Cardiac muscle

In the late presomitic stage, the cardiogenic region consists of a coherent group of cuboid cells in the ventral mesoderm (splanchnopleura). In the early somitic stage (one to two somites), the endocardial tube forms below this cell group and becomes surrounded by these cells (myocardial mantle). In the space between the myocardial mantle and the endocardial tube, heart jelly is secreted by the myocardial cells which begin to differentiate and to form myofibrils. Soon after, the heart starts to beat.
The myoblasts of the heart do not fuse into myotubes. They remain as individual cells which are paired together physically and electrically through intercellular connections. Differentiated cardiac muscle cells still divide during the fetal period. They only leave the cell cycle after birth. Thus, they are clearly different from skeletal muscles, which only begin their differentiation after they have left the cell cycle.

Smooth muscle

Smooth muscle cells develop from mesenchymal cells in the wall of viscera and blood vessels. The muscles of the sphincter and dilatator pupillae are an exception as they develop from neural crest cells of the optic cup. After an initial proliferation period, the myoblasts become elongated. Thin filaments form first, then thick filaments. The cells are capable of contraction only when the thick filaments are formed.
Smooth muscle cells can proliferate everywhere in the muscle (not only at its surface or at its ends). Mitoses can occur at every age and in cells that have already progressed in differentiation.