In cellular biology, a somatic cell (), or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell.
In contrast, gametes derive from meiosis within the of the germline and they fuse during sexual reproduction. Stem cells also can divide through mitosis, but are different from somatic in that they differentiate into diverse specialized cell types.
In , somatic cells make up all the internal organs, skin, bones, blood and connective tissue, while mammalian germ cells give rise to spermatozoa and ovum which fuse during fertilization to produce a cell called a zygote, which divides and differentiates into the cells of an embryo. There are approximately 220 types of somatic cell in the human body.
Theoretically, these cells are not germ cells (the source of gametes); they transmit their mutations, to their cellular descendants (if they have any), but not to the organism's descendants. However, in , non-differentiated somatic cells form the germ line and, in Cnidaria, differentiated somatic cells are the source of the germline. Mitotic cell division is only seen in diploid somatic cells. Only some cells like germ cells take part in reproduction.
Evolution
As
multicellularity was theorized to be evolved many times,
so did sterile somatic cells. The evolution of an immortal
germline producing specialized somatic cells involved the emergence of mortality, and can be viewed in its simplest version in
Volvocales algae.
Those species with a separation between sterile somatic cells and a germline are called
Weismann barrier. Weismannist development is relatively rare (e.g.,
,
,
Volvox), as many species have the capacity for somatic embryogenesis (e.g.,
, most
algae, and numerous
invertebrates).
[Ridley M (2004) Evolution, 3rd edition. Blackwell Publishing, p. 29-297.][Niklas, K. J. (2014) The evolutionary-developmental origins of multicellularity.]
Genetics and chromosomes
Like all cells, somatic cells contain
DNA arranged in
. If a somatic cell contains chromosomes arranged in pairs, it is called
diploid and the organism is called a diploid organism. The gametes of diploid organisms contain only single unpaired chromosomes and are called
haploid. Each pair of chromosomes comprises one chromosome inherited from the father and one inherited from the mother. In humans, somatic cells contain 46
chromosomes organized into 23 pairs. By contrast, gametes of diploid organisms contain only half as many chromosomes. In humans, this is 23 unpaired chromosomes. When two gametes (i.e. a spermatozoon and an ovum) meet during conception, they fuse together, creating a
zygote. Due to the fusion of the two gametes, a human zygote contains 46 chromosomes (i.e. 23 pairs).
A large number of species have the chromosomes in their somatic cells arranged in fours ("tetraploid") or even sixes ("hexaploid"). Thus, 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.
The frequency of spontaneous is significantly lower in advanced male than in somatic cell types from the same individual.
Cloning
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. One method of doing this is called "somatic cell nuclear transfer" and involves removing the
cell nucleus from a somatic cell, usually a skin cell. This nucleus contains all of the genetic information needed to produce the organism it was removed from. This nucleus 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, because it contains the correct amount of genetic material (a
diploid number of
chromosomes). In theory, the ovum can be implanted into the
uterus of a same-species animal and allowed to develop. The resulting animal will be a nearly genetically identical clone to the animal from which the nucleus was taken. The only difference is caused by any
DNA that is retained in the ovum, which is different from the cell that donated the nucleus. In practice, this technique has so far been problematic, although there have been a few high-profile successes, such as Dolly the Sheep (July 5, 1996 - February 14, 2003)
and, more recently,
Snuppy (April 24, 2005 - May 2015), the first cloned
dog.
Biobanking
Somatic cells have also been collected in the practice of biobanking. The cryoconservation of animal genetic resources is a means of conserving animal genetic material in response to decreasing ecological biodiversity.
As populations of living organisms fall so does their genetic diversity. This places species long-term survivability at risk. Biobanking aims to preserve biologically viable cells through long-term storage for later use. Somatic cells have been stored with the hopes that they can be reprogrammed into induced pluripotent stem cells (iPSCs), which can then differentiate into viable reproductive cells.
Genetic modifications
Development of
biotechnology has allowed for the genetic manipulation of somatic cells, whether for the modelling of chronic disease or for the prevention of malaise conditions.
Two current means of gene editing are the use of transcription activator-like effector nucleases (TALENs) or clustered regularly interspaced short palindromic repeats (CRISPR).
Genetic engineering of somatic cells has resulted in some controversies,
Cellular aging
In mammals a high level of repair and maintenance of cellular DNA appears to be beneficial early in life. However, some types of cell, such as those of the brain and muscle, undergo a transition from mitotic cell division to a post-mitotic (non-dividing) condition during early development, and this transition is accompanied by a reduction in
DNA repair capability.
This reduction may be an evolutionary adaptation permitting the diversion of cellular resources that were earlier used for DNA repair, as well as for
DNA replication and
cell division, to higher priority neuronal and muscular functions. An effect of these reductions is to allow increased accumulation of DNA damage likely contributing to cellular aging.
See also
-
Somatic cell count
-
List of biological development disorders
