Neoblast
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| 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 | |
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]
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]