Genetic analysis of developmental mechanisms in hydra. XXI. Enhancement of regeneration in a regeneration-deficient mutant strain by the elimination of the interstitial cell lineage.

The interstitial cell lineage, including interstitial stem cells, nerve cells, and nematocytes, was eliminated from a regeneration-deficient mutant strain (reg-16) of Hydra magnipapillata. The resultant interstitial cell lineage-free (or "epithelial") reg-16 animals showed a marked enhancement in the ability to regenerate head structures. The epithelial reg-16 polyps regenerated nearly the same number of tentacles as was originally present within 8 days after head removal, while interstitial cell lineage-containing (or "complete") reg-16 polyps restored less than one-third of their original tentacle number under the same conditions. Lateral tissue transplantation was used to examine the head activation and inhibition potentials. The gradients of the two potentials along the body axis of intact epithelial 105 (a wild-type strain) and intact epithelial reg-16 polyps were nearly identical to the gradients in their complete counterparts. The changes of the two potentials occurring after head removal in the epithelial 105 animals were also similar to those in the complete 105 animals. However, the postdecapitation changes in the epithelial reg-16 polyps were different from those in complete reg-16 polyps. The changes in the epithelial reg-16 animals were similar to those observed in wild-type hydra while those of complete reg-16 polyps were highly abnormal. These observations suggest that the phenotypic expression of the genetic defect present in the reg-16 mutant strain is attenuated when the interstitial cell lineage is eliminated from its tissue. The role of the interstitial cell lineage in head regeneration and the nature of the defect present in the reg-16 strain are discussed based on the observations made in this and previous related studies.