Smooth muscle

Smooth muscle cells are typically elongated cells
that are described as fusiform [L. fusus =a spindle +
forma =form] or spindle-shaped, which are tapered
at both ends. In some locations (e.g., blood vessels),
smooth muscle cells may exhibit branching.
Although they contain myofilaments, they are
nonstriated and have also been called plain
HistoNotes v9.06 8. Muscle
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muscle. Each cell is surrounded by an external
(basal) lamina and delicate reticular fibers.
1. Size. Smooth muscle myocytes vary in
length from 15 ìm to as large as 500 ìm in the
pregnant uterus. They are generally 3-8 ìm in
diameter.
2. Nucleus. The single centrally-located
nucleus is described as “cigar-shaped” in the
uncontracted myocyte. In cross-sections, this
description means that the nucleus will have a
circular profile. Recall that fibroblast nuclei are
“surfboard-shaped” so they are flattened in xs. In
contracted myocytes, nuclei take on a “corkscrew”
shape, which can readily be used as a diagnostic
feature.
3. Cytoplasm. In H&E preparations, the
cytoplasm is usually uniformly eosinophilic due to
the abundance of actin and myosin proteins. In
trichrome preparations they stain red in contrast to
the blue or green collagen fibers of CT.
4. Locations. Smooth muscle is found in
many locations throughout the systems of the body
including the walls of hollow organs (e.g., alimentary
canal, blood vessels, urinary bladder, uterus, ductus
deferens), and in the skin where bands of smooth
muscle cells can be found in the arrector pili
muscles of hair follicles.
5. Syncytial behavior. Although smooth
muscle cells are discrete units, like cardiac myocytes
they are electrically coupled with each other. Gap
junctions, which can be seen with the EM, are the
basis of this physiological approximation to a
syncytium.
6. Myofilaments. Smooth muscle cells
contain actin and myosin filaments, but they are not
arranged in the orderly arrays like those found in
striated muscle.
a. Arrangement. Actin and myosin
filaments course obliquely in the cytoplasm forming
a lattice-like arrangement.
b. Z line equivalents. Thin filaments are
attached to punctate structures called dense
bodies, which are located at multiple locations
within the cytoplasm and attached to the plasma
membrane. Thus, dense bodies serve the function of
Z lines of striated muscle.