Neoblasts and planarian stem cell systems
Neoblasts are the adult stem cells of planarian flatworms. Pietak and colleagues describe them as pluripotent cells that migrate to wounds and form a blastema, which then rebuilds the lost head, trunk or tail.8 Rink argues that the word names a whole stem cell system and not one proven cell type.3 How neoblasts know what to build remains open.9
- Earliest held
- 2009, Nogi T, Zhang D, Chan JD…
- Most discussed in
- Stem cell systems and regeneration in planaria, 2013
- In the library
- 113 passages in 23 works
- Rewritten
- 2026-10-08
Response to injury
Lobo, Beane and Levin (2012) summarise experiments that used markers for neoblasts and their divisions. Simple wounding raises neoblast mitoses throughout the animal. Only loss of tissue sends neoblasts to the wound and produces a second mitotic peak there, which forms the blastema.1 The same review lists signalling steps. ERK activity is needed for neoblasts in the blastema to stop proliferating and begin differentiating, and JNK signalling is needed for blastema formation. They also leave open what drives neoblast migration toward a wound and how positional values are encoded.2
What counts as a neoblast
Rink (2013) warns that statements about what neoblasts do often assume the cells are all alike without hard evidence. It is not known whether every dividing cell is a true stem cell or whether transient amplifying cells exist. Neither shape nor current markers draws an exact line between dividing cells and their earliest committed progeny.3 Rink therefore uses the term for pluripotent cells, their earliest postmitotic progeny and any intermediate stages. He also notes that germ line stem cells look like ordinary neoblasts. Expression of nanos distinguishes them.4
Heterogeneity and fate
Durant and colleagues (2016) report that some neoblasts express lineage-restricted transcription factors during regeneration. This suggests that neoblasts are specialised to a fate before they give rise to blastema cells.7 Neuhof, Levin and Rechavi (2016) point out that a worm has never been shown to regenerate from a single neoblast without surrounding mature tissue. They describe asexual reproduction as arising from a community of cells.6 Neoblasts in one individual also differ from one another genetically, as cited from Nishimura and colleagues (2015).
Patterning and collective control
Emmons-Bell and colleagues (2015) counted neoblasts in the anterior region of three planarian species. Worms whose gap junctions had been blocked took on the shape of another species. Their neoblasts were distributed like those of the wild-type species they resembled.5 The authors conclude that the same patterning processes organise the distribution of dividing cells. McMillen and Levin (2024) say no existing model predicts the head shape after half the neoblasts are replaced with those of another species. They call this a collective decision.9
After wounding, an increase in neoblast mitoses occurs throughout the animal; however, only tissue loss results in neoblast migration to the wound site and a second mitotic peak at the wound resulting in blastema formation
Lobo D, Beane WS, Levin M, 2012 · Modeling planarian regeneration: a primer for reverse-engineering the worm · open at passage 22What are the driving forces that permit neoblasts to migrate in the direction of the wound to initiate blastema formation and how are positional values encoded
Lobo D, Beane WS, Levin M, 2012 · Modeling planarian regeneration: a primer for reverse-engineering the worm · open at passage 42It is therefore important to stress that the term “Neoblasts” in the planarian literature and explicitly also in this text needs to be understood as a general reference to the planarian stem cell system
Rink JC, 2013 · Stem cell systems and regeneration in planaria · open at passage 8Interestingly, the factor that distinguishes somatic Neoblasts from germ line Neoblasts is expression of nanos, so far the only highly conserved germ line determinant that is not constitutively expressed in all Neoblasts
Rink JC, 2013 · Stem cell systems and regeneration in planaria · open at passage 18Remarkably, the distribution of neoblasts in pseudo worms mirrored precisely the distribution of neoblasts in the wild-type species that they resembled
Emmons-Bell M, Durant F, Hammelman J, Bessonov N, Volpert…, 2015 · Gap Junctional Blockade Stochastically Induces Different Species-Specific Head… · open at passage 16generation/regeneration of a worm from a single neoblast without a surrounding mature body has never been shown
Neuhof M, Levin M, Rechavi O, 2016 · Vertically- and horizontally-transmitted memories - the fading boundaries… · open at passage 22Some neoblasts express specific, lineage restricted transcription factors during regeneration and remodeling (Scimone et al. 2014), thus suggesting that neoblasts are specialized to a particular fate
Durant F, Lobo D, Hammelman J, Levin M, 2016 · Physiological controls of large-scale patterning in planarian regeneration: a… · open at passage 9Planaria maintain a population of pluripotent adult stem cells (neoblasts), which migrate to wounds to form a blastema
Pietak A, Bischof J, LaPalme J, Morokuma J, Levin M, 2019 · Neural control of body-plan axis in regenerating planaria · open at passage 2no existing model makes a prediction on what will happen if 50% of the neoblasts of a given planarian are replaced with those of a different species and the head is cut off
McMillen P, Levin M, 2024 · Collective intelligence: A unifying concept for integrating biology across… · open at passage 16