Somatic cells are all the cells that make up an organism, including the organ, muscle, fat, bone, and skin cells. The only exceptions are the egg and sperm cells, also called germ cells, which are involved in sexual reproduction. Although somatic cells differ greatly in their form and function, within a single organism they all contain exactly the same DNA. This is possible because different types of cells express their DNA differently. These cells are used in many types of research and are an important part of adult stem cell technology, as well as cloning science.
Most somatic cells are ordered in much the same way. Each contains a nucleus at its center, within which its chromosomes are contained. The cell is full of a liquid called cytosol, which is water and dissolved salts. Structures called organelles float in the cytosol, along with organic molecules, such as proteins and lipids. Organelles are organized groupings of molecules with specific functions, such as protein production, energy production, and breakdown of cellular waste.
The somatic cells of an animal species contain a specific number of chromosomes that must be maintained for the cells to work properly. In humans, they each contain 46 chromosomes, which includes 22 pairs of homologous chromosomes and one pair that is non-homologous. Both chromosomes in a homologous pair contain the same genes in the same order, but the exact DNA sequence of each gene differs between the pair. Each of the homologous chromosome pairs are designated by a number, from 1 to 22.
The non-homologous pair are the sex chromosomes, which are designated X or Y. The pairing of these chromosomes determines whether a fertilized egg, or zygote, develops into a female or a male. There are rare exceptions, but in general, each egg carries a single X chromosome, and each sperm cell has an X or a Y. When an egg is fertilized by an X-carrying sperm, the resulting zygote develops as female; when an egg fuses with a Y-carrying sperm, the zygote is male.
Germ cells each contain 23 chromosomes. When an egg and sperm fuse to produce a zygote, the new cell therefore has the full complement of 46, with one set of 23 from the egg and one from the sperm. All of the somatic cells that develop from the zygote have the same 46 chromosomes.
Somatic cells come in many types, each with different properties. Cells called osteoblasts create bone by depositing calcium phosphate on a collagen framework. The cells of one small part of the heart, called the sinoatrial node, are responsible for controlling the pace of the heartbeat. In the stomach, three specific cell types secrete the digestive enzymes that break down food. All of these different types have the same DNA.
This is possible because of a process called DNA expression. Even though all the cells have the same genes in the same combinations, each cell uses the genes in a different way. DNA expression refers to whether or not a specific gene is currently being used as a template to create proteins. When a gene is actively being expressed, it means that the cell in question is producing new copies of the protein associated with the gene. Each cell type in the body expresses different gene combinations, allowing the cells to have different properties even though they are all genetically identical.
Somatic cells replicate in two stages. The first stage is called mitosis, in which the DNA content of the cell is replicated. In the next stage, cytokinesis, the cell divides. This leaves each of the two new cells with one full set of 46 chromosomes, and approximately half of the organelles and cytosol of the parent cell.
Somatic Stem Cells
All somatic cells are derived from the same cell type, called a pluripotent stem cell. The richest source of these cells is in the bone marrow of large bones such as the femur. Pluripotent stem cells are unique in that they can differentiate into any other cell type in the body; however, once this differentiation occurs, the cell is fixed in its new state, and cannot turn into another cell type.
Scientists working with stem cells have discovered that it is possible to turn some types of differentiated cells back into stem cells. This is done by “reprogramming” the cells to express certain crucial genes that force the cells to return to a pluripotent state. Further research in this area might result in new ways to treat genetic diseases such as sickle-cell anemia, as well as many types of cancer.
This cell type is used in another area of experimental research called cloning. A clone is an exact copy of something; in research, it can refer to a technique called somatic-cell nuclear transfer, by which an entire organism can be produced. In this technique, the nucleus of a somatic cell is transplanted into an egg from the same species. The egg has been stripped of its own DNA, so the addition of the nucleus means the egg has a full complement of chromosomes, and does not need to be fertilized.
Once the nuclear transfer is complete, the egg is implanted into the uterus of a female of the same species. If it gestates successfully and is born, the cells of the new organism are clones of the original cell. The first animal to be successfully cloned from a somatic cell was a sheep named Dolly, who was born in 1996. Since then, animals of several other species, including rats, cats, dogs, and cattle, have been cloned.