DNA synthesis during the first stages of anterior regeneration in the polychaete annelid Owenia fusiformis (dedifferentiation and early phases of differentiation).

We have analysed DNA synthesis in early phases of regeneration in a marine Polychaete Annelid, Owenia fusiformis. The length and efficiency of the prereplicative phase was found to vary with the diurnal rhythm of activity of the animal; that is, it depends on the initial state of the cell population at the moment of the onset of proliferative stimulatin. When animals were operated on at 12 a.m., the duration of the prereplicative phase of the first cells stimulated to proliferate was found to be 12 h. The remaining cells entered the S-phase progressively in waves until the 3rd day following amputation when nearly 100% of the blastema cells were stimulated. At that time the cell-cycles of these dividing cells were found to be highly synchronized. Blastema differentiation takes place on the 4th day and is initiated by stomodeum formation. During the differentiation phase, DNA synthesis is restricted to small areas of the regenerating part. The system described is viewed as a new instrument for investigating the control of the cell cycle in synchronized and subsequently differentiating tissue cells.