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

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Stem Cells and
Tissue Renewal
chapter
22
Cells evolved originally as free-living individuals, and such cells still dominate the
Earth and its oceans. But the cells that matter most to us, as human beings, are
specialized members of a multicellular community. These cells have lost features
needed for independent survival and acquired peculiarities that serve the needs
of the body as a whole. Although they share the same genome, they are spectacularly
diverse in structure, chemistry, and behavior. There are more than 200 different
named cell types in the human body that collaborate with one another to form
many different tissues, arranged into organs performing widely varied functions.
To understand them, it is not enough to analyze cells in a culture dish: we need
also to know how they live, work, and die in their natural habitat, the intact body.
In Chapters 7 and 21, we saw how the various cell types become different in
the embryo and how cell memory and signals from their neighbors enable them
to remain different thereafter. In Chapter 19, we discussed the technology used
to build multicellular tissues—the devices that bind cells together and the extracellular
materials that give them support. But the adult body is not static: it is a
structure in dynamic equilibrium, where new cells are continually being born,
differentiating, and dying. Homeostatic mechanisms maintain a proper balance,
so that the tissue architecture is preserved despite the constant replacement of
old cells by new. In this chapter, we focus on these developmental processes that
continue throughout life. In doing so, we shall illustrate some of the diversity of
specialized cell types and see how they work together to perform their tasks.
We shall examine in particular the role played in many tissues by stem cells—
cells that are specialized to provide a fresh supply of differentiated cells where
these need to be continually replaced, or when they are required in great number
for purposes of repair and regeneration. We shall see that while many tissues
renew and repair themselves, some others do not; there, lost cells are lost forever,
causing deafness, blindness, dementia, and other ills.
In the final section of the chapter, we discuss how stem cells can be generated
and manipulated artificially, and we confront the practical question that
underlies the current storm of interest in stem-cell technology: How can we use
our understanding of the processes of cell differentiation and tissue renewal to
improve upon nature, and make good those injuries and failings of the human
body that have hitherto seemed to be beyond repair?
In This Chapter
Stem Cells and Renewal
in Epithelial Tissues
Fibroblasts and Their
Transformations: the
Connective-Tissue Cell
Family
Genesis and Regeneration
of Skeletal Muscle
Blood Vessels, Lymphatics,
and Endothelial Cells
A Hierarchical Stem-
Cell System: Blood Cell
Formation
Regeneration and Repair
Cell Reprogramming and
Pluripotent Stem Cells
Stem Cells and Renewal in Epithelial Tissues
Among all the self-renewing tissues in a mammal, the champion—for speed at
least—is the lining of the small intestine: the long, convoluted portion of the gut
tube that is chiefly responsible for absorption of nutrients from the gut lumen. To
introduce stem cells, we take the small intestine as our starting point—not only
because it renews itself at a greater rate than any other tissue in the body, but also
because the molecular mechanisms that control its organization are particularly
well understood. It thereby provides a beautiful illustration of the principles of
stem-cell systems that have broad applicability.