Somatic cells are any cells forming the body of an organism, as opposed to germline cells. In mammals, germline cells (also known as "gametes") are the spermatozoa and ova which fuse during fertilization to produce a cell called a zygote, from which the entire mammalian embryo develops. Every other cell type in the mammalian body—apart from the sperm and ova, the cells from which they are made (gametocytes) and undifferentiated stem cells—is a somatic cell: internal organs, skin, bones, blood, and connective tissue are all made up of somatic cells.
The word "somatic" is derived from the Greek word sōma, meaning "body".
Genetics and chromosome content
A simple definition of a somatic cell is that it is a non-sex cell (a very broad and not always accurate description). In humans, somatic cells contain 46 individual chromosomes, organized into 23 pairs of chromosomes. Each pair comprises a chromosome inherited from the father and the mother. Human somatic cells contain twice as many chromosomes as germline cells (sex cells). Germline cells contain only 23 chromosomes. When the germline cells meet during conception, they "fuse" together, creating a zygote. The sex of the child is dependent on the chromosome the germline cells contains (X or Y). Due to the "fusion" of the germline cells, a zygote contains 46 chromosomes (i.e. 23 pairs).
In other species, the situation is more complex. Humans, and other species whose somatic cells contain chromosomes arranged in pairs, are known as "diploid" organisms (their germline cells, which contain only single unpaired chromosomes, are known as "haploid"). However, a large number of species arrange the chromosomes in their somatic cells in fours ("tetraploid") or even sixes ("hexaploid") which means that they can have diploid or even triploid germline cells. An example of this is the modern cultivated species of wheat, Triticum Aestivum L., a hexaploid species whose somatic cells contain six copies of every chromatid.
Somatic cells and cloning technology
In recent years, the technique of cloning whole organisms has been developed in mammals, allowing almost identical genetic clones of an animal to be produced. Any retention of existing mitrochodrial DNA prevents the new cell being identical. One method of doing this is called "somatic cell nuclear transfer" and involves removing the nucleus from a somatic cell, usually a skin cell. This nucleus, which contains all of the genetic information needed to produce the organism it was removed from is then injected into an ovum of the same species which has had its own genetic material removed. The ovum now no longer needs to be fertilized as it contains the correct amount of genetic material(a diploid number of chromosomes) and, in theory, it can be implanted into the uterus of a same-species animal and allowed to develop. The resulting animal will be a genetically identical clone to the animal from which the nucleus was taken. In practice, this technique has so far been problematic, although there have been a few high profile successes, such as Dolly the Sheep and, more recently, Snuppy, the first cloned dog.