Somatic cell

See also: Somatic (biology)

In cellular biology, a somatic cell (from Ancient Greek σῶμα (sôma) 'body'), 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.^[1] Somatic cells compose the body of an organism and divide through the process of binary fission and mitotic division.

In contrast, gametes are cells that fuse during sexual reproduction and germ cells are cells that give rise to gametes. Stem cells also can divide through mitosis, but are different from somatic in that they differentiate into diverse specialized cell types.

In mammals, somatic cells make up all the internal organs, skin, bones, blood and connective tissue, while mammalian germ cells give rise to spermatozoa and ova 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.^[1]

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 sponges, 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.^[2]

Evolution

As multicellularity was theorized to be evolved many times, so did sterile somatic cells.^{[citation needed]} The evolution of an immortal germline producing specialized somatic cells involved the emergence of mortality, and can be viewed in its simplest version in volvocine algae.^[3] Those species with a separation between sterile somatic cells and a germline are called Weismannists. Weismannist development is relatively rare (e.g., vertebrates, arthropods, Volvox), as many species have the capacity for somatic embryogenesis (e.g., land plants, most algae, and numerous invertebrates).^[4][5]

Genetics and chromosomes

Like all cells, somatic cells contain DNA arranged in chromosomes. 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 mutations is significantly lower in advanced male germ cells than in somatic cell types from the same individual.^[6] Female germ cells also show a mutation frequency that is lower than that in corresponding somatic cells and similar to that in male germ cells.^[7] These findings appear to reflect employment of more effective mechanisms to limit the initial occurrence of spontaneous mutations in germ cells than in somatic cells. Such mechanisms likely include elevated levels of DNA repair enzymes that ameliorate most potentially mutagenic DNA damages.^[7]

Cloning

 

Schematic model of somatic cell nuclear transfer. This technique has been used to create clones of an organism or in therapeutic medicine.

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 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.^[8] 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 mitochondrial 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)^[9] and, more recently, Snuppy (April 24, 2005 - May 2015), the first cloned dog.^[10]

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.^[11] 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.^[12]

Genetic modifications

 

Schematic of CRISPR based gene editing technique

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.^[13][14] 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^{[citation needed]}, although the International Summit on Human Gene Editing has released a statement in support of genetic modification of somatic cells, as the modifications thereof are not passed on to offspring.^[15]

See also

• Somatic cell count
• List of biological development disorders

References

1. Campbell NA, Reece JB, Urry LA, Cain ML, Wasserman SA, Minorsky PV, Jackson RB (2009). Biology (9th ed.). Pearson Benjamin Cummings. p. 229. ISBN 978-0-8053-6844-4.
2. Chernis PJ (1985). "Petrographic analysis of URL-2 and URL-6 special thermal conductivity samples". Department Cf Energy, Mines, and Resources. Earth Physics Branch, Report. 8: 20. doi:10.4095/315247.
3. Hallmann A (June 2011). "Evolution of reproductive development in the volvocine algae". Sexual Plant Reproduction. 24 (2): 97–112. doi:10.1007/s00497-010-0158-4. PMC 3098969. PMID 21174128.
4. Ridley M (2004) Evolution, 3rd edition. Blackwell Publishing, p. 29-297.
5. Niklas, K. J. (2014) .
6. Walter CA, Intano GW, McCarrey JR, McMahan CA, Walter RB (August 1998). "Mutation frequency declines during spermatogenesis in young mice but increases in old mice". Proceedings of the National Academy of Sciences of the United States of America. 95 (17): 10015–10019. Bibcode:1998PNAS...9510015W. doi:10.1073/pnas.95.17.10015. PMC 21453. PMID 9707592.
7. Murphey P, McLean DJ, McMahan CA, Walter CA, McCarrey JR (January 2013). "Enhanced genetic integrity in mouse germ cells". Biology of Reproduction. 88 (1): 6. doi:10.1095/biolreprod.112.103481. PMC 4434944. PMID 23153565.
8. Wilmut, Ian; Bai, Yu; Taylor, Jane (2015-10-19). "Somatic cell nuclear transfer: origins, the present position and future opportunities". Philosophical Transactions of the Royal Society B: Biological Sciences. 370 (1680): 20140366. doi:10.1098/rstb.2014.0366. ISSN 0962-8436. PMC 4633995. PMID 26416677.
9. "The Life of Dolly | Dolly the Sheep". Retrieved 2023-12-09.
10. Kim, Min Jung; Oh, Hyun Ju; Kim, Geon A; Setyawan, Erif Maha Nugraha; Choi, Yoo Bin; Lee, Seok Hee; Petersen-Jones, Simon M.; Ko, CheMyong J.; Lee, Byeong Chun (2017-11-10). "Birth of clones of the world's first cloned dog". Scientific Reports. 7 (1): 15235. Bibcode:2017NatSR...715235K. doi:10.1038/s41598-017-15328-2. ISSN 2045-2322. PMC 5681657. PMID 29127382.
11. Bolton, Rhiannon L; Mooney, Andrew; Pettit, Matt T; Bolton, Anthony E; Morgan, Lucy; Drake, Gabby J; Appeltant, Ruth; Walker, Susan L; Gillis, James D; Hvilsom, Christina (2022-07-01). "Resurrecting biodiversity: advanced assisted reproductive technologies and biobanking". Reproduction and Fertility. 3 (3): R121–R146. doi:10.1530/RAF-22-0005. ISSN 2633-8386. PMC 9346332. PMID 35928671.
12. Sun, Yanyan; Li, Yunlei; Zong, Yunhe; Mehaisen, Gamal M. K.; Chen, Jilan (2022-10-09). "Poultry genetic heritage cryopreservation and reconstruction: advancement and future challenges". Journal of Animal Science and Biotechnology. 13 (1): 115. doi:10.1186/s40104-022-00768-2. ISSN 2049-1891. PMC 9549680. PMID 36210477.
13. Jarrett KE, Lee CM, Yeh YH, Hsu RH, Gupta R, Zhang M, et al. (March 2017). "Somatic genome editing with CRISPR/Cas9 generates and corrects a metabolic disease". Scientific Reports. 7: 44624. Bibcode:2017NatSR...744624J. doi:10.1038/srep44624. PMC 5353616. PMID 28300165.
14. "NIH Commits $190M to Somatic Gene-Editing Tools/Tech Research". 24 January 2018. Retrieved 5 July 2018.
15. "Why Treat Gene Editing Differently In Two Types Of Human Cells?". 8 December 2015. Retrieved 5 July 2018.