Mitosis is a fundamental biological process where a single parent cell divides into two genetically identical daughter cells. This process involves the separation of replicated chromosomes into new nuclei. As part of the cell cycle in eukaryotic cells, mitosis ensures each new cell receives complete genetic information from the parent, maintaining genetic stability. This mechanism is important for the growth and maintenance of living organisms.
Mitosis in Somatic Cells
Mitosis primarily occurs in somatic cells, which constitute all the non-reproductive cells of an organism. These cells make up the vast majority of the body’s tissues and organs. The division of somatic cells by mitosis is important for several biological functions, including growth, repair, and maintenance.
During an organism’s development, mitosis increases the total number of cells, allowing for growth from a single-celled zygote into a multicellular body. For instance, a growing child’s bones and muscles expand due to the continuous mitotic division of their respective cells.
Mitosis also plays an important role in repairing damaged tissues and replacing old or worn-out cells. For example, skin cells regularly undergo mitosis to replace those that are shed. Cells lining the digestive tract divide frequently to maintain the intestinal lining. Hair follicle cells also divide continuously, contributing to hair growth. Bone marrow cells divide to produce various blood cells, including red blood cells and immune cells, which require constant replenishment.
Mitosis in Germline Cells
While mature sperm and egg cells (gametes) are produced through meiosis, their precursor cells, called germline stem cells, undergo mitosis. These germline stem cells reside in microenvironments within the gonads. In males, spermatogonia are germline stem cells, and in females, oogonia are the female equivalent.
The purpose of mitosis in these germline stem cells is to proliferate and maintain a continuous supply of cells that can eventually enter meiosis. For instance, spermatogonia undergo mitotic divisions to produce more spermatogonia, some of which then differentiate and proceed through meiosis to form mature sperm.
This mitotic process in germline stem cells is distinct from meiotic divisions that reduce the chromosome number by half to form haploid gametes. Mitotic divisions maintain the diploid chromosome number in the stem cell population. Only after sufficient proliferation do these cells commit to the meiotic pathway, leading to the formation of genetically diverse gametes.
Cells with Limited or No Mitotic Activity
Not all cells in the body continuously undergo mitosis. Some specialized cells, once mature, either rarely or never divide. This characteristic is linked to their specific functions and the complexity of their cellular structure.
Mature neurons, or nerve cells, are an example; they are terminally differentiated and lose their ability to divide after birth. This lack of mitotic activity relates to their intricate, interconnected structures essential for transmitting electrical signals. Similarly, mature skeletal muscle cells and cardiac muscle cells exhibit limited or no mitotic activity. These cells are specialized for contraction and have complex internal organization, making cell division challenging once established.
Mature red blood cells also do not undergo mitosis. Unlike most other cells, they lack a nucleus and other organelles necessary for cell division. Their main function is oxygen transport, and their anucleated state maximizes space for hemoglobin. These cells are continuously produced by mitotic divisions of precursor cells in the bone marrow. The inability of these specialized cells to divide means that damage or loss to these tissues results in permanent functional impairment or requires other repair mechanisms.