Essential Cell Biology 5th edition

718 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
is formed from human ES cells, including a multilayered retina similar in
organization to that seen in the developing human eye in vivo.
Mouse and human iPS cells, and precursor cells derived from them, have
now been used to form organoids that resemble a variety of developing
organs, including the human brain, arguably the most complex and
sophisticated structure on Earth. Such organoids provide powerful models
for studying organ development in a culture dish, where one can
identify and manipulate the genes involved and explore the roles of cell–
cell interactions in ways not possible in an intact organism. In addition,
organoids can be used to investigate how developmental pathways can
be derailed by disease. For example, brain organoids have been produced
using human iPS cells derived from an individual with microcephaly, a
condition characterized by severely stunted brain growth and development.
Careful analysis of these developing brain organoids revealed that
the microcephaly in this case was probably caused by the premature
cessation of proliferation and differentiation of the brain precursor cells,
resulting in a decreased production of brain cells.
The development of iPS cells and organoid technology has opened up an
entirely new way to study human development and disease. It also opens
up promising avenues for treatment.
CANCER
Humans pay a price for having tissues that can renew and repair themselves.
The delicately balanced mechanisms that control these processes
can break down, leading to catastrophic disruption of tissue structure.
Foremost among the diseases of tissue renewal is cancer, which stands
alongside infection, malnutrition, war, and heart disease as a major
cause of death in human populations. In Europe and North America, for
example, at least one in five of us will die of cancer.
Cancer arises from violations of the basic rules of social cell behavior. To
make sense of the origins and progression of the disease, and to devise
treatments, we have to draw upon almost every part of our knowledge
of how cells work and interact in tissues. In this section, we examine
the causes and mechanisms of cancer, the types of cell misbehavior that
contribute to its progress, and the ways in which we hope to use our
understanding to defeat these misbehaving cells and, hence, the disease.
Although there are many types of cancer, each with distinct properties,
we will refer to them collectively by the umbrella term “cancer,” as they
are united by certain common principles and abnormalities.
Cancer Cells Proliferate Excessively and Migrate
Inappropriately
As tissues grow and renew themselves, each individual cell must adjust
its behavior according to the needs of the organism as a whole. The cell
must divide only when new cells of that type are needed, and refrain from
dividing when they are not; it must live as long as it is needed, and be
removed when it is not; it must maintain its specialized character; and it
must occupy its proper place and not stray into inappropriate territories.
In a large organism, no significant harm is done if an occasional single
cell misbehaves. But a potentially devastating breakdown of order occurs
when a single cell suffers genetic alterations that allow it to survive and
divide when it should not, producing daughter cells that behave in the
same antisocial way. Such a relentlessly expanding clone of abnormal
cells can disrupt the organization of the tissue, and eventually that of the
body as a whole. It is this catastrophe that occurs in cancer.