We may not immediately think of our heart as a collection of individual cells. But it is the complex interaction of numerous cell types that give the heart its ability to pump blood. Some cells form heart connective tissue, other cells grow into heart valves. And muscle cells give the heart its ability to beat and pump blood throughout the body.
You can dissolve an embryonic heart into its individual cell types with trypsin, an enzyme that destroys the protein glue between the cells. Plate these cells in a dish and you will see some cells - called myocytes - that beat independently. The cells shown here are from the chick embryo.
A single cell beats when a complex series of gates - called ion channels - open and close in an organized manner. Cell physiologists can measure how these ion channels work using a technique called the patch clamp.
As long as the beating cells do not touch one another, their beats are independent - some are faster, some are slower. But after two or three days, the myocytes form interconnected sheets of cells (monolayers, shown right) that beat in unison. Pores (gap junctions) open between adjacent touching cells, making their cytoplasms interconnected. It is these gap junctions that ensure that the connected cells work as one.
If the cells of the adult don't beat in unison, heart arrythmias can occur. Electronic pacemakers may sometimes be used in a patient whose heart doesn't beat in rhythm.