Regeneration and pattern formation in planarians

In planarians, blastema cells do not divide, and growth of blastema is thought to result from the steady accumulation, beneath the wound epithelium, of undifferentiated cells produced by cell division in the stump. However, whether these cells come only from local stump sources or whether cells placed far from the wound can also participate, after long-range migrations, in the growth of blastema, is still uncertain. To study this problem, we have analysed three basic parameters of the process of regeneration: cell kinetics of blastema growth; number of cells produced by mitosis in the stump areas near the wound (postblastema); and rates of movement ('migration') of undifferentiated cells using grafting procedures with nuclear and chromosomal markers. The results show that: (1) cells near the wound area spread (move) ar^iigher rates than cells placed far from it (90-140 urn day"' versus 40-50jonday~); (2) cells originally placed farther than 500 fan from the wound boundary are hardly represented within 3and 5-day-old blastemata; and (3) the number of cells produced by mitosis within a 200-300 /on postblastema area around the wound seem sufficient to explain, provided their rates of movement are taken into account, the increasing number of blastema cells. From this, it is concluded that blastema cells in planarians originate from local stump areas, and that mitotic activity jointly with local cell movement within a 200-300 fim postblastema area around the wound match the increasing number of blastema cells during regeneration. The implications of these results for blastema growth and pattern formation mechanisms are discussed.