Mitosis is how parent somatic cells multiply into daughter cells. It's the means by which organs and tissues repair and regenerate. Mitosis allows infants to grow into adults and is the mechanism by which organs and tissues initially grow and differentiate from stem cells in the developing embryo. The embryonic and fetal stages are the life stages when mitosis occurs most rapidly.
Copy and Divide
During mitosis the somatic cell nucleus copies its DNA then divides once. These nuclei form the basis for two genetically-identical daughter cells. Mitosis occurs over five distinct stages preceded by interphase, a resting period where the cell prepares for mitosis, and followed by cytokinesis, when the cell's cytoplasm divides in two and joins the daughter nuclei to pinch off into distinct cells.
Phases and Changes
Mitosis begins with prophase, when the cell's chromatin gathers together into orderly chromosomes and centrioles position themselves at opposite poles of the cell. Fibers from the centrioles' centromeres form the mitotic spindle, where the action of nuclear division takes place. Prometaphase begins when the nuclear membrane disappears and the chromosomes start heading toward the center of the cell. Metaphase is distinguished by the metaphase plate: the orderly line of chromosomes along the cell's middle. During anaphase the paired chromosomes separate and one homolog moves to each pole. Their arrival at the poles signals telophase, when new nuclear membranes form around each set of daughter chromatids, then the chromosomes leave their orderly chains to become an indistinguishable mass of chromatin once more.
When an ovum and a spermatozoan join they form a zygote, or fertilized egg. This egg performs its first mitotic divisions and becomes a blastocyst, the hollow ball of cells that may go on to become an embryo and fetus. The blastocyst has outer and inner cell masses. The inner cell mass is made up of embryonic stem cells, which rapidly divide and can differentiate into any type of cell. The cells that make up the blastocyst are called blastomeres. The first ones are large, but as mitosis continues they decrease in size and get packed together, so even though mitosis is rapid, the blastula doesn't get bigger. Instead it turns into a gastrula, the first true embryonic stage. The gastrula has three layers of somatic stem cells, which differentiate through mitosis into all the cells comprising the inside, middle and outside of the body. The incredibly rapid mitosis of prenatal life allows a single cell to grow into a complete neonate in a very short period of time, around 40 weeks in the case of human beings, much faster in many other species.
The Rest of Life
Mitosis continues throughout life, though at a far slower rate than in the growing embryo and fetus. It's responsible for growth, healing after injury and illness, and replacing dead cells. In adults it's most rapid in the fast growing, frequently replaced tissues of the body, including hair, skin, fingernails, and gastric and uterine linings.