No head regeneration here: regeneration capacity and stem cell dynamics of Theama mediterranea (Polycladida, Platyhelminthes)

Abstract

Research on the regeneration potential of flatworms (Platyhelminthes) has been mainly undertaken with planarians (Tricladida), where most species can regenerate a head and no proliferation takes place in the blastema, i.e. the early undifferentiated regenerative tissue. Only few studies are available for an early-branching group within the Platyhelminthes, the Polycladida. Head regeneration in polyclads is not possible, with a single exception from a study performed more than 100 years ago: Cestoplana was reported to be able to regenerate a head if cut a short distance behind the brain. Here, we show that ‘Cestoplana’ was misdetermined and most likely was the small interstitial polyclad Theama mediterranea. We revisited regeneration capacity and dynamics of T. mediterranea with live observations and stainings of musculature, nervous system, and proliferating and differentiating stem cells. In our experiments, after transversal amputation, only animals retaining more than half of the brain could fully restore the head including the brain. If completely removed, the brain was never found to regenerate to any extent. Different from planarians, but comparable to other free-living flatworms we detected cell proliferation within the posterior regeneration blastema in T. mediterranea. Similar to other free-living flatworms, proliferation did not occur within, but only outside, the differentiating organ primordia. Our results strongly imply that brain regeneration in the absence of the latter is not possible in any polyclad studied so far. Also, it appears that proliferation of stem cells within the regeneration blastema is a plesiomorphy in flatworms and that planarians are derived in this character.

Regeneration of the flatworm Prosthiostomum siphunculus (Polycladida, Platyhelminthes)

“…The acotylean Notocomplana humilis was found to regenerate the anterior half of the brain (Ishida 1998) as did the cotylean Theama mediterranea (Bertemes et al 2020). To date, T. mediterranea remains the sole cotylean shown to regenerate the anterior half of its brain as we were not able to observe brain regeneration after horizontal cuts through the brain of P. siphunculus.…”

“…In triclads, the brain induces eye regeneration and eyes cannot regenerate in the absence of the brain (Brøndsted 1969). Interestingly, in the polyclad T. mediterranea, some eyes were regenerated even in the complete absence of the brain if cut just posterior of the brain (Bertemes et al 2020). This indicates that also (nerve) cells outside the brain capsule can induce eye regeneration, at least in T. mediterranea.…”

“…but rather outside at the boundary of the wound site (Saló and Baguñà 1984). In this regard, Tricladida differ from any other studied free-living flatworm taxa, which all show a high abundance of proliferating cells within the regeneration blastema, which suggests that this proliferation-free blastema is an apomorphy of the Tricladida (Egger et al 2009a;Dirks et al 2012;Girstmair et al 2014;Bertemes et al 2020). The only proliferating cells in flatworms are undifferentiated mesenchymal cells called neoblasts.…”

“…While planarians have been the focus of many detailed regeneration studies, polyclads have not been studied to this extent. To this date, only few studies focus on polyclad regeneration in high detail, with acotylean species being much more often the focus of such studies than cotylean species (Bertemes et al 2020). Hence, not much is known about the regeneration capacity of cotylean species.…”

“…Until now, the regeneration capacity of only nine cotylean species has been mentioned in previous studies. Out of these, only two species were studied in high detail regarding their regeneration capacities: Thysanozoon brocchi (Monti 1900;Levetzow 1939) and Theama mediterranea (Child 1905b;1905c;1905d;Bertemes et al 2020).…”