Ganong's Review of Medical Physiology, 23rd Edition

36
SECTION I
Cellular & Molecular Basis of Medical Physiology

makes them so stable that organelles cannot move. Mitotic
spindles cannot form, and the cells die.
Intermediate filaments
(Figures 2–5 and 2–6) are 8 to 14 nm
in diameter and are made up of various subunits. Some of these
filaments connect the nuclear membrane to the cell membrane.
They form a flexible scaffolding for the cell and help it resist
external pressure. In their absence, cells rupture more easily, and
when they are abnormal in humans, blistering of the skin is com-
mon. The proteins that make up intermediate filaments are cell-
type specific, and are thus frequently used as cellular markers.
For example, vimentin is a major intermediate filament in fibro-
blasts, whereas cytokeratin is expressed in epithelial cells.
Microfilaments
(Figures 2–5 and 2–6) are long solid fibers
with a 4 to 6 nm diameter that are made up of
actin.
Although
actin is most often associated with muscle contraction, it is

present in all types of cells. It is the most abundant protein in

mammalian cells, sometimes accounting for as much as 15% of

the total protein in the cell. Its structure is highly conserved; for

example, 88% of the amino acid sequences in yeast and rabbit

actin are identical. Actin filaments polymerize and depolymerize

in vivo, and it is not uncommon to find polymerization occurring

at one end of the filament while depolymerization is occurring at

the other end.

Filamentous (F) actin

refers to intact microfila-

ments and

globular (G) actin

refers to the unpolymerized pro-

tein actin subunits. F-actin fibers attach to various parts of the

cytoskeleton and can interact directly or indirectly with mem-

brane-bound proteins. They reach to the tips of the microvilli on

the epithelial cells of the intestinal mucosa. They are also

abundant in the lamellipodia that cells put out when they crawl

along surfaces. The actin filaments interact with integrin

FIGURE 2–5
Cytoskeletal elements of the cell.
Artistic impressions that depict the major cytoskeletal elements are shown on the left, with
basic properties of these elements on the right.
(Reproduced with permission from Widmaier EP, Raff H, Strang KT:
Vander’s Human Physiology: The Mechanisms of Body
Function
, 11th ed. McGraw-Hill, 2008.)

Cytoskeletal filaments Diameter (nm) Protein subunit

Microfilament 7 Actin

Intermediate filament 10 Several proteins

Microtubule 25 Tubulin

FIGURE 2–6
Microfilaments and microtubules.
Electron micrograph
(Left)
of the cytoplasm of a fibroblast, displaying actin microfila-
ments (MF) and microtubules (MT).
(Reproduced, with permission, from Junqueira LC, Carneiro J:
Basic Histology
, 10th ed. McGraw-Hill, 2003.)
Fluorescent micro-
graphs of airway epithelial cells displaying actin microfilaments stained with phalloidin
(Middle)
and microtubules visualized with an antibody to
β
-tubulin
(Right)

. Both fluorescent micrographs are counterstained with Hoechst dye (blue) to visualize nuclei. Note the distinct differences in
cytoskeletal structure.