Image courtesy of Alex Lin and Bret Pearson. If the pharynx is removed, the actively dividing neoblasts and their progeny will migrate to the damaged area and begin organ regeneration. The image distinguishes between neoblasts that are in the process of dividing to form new cells (green dots) and those that are not dividing (red dots) in a freshwater planarian called Schmidtea mediterranea. Neoblasts are found throughout the body but are excluded from the pharynx. Blocking stem cell function by irradiation, or by RNA interference knockdown of stem cell-specific genes, prevented regeneration, which suggests that neoblasts have a crucial role in the regeneration of the pharynx. found that incubating flatworms in sodium azide caused the pharynx to be ejected from the body without affecting the rest of the worm, thus allowing them to monitor the process of pharynx regeneration. However, a worm without a pharynx can rapidly regenerate this rather complex organ. The pharynx itself contains no stem cells and cannot regenerate the rest of the worm ( Figure 1). Are there different types of stem cell that form different tissues? Do signals produced by nearby cells cause specific tissues to form? Or is a combination of both stem cell bias and local signalling used? Now, in eLife, Alejandro Sánchez Alvarado and colleagues at the Stowers Institute for Medical Research-including Carolyn Adler as first author-have provided new insights into this question by developing a method to specifically remove the pharynx, the feeding organ of the worm, to study organ-specific regeneration ( Adler et al., 2014). It is still not entirely clear how the stem cells regenerate specific organs. These cells are distributed throughout the body and, when part of the worm has been amputated, they are activated to reform the tissues that have been removed ( Wagner et al., 2011). Regeneration in planarians depends on the presence of stem cells called neoblasts.
In recent years this system has been rediscovered as an experimental model for probing how and why tissues regenerate, with the hope that this will help us to improve tissue repair in our own bodies. In a manner reminiscent of the Sorcerer’s Apprentice, chopping one worm into little pieces will result in a dish full of tiny worms regenerated from the fragments in just a few days. Planarian flatworms are well known for their amazing regenerative capacity.
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