Essential Cell Biology 5th edition

Extracellular Matrix and Connective Tissues
695
what directs the enzyme complexes? Just beneath the plasma membrane,
microtubules are aligned exactly with the cellulose microfibrils outside
the cell. The microtubules serve as tracks that help guide the movement
of the enzyme complexes (Figure 20–7). In this curiously indirect way,
the cytoskeleton controls the shape of the plant cell and the modeling of
the plant tissues. We will see that animal cells use their cytoskeleton to
control tissue architecture in a much more direct manner.
Animal Connective Tissues Consist Largely of
Extracellular Matrix
It is traditional to distinguish four major types of tissues in animals: connective,
epithelial, nervous, and muscular. But the basic architectural
distinction is between connective tissues and the rest. In connective
tissues, extracellular matrix is abundant and carries the mechanical
load. In other tissues, such as epithelia, extracellular matrix is sparse,
and the cells are directly joined to one another and carry the mechanical
load themselves. We discuss connective tissues first.
Animal connective tissues are enormously varied. They can be tough and
flexible like tendons or the dermis of the skin; hard and dense like bone;
resilient and shock-absorbing like cartilage; or soft and transparent like
the jelly that fills the interior of the eye. In all these examples, the bulk of
the tissue is occupied by extracellular matrix, and the cells that produce
(A)
(B)
turgor
pressure
cellulose microfibrils
Figure 20–6 The orientation of cellulose
microfibrils within the plant cell wall
influences the direction in which the cell
elongates. ECB5 The e20.06/20.06 cells in (A) and (B) start
off with identical shapes (shown here as
cubes) but with different orientations of
cellulose microfibrils (blue) in their walls.
Although turgor pressure is uniform in all
directions, each cell tends to elongate in a
direction perpendicular to the orientation
of the microfibrils, which have great tensile
strength. The final shape of an organ, such
as a shoot, is determined by the direction in
which its cells expand.
(A)
cellulose synthase complex
makes many cellulose
molecules and assembles
them into a microfibril
(C)
connector
protein
200 nm
(B)
cellulose microfibril being
added to preexisting wall
glucose supplied
from cytosol
CYTOSOL
microtubule attached
to plasma membrane
plasma membrane
0.1 µm
1 µm
Figure 20–7 Microtubules help direct the
deposition of cellulose in the plant cell
wall. Electron micrographs show (A) oriented
cellulose microfibrils in a plant cell wall and
(B) microtubules just beneath a plant cell’s
plasma membrane. (C) The orientation of
the newly deposited extracellular cellulose
microfibrils (dark blue strands) is determined
by the orientation of the underlying
intracellular microtubules (dark green). The
large cellulose synthase enzyme complexes
(light blue) are integral membrane proteins
that continuously synthesize cellulose
microfibrils on the outer face of the plasma
membrane. The distal ends of the stiff
microfibrils become integrated into the
texture of the cell wall (not shown), and their
elongation at the other end pushes the
synthase complex along in the plane of the
plasma membrane (blue arrow). The cortical
array of microtubules attached to the plasma
membrane by transmembrane proteins
(light green vertical bars) helps determine
the direction in which the microfibrils are
laid down. (A, courtesy of Brian Wells and
Keith Roberts; B, courtesy of Brian Gunning:
from Plant Cell Biology on DVD, Information
for Students and a Resource for Teachers.
Springer 2009.)