Cells look different because they all perform different functions in the body, and therefore are made up of different components. A brain cell, for example, performs differently than a muscle cell. Therefore, the proteins that make up the cells are different, giving each cell a unique appearance.
Most of the cells in a person's body all have the same parts. These include the cell membrane, cytoplasm, mitochondria, ribosomes and a nucleus. In the nucleus of each cell is deoxyribonucleic acid, or DNA, which is basically the same in each cell. The difference comes in when genes get turned on or off, depending on the function of the cell. As the cell matures, the protein makeup determines how the cell will look and perform. For example, a cell in the eye and a cell in the lung differs because the gene for breathing is turned off in the eye, and the gene for sight is turned on. The opposite is true in a lung cell.
A human being starts with just one cell, a fertilized egg. That one cell divides into two, then four, then eight and increases exponentially. Ultimately, about 100 trillion cells make up the human body. As the cells divide, the protein makeup changes and a cell is designated for its specific function such as a neuron, a white fat cell or a red skeletal cell.