Essential Cell Biology 5th edition

CHAPTER TWENTY
20
Cell Communities: Tissues,
Stem Cells, and Cancer
Cells are the building blocks of multicellular organisms. Although this
seems a relatively simple statement, it raises deep questions. Cells are
not like bricks: they are small and squishy and enclosed in a flimsy membrane
less than a hundred-thousandth of a millimeter thick. How, then,
can cells be joined together robustly to construct a giraffe’s neck, a redwood
tree, or muscles that can support an elephant’s weight? How are all
the different cell types in a plant or an animal produced, and how do they
assemble so that each is in its proper place (Figure 20–1)? Most mysterious
of all, if cells are the building blocks, where is the builder and where
are the architect’s plans?
Most of the cells in multicellular organisms are organized into cooperative
assemblies called tissues, such as the nervous, muscular, epithelial,
and connective tissues found in vertebrates; tissues, in turn, are organized
into organs, such as heart, lung, brain and kidney (Figure 20–2). In
this chapter, we begin by discussing the architecture of tissues from a
mechanical point of view. We see that tissues are composed not only of
cells, with their internal framework of cytoskeletal filaments (discussed
in Chapter 17), but also of extracellular matrix, the material that cells
secrete around themselves; it is this matrix that gives supportive tissues
such as bone or wood their strength. At the same time, cells can also
attach to one another directly. Thus, we also discuss the cell junctions
that link cells together in the flexible epithelial tissues of animals. These
junctions transmit forces either from the cytoskeleton of one cell to that
of the next, or from the cytoskeleton of a cell to the extracellular matrix.
EXTRACELLULAR MATRIX
AND CONNECTIVE TISSUES
EPITHELIAL SHEETS AND
CELL JUNCTIONS
STEM CELLS AND TISSUE
RENEWAL
CANCER
But there is more to the organization of tissues than mechanics. Just as
a building needs plumbing, telephone lines, and other fittings, so an animal
tissue requires blood vessels, nerves, and other components formed