Nerve repair and behavioral recovery following brain transplantation in Notoplana acticola, a polyclad flatworm

Regeneration of the flatworm Prosthiostomum siphunculus (Polycladida, Platyhelminthes)

Fueled by the discovery of head regeneration in triclads (planarians) two and a half centuries ago, flatworms have been the focus of regeneration research. But not all flatworms can regenerate equally well and to obtain a better picture of the characteristics and evolution of regeneration in flatworms other than planarians, the regeneration capacity and stem cell dynamics during regeneration in the flatworm order Polycladida are studied. Here, we show that as long as the brain remained at least partially intact, the polyclad Prosthiostomum siphunculus was able to regenerate submarginal eyes, cerebral eyes, pharynx, intestine and sucker. In the complete absence of the brain only wound closure was observed but no regeneration of missing organs. Amputated parts of the brain could not be regenerated. The overall regeneration capacity of P. siphunculus is a good fit for category III after a recently established system, in which most polyclads are currently classified. Intact animals showed proliferating cells in front of the brain which is an exception compared with most of the other free-living flatworms that have been observed so far. Proliferating cells could be found within the regeneration blastema, similar to all other flatworm taxa except triclads. No proliferation was observed in epidermis and pharynx. In pulse-chase experiments, the chased cells were found in all regenerated tissues and thereby shown to differentiate and migrate to replace the structures lost upon amputation.

Transplantation and restoration of functional synapses between an identified neuron and its targets in the intact brain of Lymnaea stagnalis

Most information available to date regarding the cellular and synaptic mechanisms of target cell selection and specific synapse formation has primarily come from in vitro cell culture studies. Whether fundamental mechanisms of synapse formation revealed through in vitro studies are similar to those occurring in vivo has not yet been determined. Taking advantage of the regenerative capabilities of adult molluscan neurons, we demonstrate that when transplanted into the host ganglia an identified neuron reestablishes its synaptic connections with appropriate targets in vivo. This synaptogenesis, however, was possible only if the targets were denervated from the host cell. Specifically, the giant dopamine neuron right pedal dorsal 1 (RPeD1) located in the pedal ganglia was isolated from a donor brain and transplanted into the visceral ganglia of the recipient brain. We discovered that within 2-4 days the transplanted RPeD1 exhibited extensive regeneration. However, simultaneous intracellular recordings failed to reveal synapses between the transplanted cell and its targets in the visceral ganglia, despite physical overlap between the neurites. To test whether the failure of a transplanted cell to innervate its target was due to the fact that the targets continued to receive input from the native RPeD1, the latter soma was surgically removed prior to the transplantation of RPeD1. Even after the removal of host soma, the transplanted RPeD1 failed to innervate the targets such as visceral dorsal 4 (VD4)-despite extensive regeneration by the transplanted cell. However, when RPeD1 axon was allowed to degenerate completely, the transplanted RPeD1 successfully innervated all of its targets and these synapses were similar to those seen between host RPeD1 and its targets. Taken together, our data demonstrate that the transplanted cells will innervate their potential targets only if the targets were denervated from the host cell. These data also lend support to the idea that, irrespective of their physical location in the brain, the displaced neurons are able to regenerate, recognize their targets, and establish specific synapses in the nervous system.

Photonegative response in brown planaria (Dugesia tigrina) following regeneration

The photonegative response was investigated in regenerated brown planaria (Dugesia tigrina). Old, middle, and young animals were tested, bisected into heads and tails, and allowed to regenerate. Different regeneration times affected function, with generally older planaria achieving full functionality of the photonegative response before younger planaria. Counterintuitively, the heads of the middle and young lost this function initially (despite only needing to regenerate their tails) and regained it over time. The size of the animal was not a factor in the photonegativity response. The biological basis for the photonegative response is complex, requiring nervous and locomotive system function and integration. Using the heads from young planaria may serve as a model in aging, degenerative diseases, or environmental toxins.