Essential Cell Biology 5th edition

692 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
Figure 20–1 Multicellular organisms are built from organized
collections of cells. This thin section shows cells in the urine-collecting
ducts of a human kidney. Each duct is made of closely packed
“principal” cells, which form an epithelial tube, seen here in cross
section as rings of cells. The ducts are embedded in an extracellular
matrix populated by other types of cells. (Jose Luis Calvo/Shutterstock.)
from a variety of specialized cell types. All the tissue components have
to be appropriately organized and functionally coordinated, and many of
them require continual maintenance and renewal. Cells die and have to
be replaced with new cells of the right type, in the right places, and in the
right numbers. In the third section of this chapter, we discuss how these
processes are organized, as well as the crucial role that stem cells—selfrenewing,
undifferentiated cells—play in the renewal and repair of some
tissues.
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Disorders of tissue renewal are a major medical concern, and those due
to the misbehavior of mutant cells underlie the development of cancer.
We discuss cancer in the final section of this chapter and of the book as
a whole. The study of cancer requires a synthesis of knowledge of cells
and tissues at every level, from the molecular biology of DNA repair to
the principles of natural selection and the social interactions of cells in
tissues. Many fundamental advances in cell biology have been driven by
cancer research, and basic cell biology in return continues to deepen our
understanding of cancer and provide us with renewed optimism about
its treatment.
EXTRACELLULAR MATRIX AND CONNECTIVE
TISSUES
Plants and animals have evolved their multicellular organization independently,
and their tissues are constructed on different principles.
Animals prey on other living things—and often are preyed on by other
animals—and for these reasons they must be strong and agile: they must
possess tissues capable of rapid movement, and the cells that form those
tissues must be able to generate and transmit forces and to change shape
quickly. Plants, by contrast, are sedentary: their tissues are more or less
rigid, although their cells are weak and fragile if isolated from the stiff
supporting matrix that surrounds them.
In plants, the supportive matrix is called the cell wall, a boxlike structure
that encloses, protects, immobilizes, and shapes each cell (Figure 20–3).
LUMEN OF GUT
epithelial
cell
epithelium
Figure 20–2 Cells are organized into
tissues, and tissues often assemble into
organs. Simplified drawing of a cross
section through part of the wall of the
intestine of a mammal. This long, tubelike
organ is constructed from epithelial tissues
(red ), connective tissues (green), and muscle
tissues (yellow). Each tissue is an organized
assembly of cells, held together by cell–cell
adhesions, extracellular matrix, or both.
smooth
muscle
connective
tissue
circular
fibers
longitudinal
fibers
connective
tissue
epithelium
fibroblast
smooth
muscle
cells
epithelial cell