Complex sentence: It is hypothesized that endothelial progenitor cells are mobilized after a myocardial infarction, and that they function to restore the lining of blood vessels that are damaged during the heart attack.
Split into simpler sentences: It is hypothesized that endothelial progenitor cells are mobilized after a myocardial infarction. They appear to help restore the linig of blood vessels that are damaged during the heart attack.