Histologically, there are three types of muscle fibres: Skeletal muscles, Cardiac muscles and Smooth muscles.
Skeletal muscle fibres bear obvious striations, have many peripherally loculated nuclei, are of the same thickness throughout their length and do not branch. eg) muscles attached to the skeleton.
Cardiac muscle fibres are striated, have one or two centrally located nuclei, branch and anastomose with other fibres, and are joined to one another by intercalated discs- eg) cardiac muscles.
Smooth muscle is intrinsic muscle of the internal organs and blood vessels. No striations are present due to the different arrangement of actin and myosinfilaments. Like cardiac muscle, smooth muscle fibres are intrinsically contractile but responsive to autonomic and hormonal stimuli. They are specialized for slow, prolonged contraction. They are generally arranged in bundles or sheets. Each fibre is fusiform in shape with a thicker central portion and tapered at both ends. The SINGLE NUCLEUS is located in the central part of the fibre. Also smooth muscles often have a cork screw shape in longitudinal section due to contraction of the muscle fibre during fixation.
One distinguishing physiological feature of smooth muscle is its ability to secrete connective tissue matrix. In the walls of blood vessels and the uterus in particular, smooth muscle fibres secrete large amount of collagen and elastin.
To be more brief, smooth muscle fibres (eg) of the UTERUS are fusiform with taped ends, have a single centrally located nucleus and do not exhibit striations.
Cardiac fibromas could resemble a uterine fibroid in naked eye examination. Histologically the tumor, benign though, consists of fibrous tissue of various degree of cellularity in different areas.
The smooth muscles of uterine fibroid tends to die out with the passage of time.
I hope this gives us a clue how fibroid uterus is different from cardiac fibromas and why they can not interchange in their occurrence.
Thank you