Neoblast

Neoblast
Distribution of neoblasts in the planarian Schmidtea mediterranea. Neoblasts are found throughout the body, except for the pharynx (blue arrow) and the head tip. Neoblasts are labeled in red. Green labeling shows neoblasts in division.
Details
Gives rise to	Blastema
Anatomical terminology [edit on Wikidata]

Neoblasts (ˈniːəʊˌblæst) are adult stem cells found in planarian flatworms. They are the only dividing planarian cells, and they produce all cell types, including the germline.^[1][2] Neoblasts are abundant in the planarian parenchyma, and comprise up to 30 percent of all cells.^[1] Following injury, neoblasts rapidly divide and generate new cells, which allows planarians to regenerate any missing tissue.^[1]

Characteristics

Neoblasts are somatic adult stem cells that are abundant in planarians. Morphologically, neoblasts are round and small, 5 to 10 μm, and have a large nucleus and scant cytoplasm.^[1] They are the only dividing planarian cells.^[3] Neoblasts are found in the planarian parenchyma across the entire body, outside organ systems.^[1] The only regions that lack neoblasts completely are the pharynx and the head tip.^[4]

Blastema formation

New cells produced by dividing neoblasts form the regenerating blastema.^[5] Hours following the injury, a wound response is initiated.^[5] The initial wound response is characterized by an increase in the number of cell divisions and by expression of injury-response genes.^[6] The expression of genes that are required for regenerating the specific damaged tissues is observed few days following the injury.^[5] The changes in gene expression are followed by the rapid growth of the blastema, and with emergence of new functional tissues.^[5]

Molecular characteristics

Components of chromatoid bodies

Neoblasts have chromatoid bodies, which are electronically dense structures composed of ribonucleoprotein complexes that are possibly responsible for maintaining neoblasts. Two protein components have been found within the chromatoid bodies DjCBC-1 and SpolTud-1, which are homologous to proteins involved in the proliferation of germline cells in other organisms.^[7]

Piwi and the interaction of small RNAs in neoblasts

The Argonaute Piwi sub-family of proteins and the small RNAs that interact with them are essential for germline cell development, cell turnover, epigenetic regulation, and repression of transposable elements. Neoblasts express three Piwi homologs, and expression of the Piwi homolog smedwi-1 is used to distinguish neoblasts from other somatic cells.^[8] The expression of two other Piwi homologs, smedwi-2 and smedwi-3, is essential for neoblasts.^[8][9] Inhibition of smedwi-2 or smedwi-3 gene expression blocks regeneration, impairs tissue maintenance and leads to death.^[8][9]

Neoblast specialization

The gene smedwi-1 is expressed by all neoblasts.^[6]

There are two distinct populations of neoblasts, called zeta and sigma.^[6] Zeta and sigma neoblasts are morphologically similar, but they are characterized by expression of different genes. Sigma neoblasts produce brain, intestine, muscle, excretory, pharynx, and eye cell types. They also lead to cells that become zeta neoblasts. Zeta neoblasts then develop the other epidermal cell types.^[6]

Signaling pathways affecting neoblasts

Wnt signaling pathway activity regulates the planarian anterior-posterior axis. Analysis of the gene Smed-betacatenin-1, encoding a Wnt pathway effector, has revealed its role in regulating the anterior-posterior axis.^[10] Smed-betacatenin-1 expression is required for producing tissues with posterior identity, and inhibition of Smed-betacatenin-1 expression results in animals regenerating anterior tissues (e.g., head) instead of posterior (e.g. tail).^[10]

History

Regeneration research using planarians began in the late 1800s and was popularized by T.H. Morgan at the beginning of the 20th century.^[11] Alejandro Sanchez-Alvarado and Philip Newmark transformed planarians into a model genetic organism in the beginning of the 20th century to study the molecular mechanisms underlying regeneration.^[12] Morgan found that a piece corresponding to 1/279th of a planarian^[11] or a fragment with as few as 10,000 cells could regenerate into a new worm within one to two weeks.^[13] Morgan also found that if both the head and the tail were cut off a flatworm the middle segment would regenerate a head from the former anterior end and a tail from the former posterior end.

