cells, heart (cardiac) muscle cells,
smooth muscle cells, and myoepithelial cells (Figure 22–18). These differ in function,
structure, and development. Although all of them generate contractile forces
by using organized filament systems based on actin and myosin II, the actin and
myosin molecules employed have somewhat different amino acid sequences, are