Homeostatic recovery of interstitial cell populations in Hydra

The growth of interstitial cell populations in Hydra magnipapillata was examined following transplantation of small numbers of interstitial cells into "epithelial animals" which lacked all cell types in the interstitial cell lineage. The distribution pattern of transplanted interstitial cells during the growth phase was examined by staining whole animals with toluidine blue and cell numbers were determined by maceration. The following results were obtained: (1) Transplanted interstitial cells formed a contiguous patch which spread distoproximally but not circumferentially. (2) The displacement of interstitial cells from parents to buds was a random process; buds incorporated interstitial cells only when they were formed in the vicinity of the patch. (3) Interstitial cells increased exponentially in number with a doubling time of 1.8 days for at least 10 days after transplantation, which is faster than the normal doubling time of 2.8 days. (4) The self-renewal probability at low interstitial cell levels was estimated to be 0.72, which was higher than the normal value of 0.64. This increase was attained by lowering the fraction of nematocyte differentiation. These results indicate that the homeostatic recovery of interstitial cell populations is attained by increasing the self-renewal probability rather than by preferential retention of interstitial cells in parent animals at the expense of buds (Heimfeld, 1985).