Essential Cell Biology 5th edition

710 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
of a variety of distinct cell types, each expressing different sets of genes
and arranged in a precise, intricate, three-dimensional pattern. No one
builds these amazing organisms: they self-assemble during development.
Although the final structure of an animal’s body may be enormously complex,
it is generated by a limited repertoire of cell activities. Examples
of all these activities have been discussed in earlier pages of this book.
Cells grow, divide, migrate, establish connections with other cells, and
die. They form mechanical attachments and generate forces that bend
epithelial sheets (see Figure 20−25). They differentiate by switching on or
off the production of specific sets of proteins and regulatory RNAs. They
produce molecular signals to influence neighboring and distant cells, and
they respond to signals that other cells deliver to them. They remember
the effects of previous signals they have received, and so progressively
become more and more specialized in the characteristics they adopt.
The genome, identical in virtually every cell, defines the rules by which
these various cell activities are called into play. Through its operation in
each cell individually, the genome guides the whole intricate process by
which a multicellular organism assembles itself, starting from a fertilized
egg. Movie 1.1, Movie 20.3, and Movie 20.4 offer some visual examples
of how development unfurls for the embryos of a frog, a fruit fly, and a
zebrafish, respectively.
For developmental biologists, the challenge is to explain how genes
orchestrate the entire sequence of interlocking events that lead from the
egg to the adult organism. We will not attempt to set out an answer to
this problem here: we do not have space to do it justice, even though
a great deal of the genetic and cell-biological basis of development is
now understood. But the same basic activities that combine to create the
organism during development continue even in the adult body, where
fresh cells are continually generated in precisely controlled patterns. It
is this more limited topic that we discuss in this section, focusing on the
organization and maintenance of the tissues of adult vertebrates.
Tissues Are Organized Mixtures of Many Cell Types
Although the specialized tissues in our body differ in many ways, they
all have certain basic requirements, usually fulfilled by a mixture of cell
types, as illustrated for the skin in Figure 20–33. As discussed earlier, all
tissues need mechanical strength, which is often supplied by a supporting
framework of connective tissue laid down and inhabited by fibroblasts
and related cell types. In this connective tissue, blood vessels lined with
endothelial cells satisfy the need for oxygen, nutrients, and waste disposal.
Likewise, most tissues are innervated by nerve cell axons, which
are ensheathed by Schwann cells, some of which wrap around large
axons to provide electrical insulation. Macrophages dispose of dead and
damaged cells and other unwanted debris, and, together with lymphocytes
and other white blood cells, they help combat infection. Most of
these cell types originate outside the tissue and invade it, either early in
the course of its development (endothelial cells, nerve cell axons, and
Schwann cells) or continuously throughout life (macrophages and other
cells derived from the blood).
A similar supporting apparatus is required to maintain the principal specialized
cells of many tissues: the contractile cells of muscle, the secretory
cells of glands, or the blood-forming cells of bone marrow, for example.
Almost every tissue is therefore an intricate mixture of many cell types
that must remain different from one another while coexisting in the same
environment. Moreover, in almost all adult tissues, cells are continually
dying and being replaced; throughout this hurly-burly of cell replacement
and tissue renewal, the organization of the tissue must be preserved.