Essential Cell Biology 5th edition

714 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
dead, flattened cells
packed with keratin
filaments (scales)
DEAD
CELLS
ARE
SHED
epidermis
dead
cells
EPIDERMIS
(epithelium)
basal layer
dermis
DERMIS
(connective
tissue)
CELLS
ARE
BORN
(A)
30 µm basal lamina dividing basal cell
(B)
100 mm
Figure 20–36 The epidermis of the skin
is a stratified epithelium renewed from
stem cells in its basal layer. (A) The basal
layer contains a mixture of stem cells and
dividing precursor cells that are produced
from the stem cells. On emerging from
the basal layer, the precursor cells stop
dividing and move outward, progressively
differentiating as they go. Eventually,
the cells undergo a special form of cell
death: the nucleus and other organelles
disintegrate, and the cell shrinks to the form
of a flattened scale, packed with keratin
filaments. These scales are ultimately shed
from the skin surface. (B) Light micrograph
of a cross section through the sole of a
human foot, stained with hematoxylin
and eosin.
eosinophil
platelet
basophil
eosinophil
neutrophil
blood-cell formation, or hemopoiesis, provides an extreme example of
this phenomenon. All of the different cell types in the blood—both the red
blood cells that carry oxygen and the many types of white blood cells that
fight infection (Figure 20–37)—ultimately derive from a shared hemopoietic
stem
ECB5 e20.37-20.37
cell found in the bone marrow (Figure 20–38).
Specific Signals Maintain Stem-Cell Populations
Every stem-cell system requires control mechanisms to ensure that new
differentiated cells are generated in the appropriate places and in the
right numbers. The controls depend on extracellular signals exchanged
between the stem cells, their progeny, and other cell types. These signals,
and the intracellular signaling pathways they activate, fall into a surprisingly
small number of families, corresponding to half-a-dozen basic
signaling mechanisms, some of which are discussed in Chapter 16. These
few mechanisms are used again and again—in different combinations—
evoking different responses in different contexts in both the embryo and
the adult.
Almost all these signaling mechanisms contribute to the task of maintaining
the complex organization of a stem-cell system such as that of
the intestine. In this system, a class of signal molecules known as the
Wnt proteins serves to promote the proliferation of the stem cells and
precursor cells at the base of each intestinal crypt (Figure 20–39). Cells
in the crypt produce, in addition, other signals that act at longer range
to prevent activation of the Wnt pathway outside the crypts. The crypt
cells also exchange yet other signals that control cell diversification, so
that some precursor cells differentiate into secretory cells while others
become absorptive cells.
monocyte
lymphocyte
erythrocyte
20 µm
Figure 20–37 Blood contains many circulating cell types, all derived
from a single type of stem cell. A sample of blood is smeared onto
a glass cover slip, fixed (see Panel 1−1, p. 12), and stained with a dye
that mainly stains the nucleus blue and cytoplasm red. Microscopic
examination reveals numerous small erythrocytes (red blood cells),
which lack a nucleus and DNA. The nucleated cells are different types
of white blood cell: lymphocytes, eosinophils, basophils, neutrophils,
and monocytes. Blood smears of this kind are routinely used as a
clinical test in hospitals to look for increases or decreases in specific
types of blood cells; for example, an increase in specific types of white
blood cells could signal infection, inflammatory disorders, or leukemia.
(Courtesy of Peter Takizawa.)