All cells come from preexisting cells. A cell divides its DNA in a process known as mitosis and its cytoplasmic contents in a process known as cytokinesis, producing two new daughter cells while all life begins as a single cell, the human body is not just one cell type. Cells that can divide without becoming a specific cell type (i.e. skin cells) are known as stem cells. When stem cells undergo mitosis and divide, they produce two cells. One cell remains a stem cell and the other is a partially specialized cell known as a progenitor cell. Cells undergo a process of specialization or determination called differentiation to become neurons, muscle cells, or skin cells. Stem cells are the source of all different cell types.