Essential Cell Biology 5th edition

Stem Cells and Tissue Renewal
711
epithelium of
EPIDERMIS
loose connective
tissue of DERMIS
dense connective
tissue of DERMIS
fatty connective tissue
of HYPODERMIS
dead
cells
epidermis
loose connective tissue
of dermis
sensory nerves
blood vessel
dense connective tissue
of dermis
Figure 20–33 Mammalian skin is made
of a mixture of cell types. Schematic
diagrams showing the cellular architecture
of the main layers of thick skin. Skin can
be viewed as a large organ composed
of two main tissues: epithelial tissue (the
epidermis) on the outside, and connective
tissue on the inside. The outermost layer
of the epidermis consists of flat, dead
cells, whose intracellular organelles have
disappeared (see Figure 20–36). The
underlying connective tissue consists of the
tough dermis (from which leather is made)
and the deeper, fatty hypodermis. The
dermis and hypodermis are richly supplied
with blood vessels and nerves; some of the
nerves extend into the epidermis, as shown.
keratinocytes
melanocyte producing
pigment granules
Langerhans cell
(involved in immune
responses)
collagen
fiber
macrophage
fibroblast
lymphocyte
endothelial cell
forming capillary
fibroblasts
elastic fiber
collagen fibers
Three main factors contribute to this stability.
1. Cell communication: each type of specialized cell continually monitors
ECB5 e20.34-20.34
its environment for signals from other cells and adjusts its behavior
accordingly; the proliferation and even the survival of most vertebrate
cells depends on such social signals (discussed in Chapters 16 and
18). This communication ensures that new cells are produced and
survive only when and where they are required.
2. Selective cell adhesion: because different cell types have different
cadherins and other cell adhesion molecules in their plasma membrane,
they tend to stick selectively, by homophilic binding, to other cells of
the same type. They may also form selective attachments to certain
other cell types and to specific extracellular matrix components. The
selectivity of these cell adhesions keeps cells in their proper positions.
3. Cell memory: as discussed in Chapter 8, specialized patterns of
gene expression, evoked by signals that acted during embryonic
development, are afterward stably maintained, so that cells
autonomously preserve their distinctive character and pass it on to
their progeny. A fibroblast divides to produce more fibroblasts, an
endothelial cell divides to produce more endothelial cells, and so on.
Different Tissues Are Renewed at Different Rates
Human tissues vary enormously in their rate and pattern of cell turnover.
At one extreme is the intestinal epithelium, in which cells are replaced
every three to six days. At the other extreme is nervous tissue, in which
most of the nerve cells last a lifetime without replacement. Between
these extremes there is a spectrum of different speeds and styles of tissue
renewal. Bone (see Figure 20–8) has a turnover time of about ten
years, and it involves renewal of the matrix as well as of cells: old bone