Regeneration and Pattern Formation in Planarians--Cells,Molecules and Genes
The process of regeneration in freshwater planarians (Platyhelminthes ; Turbellaria ; Tricladida) is reviewed. Long-standing questions such as time and role of wound healing, the origin of blastema cells and the time and mode of determination of lost structures have come to be solved in recent years due to the use of old and new cell labelling and transplantation methods. In most turbellarian species wound healing occurs in less than 1 hour. Sound evidence has been produced to support the origin of blastema cells from undifferentiated stem-cells or neoblasts, and determination of lost structures takes place during the first two days of regeneration following a disto-proximal sequence. These results suggest that epithelial-mesenchymal interactions, brought up by wound healing, may be instrumental, through cell-cell interactions or activating transient morphogenetic gradients, in setting the early pattern of lost structures upon the equivalent blastema and postblastema cells. We are fully ignorant, however, on the molecular nature of substances playing such roles as well as on how positional information along the antero-posterior and dorso-ventral axes is set and maintained in the intact organism and reset again and transmitted to blastema cells in regenerating organisms. Molecular biology and recombinant DNA techniques brought into the field of planarian regeneration in the last 10years have opened new research perspectives but produced limited results. As expected, signals, receptors and transducing molecules which activate cell proliferation and differentiation in planarians were found to be not different to those known to operate in other activated systems. Instead, monoclonal antibodies (mAbs) against positional specific antigens have recently been obtained and found to be very sensitive to changes in positional values during regeneration. Homeobox containing genes, namely those belonging to the HOM/Hox Antennapedia class have been detected and sequenced in different species. Whether or not they are clustered in the genome, how are they expressed along the antero-posterior body axis in both intact and regenerating organisms, and how are they linked to the positional markers detected by mAbs are key questions to answer and a matter of intense research. Finally, the recent finding of active transposable elements in some species of planarians opens the way to obtain transformed neoblasts and, ultimately, transgenic planarians. When available, this would represent an extremely useful tool to study cell lineage, as well as to test the function of cloned genes and to induce loss and gain of function mutations. Based on these results and perspectives, a new research agenda is suggested.
- Zoological Science
Zoological Science 11(6), 781-795, 1994-12
Zoological Society of Japan