105
CHAPTER THREE
Cells
Cells that retain the ability to divide repeatedly enable
the body to grow and injuries to heal
(fig. 3.40)
. A
stem
cell
divides mitotically to yield either two daughter cells like
itself, or one daughter cell that is a stem cell and one that
is partially specialized. One deF ning characteristic of a stem
cell is its ability, called self-renewal, to divide to give rise to
other stem cells. A stem cell can also differentiate as any of
many cell types, given appropriate biochemical signals.
A partly specialized cell that is the daughter of a stem cell
is intermediate between a stem cell and a fully differentiated
cell and is termed a
progenitor cell.
A progenitor is said to be
“committed” because its daughter cells can become any of a
restricted number of cell types. ±or example, a neural stem
cell divides to give rise to cells that become part of neural tis-
sue (neurons and neuroglial), but not part of muscle or bone
tissue. All of the differentiated cell types in a human body can
be traced back through lineages of progenitor and stem cells.
Stem cells and progenitor cells are described in terms of
their potential—according to the possible fates of their daugh-
ter cells. A fertilized egg and cells of the very early embryo,
when it is a small ball of cells, are
totipotent,
which means
that they can give rise to every cell type
(F g. 3.41)
. In contrast,
chemicals or radiation, may induce cancer by altering (mutat-
ing) oncogenes and tumor suppressor genes in body (somatic)
cells. Cancer may also be the consequence of a failure of normal
programmed cell death (apoptosis), resulting in overgrowth.
PRACTICE
28
How do cells vary in their rates of division?
29
Which factors control the number of times and the rate at which
cells divide?
30
How can too infrequent or too frequent cell division aF
ect health?
31
What is the diF
erence between a benign and a malignant tumor?
32
What are two ways that genes cause cancer?
3.6
STEM AND PROGENITOR CELLS
Cells come from preexisting cells, by the processes of mitosis
and cytokinesis. Cell division explains how a fertilized egg
develops into an individual consisting of trillions of cells, of
at least 260 specialized types. The process of specialization
is called
differentiation.
Cancer trigger:
inherited mutation
or environmental
insult that causes
mutation
Oncogene turned on
Tumor suppressor
gene turned off
or
Epithelial
cell
Nucleus
Blood
vessel
Healthy, specialized cells
In a healthy cell, oncogenes are not overexpressed, and tumor
suppressor genes are expressed. As a result, cell division rate is
under control. Cancer begins in a single cell when an oncogene
is turned on or a tumor suppressor gene is turned off, lifting controls
on cell division and making the cell “immortal.” This initial step may
result from an inherited mutation, or from exposure to radiation,
viruses, or chemicals that cause mutation in a somatic (nonsex) cell.
(a)
Other mutations
Malignancy often results from a
series of mutations. An affected
cell divides more often than the
cell type it descends from and
eventually loses its specialized
characteristics.
(b)
Invasion and metastasis
Cancers grow and spread by inducing
formation of blood vessels to nourish
them and then breaking away from their
original location. The renegade cells often
undergo further genetic change and surface
alterations as they travel. This changeable
nature is why many treatments eventually
cease to work or a cancer recurs in a new place.
(c)
To other tissues
Loss of cell
division control
Loss of
specialization
Tumor
cell
Capillary
Tumor
FIGURE 3.39
Steps in the development of cancer.
previous page 135 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 137 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off