Schmidtea mediterranea has emerged as the species of choice for research due to its diploid chromosomes and the existence of both asexual and sexual strains.^[14] Recent genetic screens utilizing double-stranded RNA technology have uncovered 240 genes that affect regeneration in S. mediterranea. Many of these genes have orthologs in the human genome.^[15]

Application

The study of neoblasts helps uncover the mechanisms and functioning of stem cells and tissue degeneration. Planarians can regenerate any body part from small pieces in a few days and have many adult stem cells. They are easy to culture and grown to large populations. Their proteins are similar to human proteins. RNA interference is done by feeding, injecting, or soaking them in double-stranded RNA. The genome of Schmidtea mediterranea has been sequenced. In humans, no known pluripotent stem cells remain after birth.^[16]

A collaborative research community on planarian research, EuroPlanNet, was launched in May 2010.^[16]

References

1. Rink JC (2018). "Stem Cells, Patterning and Regeneration in Planarians: Self-Organization at the Organismal Scale". In Rink JC (ed.). Planarian Regeneration. Methods in Molecular Biology. Vol. 1774. New York, NY: Springer New York. pp. 57–172. doi:10.1007/978-1-4939-7802-1_2. ISBN 978-1-4939-7800-7. PMID 29916155. Retrieved 3 December 2023.
2. Sánchez Alvarado, Alejandro; Kang, Hara (25 April 2005). "Multicellularity, stem cells, and the neoblasts of the planarian Schmidtea mediterranea". Experimental Cell Research. 306 (2): 299–308. doi:10.1016/j.yexcr.2005.03.020. PMID 15925584.
3. Reddien PW, Sánchez Alvarado A (2004). "Fundamentals of planarian regeneration". Annual Review of Cell and Developmental Biology. 20: 725–757. doi:10.1146/annurev.cellbio.20.010403.095114. PMID 15473858.
4. Reddien PW (March 2013). "Specialized progenitors and regeneration". Development. 140 (5): 951–957. doi:10.1242/dev.080499. PMC 3583037. PMID 23404104.
5. Reddien PW (October 2018). "The Cellular and Molecular Basis for Planarian Regeneration". Cell. 175 (2): 327–345. doi:10.1016/j.cell.2018.09.021. PMC 7706840. PMID 30290140.
6. Barresi M, Gilbert S (July 2019). Developmental Biology (12th ed.). Oxford University Press. ISBN 978-1605358222.
7. Yoshida-Kashikawa M, Shibata N, Takechi K, Agata K (December 2007). "DjCBC-1, a conserved DEAD box RNA helicase of the RCK/p54/Me31B family, is a component of RNA-protein complexes in planarian stem cells and neurons". Developmental Dynamics. 236 (12): 3436–3450. doi:10.1002/dvdy.21375. PMID 17994545. S2CID 35919013.
8. Reddien, Peter W.; Oviedo, Néstor J.; Jennings, Joya R.; Jenkin, James C.; Alvarado, Alejandro Sánchez (25 November 2005). "SMEDWI-2 Is a PIWI-Like Protein That Regulates Planarian Stem Cells". Science. 310 (5752): 1327–1330. Bibcode:2005Sci...310.1327R. doi:10.1126/science.1116110. ISSN 0036-8075. PMID 16311336.
9. Palakodeti, Dasaradhi; Smielewska, Magda; Lu, Yi-Chien; Yeo, Gene W.; Graveley, Brenton R. (2 May 2008). "The PIWI proteins SMEDWI-2 and SMEDWI-3 are required for stem cell function and piRNA expression in planarians". RNA. 14 (6): 1174–1186. doi:10.1261/rna.1085008. ISSN 1355-8382. PMC 2390803. PMID 18456843.
10. Petersen CP, Reddien PW (January 2008). "Smed-betacatenin-1 is required for anteroposterior blastema polarity in planarian regeneration". Science. 319 (5861): 327–330. Bibcode:2008Sci...319..327P. doi:10.1126/science.1149943. PMID 18063755. S2CID 37675858.
11. Morgan TH (1900). "Regeneration in Planarians". Archiv für Entwicklungsmechanik der Organismen. 10 (1): 58–119. doi:10.1007/BF02156347. hdl:2027/hvd.32044107333064. S2CID 33712732.
12. Sánchez Alvarado A, Newmark PA (1998). "The use of planarians to dissect the molecular basis of metazoan regeneration". Wound Repair and Regeneration. 6 (4): 413–420. doi:10.1046/j.1524-475x.1998.60418.x. PMID 9824561. S2CID 8085897.
13. Montgomery JR, Coward SJ (July 1974). "On the minimal size of a planarian capable of regeneration". Transactions of the American Microscopical Society. 93 (3): 386–391. doi:10.2307/3225439. JSTOR 3225439. PMID 4853459.
14. Newmark PA, Sánchez Alvarado A (March 2002). "Not your father's planarian: a classic model enters the era of functional genomics". Nature Reviews. Genetics. 3 (3): 210–219. doi:10.1038/nrg759. PMID 11972158. S2CID 28379017.
15. Castaneda B. "Regeneration in S. mediterranea". Developmental Biology. Georgia Tech. Retrieved 31 March 2014.
16. Gentile L, Cebrià F, Bartscherer K (January 2011). "The planarian flatworm: an in vivo model for stem cell biology and nervous system regeneration". Disease Models & Mechanisms. 4 (1): 12–19. doi:10.1242/dmm.006692. PMC 3014342. PMID 21135057. S2CID 2478930.