Smooth muscle ultrastructure – still shrouded in mystery?

Smooth muscle is fundamental to life, as its function underpins circulation, nutrition, and reproduction. However, in contrast to striated muscle, the arrangement of its contractile filaments, composed of myosin and actin, is not readily apparent by light microscopy and, even electron microscopy fails to reveal an obviously ordered structural organisation. The aim of this review is to bring together classic ultrastructural data with more recent immunofluorescent analyses to develop a model how the protoplasma of a smooth muscle cell might be organised.