Regulation of the self-renewal probability in Hydra stem cell clones.

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Abstract
Hydra interstitial stem cells continuously give rise to daughter stem cells as well as precursors for nerve and nematocyte differentiation. Growth of the stem cell population is controlled by the self-renewal probability (Ps); Ps is the fraction of stem cell daughters that remain stem cells in each generation. Ps for Hydra interstitial stem cells was determined using a novel technique based on the cell composition of clones. Stem cell clones were grown in aggregates of nitrogen mustard-inactivated Hydra tissue. They contained several hundred cells after 14 days of growth, including stem cells, differentiating nematocytes and differentiating nerve cells. Clone size, size variability and the ratio of differentiating cells to stem cells were sensitive measures of P2. Standard curves relating these parameters to Ps were prepared using computer simulations of clone growth. Comparison of the experimentally observed parameters of clones to these curves indicated that P2 decreased from 0.8 in 5- to 6-day clones to 0.6 in 10- to 12-day clones. The decrease in Ps coincided with increase in clone size and suggested that P2 may be regulated by the density of stem cells in clones. Such a mechanism could be responsible for the observed homeostasis of stem cell populations in vivo.