Cellular aspects of regeneration hormone influence in Nereis diversicolor.

The histology of caudal regeneration in Nereis diversicolor is described with special reference to the role of the cerebral 'regeneration' hormone. Wound healing is complete after 8 days; the regenerated stump of tissue then consists of the pygidium, associated anal cirri and a prepygidial region between the pygidium and the posterior margin of the last intact segment. The term segment blastema is used to refer to the cells from which all segmental structures are derived. The segment blastema is situated in the ventral posterior margin of the prepygidial region, and consists of transverse bands of cells with large nuclei (12 fim diameter) and prominent nucleoli. Segment anlagen are formed immediately in front of the segment blastema; the development of the anlagen is described, and six stages of development are defined. The extent of caudal regeneration in different animals is best compared with reference to the total number and stage of development reached by the segment anlagen. During normal regeneration at 18 °C, segment anlagen are first formed after 8 days, and they continue to be formed at the same rate until at least the 21st day after caudal ablation. The oldest segments reach stage 6 after about 15 days of regeneration. Wound healing, pygidium formation, and cirrus development and establishment of the segment blastema all occur in decerebrate animals following the loss of caudal segments, but segment anlagen formation is almost completely inhibited. Implantation of ganglia taken from intact donor animals into the coelom of decerebrate animals which have lost caudal segments initiates segment anlagen formation. The segment blastema of decerebrate animals remains competent to respond in this way for at least 15 days. The regeneration hormone produced by the cerebral ganglion is essential for continued segment anlagen production throughout the 2nd and 3rd weeks of regeneration at 18 °C. Delayed decerebration leads to an arrest of anlagen production, and anlagen younger than stage 3 fail to develop further. Older anlagen (stages 4 and 5) are independent of the regeneration hormone and can continue to differentiate in its absence.