Reproductive cells undergo meiosis. Non-reproductive cells, or somatic cells, undergo mitosis. When you consider the fact that there are trillions more somatic cells than reproductive cells in the human body, it becomes clear that mitosis occurs much more often than meiosis. Mitosis and meiosis share similarities in that one cell splits to make more cells, but the differences between them are crucial for understanding why these two fundamental biological processes lead to the beautiful complexity of life forms.
Reproductive Cells Are Meiotic
The only meiotic cells in the human body are the reproductive cells found in the testes of men and the ovaries of women. Human reproductive cells begin with a full set of human chromosomes -- that is, 23 pairs of them. They undergo meiosis to produce sperm or eggs, but these daughter cells are not identical to each other nor to the mother cell that gave rise to them. Sperm and egg cells have only half the number of chromosomes that the mother cells have. When mother cells divide during meiosis, many different combinations of chromosomes can result in the daughter cells -- 8,388,608 of them. With more than 8 million possible unique egg cells in a woman and more than 8 million possible unique sperm cells in a man, over 70 trillion genetically unique individuals can result. Meiosis is restricted to the reproductive organs and does not occur in the rest of the body.
Somatic Cells Are Mitotic
Cells that do not produce sperm or eggs are called somatic cells. Somatic cells cannot undergo meiosis; they can only undergo mitosis. The purpose of mitosis is for cells to produce identical copies of themselves. Somatic cells are the ones that make up various tissues of the body. They are responsible for functions such as absorbing digested food, lining airways, growing hair, and forming new skin. The majority of the body’s cells are somatic. After an egg cell is fertilized and begins to develop into an embryo, the cells not only divide but also differentiate and take on new identities, becoming different tissues and organs that make up the diverse parts of the body.
Somatic Cells Outnumber Reproductive Cells
The human body has 60-90 trillion cells. The somatic tissues and organs (such as the liver, intestines, skin, and bones) collectively have much greater mass than the testes or ovaries, meaning they have many more cells. Most somatic cells -- with the exception of nerve cells, striated muscle cells, and some other cell types in adult humans -- repeatedly undergo mitosis. Thus, more mitosis occurs in the human body than meiosis.
Meiosis in Females and Males
A woman has about one million egg cells in her ovaries at the time of her birth. Egg cells develop when “mother” egg cells called oogonia undergo meiosis. It used to be believed that after birth, a woman’s ovaries could no longer produce new eggs, but scientists now debate this. However, it is still believed that most of a woman's egg cells have developed to the "pre-egg" stage (an early stage of meiosis) by the time she is born. The pre-egg cell does not actually complete meiosis until after fertilization. This is another reason mitosis happens more frequently than meiosis, especially in women. Nearly all of a woman’s somatic cells continue to divide throughout her lifetime, so mitosis occurs continually. In males, meiosis begins during adolescence and continues occurs throughout his lifetime, meaning he continually produces sperm.