give one example of a contractile cell

A contractile cell is a cell that contracts. The best example is a muscle cell. When stimulated, a muscle cells contracts from an elongated shape to a more compact one. In order to do this, a large amount of intracellular calcium is required to be tightly regulated. The organelle that does this is the sarcoplasmic reticulum, a specialized kind of endoplasmic reticulum that stores and releases calcium in response to contractile signals from the muscle cell.