Essential Cell Biology 5th edition

Intermediate Filaments
577
Intermediate Filaments Strengthen Cells Against
Mechanical Stress
Intermediate filaments are particularly prominent in the cytoplasm of cells
that are subject to mechanical stress. They are present in large numbers,
for example, along the length of nerve cell axons, providing essential
internal reinforcement to these extremely long and fine cell extensions.
They are also abundant in muscle cells and in epithelial cells such as
those of the skin. In all these cells, intermediate filaments distribute the
effects of locally applied forces, thereby keeping cells and their membranes
from tearing in response to mechanical shear. A similar principle
is used to strengthen composite materials such as fiberglass or reinforced
concrete, in which tension-bearing linear elements such as carbon fibers
(in fiberglass) or steel bars (in concrete) are embedded in a space-filling
matrix to give the material strength.
Intermediate filaments can be grouped into four classes: (1) keratin filaments
in epithelial cells; (2) vimentin and vimentin-related filaments in
connective-tissue cells, muscle cells, and supporting cells of the nervous
system (glial cells); (3) neurofilaments in nerve cells; and (4) nuclear
lamins, which strengthen the nuclear envelope. The first three filament
types are found in the cytoplasm, whereas the fourth is found in the
nucleus (Figure 17–5). Filaments of each class are formed by polymerization
of their corresponding intermediate filament subunits.
The keratin filaments are the most diverse class of intermediate filament.
Every kind of epithelium in the vertebrate body—whether in the
tongue, the cornea, or the lining of the gut—has its own distinctive mixture
of keratin proteins. Specialized keratins also occur in hair, feathers,
and claws. In each case, the keratin filaments are formed from a mixture
of different keratin subunits. Keratin filaments typically span the interiors
of epithelial cells from one side of the cell to the other, and filaments in
adjacent epithelial cells are indirectly connected through desmosomes
(see Figure 17–3B). The ends of the keratin filaments are anchored to the
desmosomes, and the filaments associate laterally with other cell components
through the globular head and tail domains that project from their
surface. These strong cables, formed by the filaments throughout the epithelial
sheet, distribute the stress that occurs when the skin is stretched.
The importance of this function is illustrated by the rare human genetic
disease epidermolysis bullosa simplex, in which mutations in the keratin
genes interfere with the formation of keratin filaments in the epidermis.
As a result, the skin is highly vulnerable to mechanical injury, and even a
gentle pressure can rupture its cells, causing the skin to blister. The disease
can be reproduced in transgenic mice expressing a mutant keratin
gene in their skin (Figure 17–6).
INTERMEDIATE FILAMENTS
CYTOPLASMIC
NUCLEAR
keratin filaments
in epithelial cells
vimentin and
vimentin-related
filaments
in connectivetissue
cells,
muscle cells,
and glial cells
neurofilaments
in nerve cells
nuclear lamins
in all
animal cells
Figure 17–5 Intermediate filaments are
divided into four major classes. These
classes can include numerous subtypes.
Humans, for example, have more than
50 keratin genes.