Cardiogenesis is an exquisitely sensitive process. Any perturbation in the cells that contribute to the building of the heart leads to cardiac malformations, which frequently result in the death of the embryo. Previously, the myocardium was thought to be derived from a single source of cells. However, the recent identification of a second source of myocardial cells that make an important contribution to the cardiac chambers has modified the classical view of heart formation. It also has an important influence on the interpretation of mutant phenotypes in the mouse, with consequences for the classification and prognosis of human congenital heart defects.

Muscle satellite cells contribute to muscle regeneration. We have used a Pax3(GFP/+) mouse line to directly isolate (Pax3)(green fluorescent protein)-expressing muscle satellite cells, by flow cytometry from adult skeletal muscles, as a homogeneous population of small, nongranular, Pax7+, CD34+, CD45-, Sca1- cells. The flow cytometry parameters thus established enabled us to isolate satellite cells from wild-type muscles. Such cells, grafted into muscles of mdx nu/nu mice, contributed both to fiber repair and to the muscle satellite cell compartment. Expansion of these cells in culture before engraftment reduced their regenerative capacity.