From your first part of the question, Muscle cells are the cells that make up our muscles. They are specialized cells that have fibers in them that contract as you move. This contraction causes the cell to change shape. The main function of muscle cells is to allow for body movement. Almost every part of the body has a muscle cell. Muscle cells contain mitochondria, nucleus and cell membrane just like all other cells.
Well, for the second question, The myofibres cause skeletal and cardiac muscles to appear striated because they are cyndrical and long, extending across the entire surface of the muscle. There are two types of striated muscle: skeletal muscle and cardiac muscle.