Essential Cell Biology 5th edition

Extracellular Matrix and Connective Tissues
693
(A)
20 µm
(B)
2 µm
Figure 20–3 Plant tissues are strengthened
by cell walls. (A) A cross section of part of
the stem of the flowering plant Arabidopsis
is shown, stained with fluorescent
dyes that label two different cell wall
polysaccharides—cellulose in blue, and
pectin in green. The cells themselves are
unstained and invisible in this preparation.
Regions rich in both cellulose and pectin
appear white. Pectin predominates in the
outer layers of cells, which have only primary
cell walls (deposited while the cell is still
growing). Cellulose is more plentiful in
the inner layers, which have thicker, more
rigid secondary cell walls (deposited after
cell growth has ceased). (B) Cells and their
primary cell walls are clearly seen in this
electron micrograph of the young cells in
the root of the same plant. These cells are
much smaller than those in the stem, as can
be seen by the different scale bars in the two
micrographs. (Courtesy of Paul Linstead.)
Plant cells themselves synthesize, secrete, and control the composition
of this extracellular matrix: a ECB5 cell wall 20.03/20.03 can be thick and hard, as in wood,
or thin and flexible, as in a leaf. But the principle of construction is the
same in either case: many tiny boxes are cemented together, with a delicate
cell living inside each one. Indeed, as we noted in Chapter 1, it was
the close-packed mass of microscopic chambers that Robert Hooke saw
in a slice of cork three centuries ago that inspired the term “cell.”
Animal tissues are more diverse. Like plant tissues, they consist of both
cells and extracellular matrix, but these components are organized in
many different ways. In specialized connective tissues, such as bone or
tendon, extracellular matrix is plentiful and mechanically all-important;
in other tissues, such as muscle or the epidermis of the skin, extracellular
matrix is scanty, and the cytoskeletons of the cells themselves carry the
mechanical load. We begin this section with a brief discussion of plant
cells and tissues before considering those of animals.
Plant Cells Have Tough External Walls
A naked plant cell, artificially stripped of its wall, is a delicate and vulnerable
thing. With care, it can be kept alive in culture; but it is easily
ruptured, and even a small decrease in the osmotic strength of the culture
medium can cause the cell to swell and burst. Its cytoskeleton lacks
the tension-bearing intermediate filaments found in animal cells, and as
a result, it has virtually no tensile strength. An external wall, therefore,
is essential.
The plant cell wall has to be tough, but it does not necessarily have to
be rigid. Osmotic swelling of the cell, limited by the resistance of the cell
wall, can keep the chamber distended, and a mass of such swollen chambers
cemented together forms a semirigid tissue. Such is the state of a
crisp lettuce leaf. If water is lacking, the cells shrink and the leaf wilts.
Most newly formed cells in a plant initially make relatively thin primary
cell walls, which can slowly expand to accommodate cell growth (see
Figure 20–3B). The driving force for cell growth is the same as that keeping
the lettuce leaf crisp—a swelling pressure, called the turgor pressure,
that develops as the result of an osmotic imbalance between the interior