What is a defining histologic feature of smooth muscle tissue?

• A. Spindle-shaped cells with single central nucleus arranged in sheets or bundles; no striations.
• B. Cylindrical cells with multiple nuclei and obvious striations.
• C. Branched cells with intercalated discs.
• D. Cells with cross-striations arranged in sarcomeres.

Answer: (A)

Smooth muscle is defined histologically by spindle-shaped (fusiform) cells with a single central nucleus, arranged in sheets or bundles. These cells lack striations because their contractile proteins aren’t organized into sarcomeres, allowing slower, sustained contractions characteristic of smooth muscle in the walls of hollow organs and blood vessels.

In contrast, the other muscle types have clear features of striations and different cell shapes: skeletal muscle shows cylindrical, multinucleated fibers with obvious striations due to organized sarcomeres; cardiac muscle shows branched cells with intercalated discs, also with striations from sarcomeric organization.