Interstitial stem cells in Hydra are a remarkably accessible stem cell system for analyzing stem cell behavior in vivo. Hydra are small fresh water polyps which can be easily maintained in laboratory culture. They grow by asexual budding. Adult polyps contain 50-100.000 cells and give rise to buds with 20-30.000 cells. In well-fed cultures adult polyps form one bud per day.
Adult Hydra polyps contain 3-4000 interstitial stem cells scattered throughout the body column. Interstitial stem cells differentiate to nerve cells, nematocytes and gland cells continuously in asexually budding hydra and also differentiate sperm and egg during sexual reproduction. We have recently developed a transgenic stem cell lineage expressing GFP under the control of the EF1-alpha promoter. Both interstitial stem cells and differentiated products express the GFP label and can be clearly visualized in vivo in normal polyps. This permits imaging of stem cell lineages as they develop in vivo.
Previous work using the vital carbocyanin dye DiI has shown that stem cells divide asymmetrically to yield one stem cell and one differentiated daughter cell and also symmetrically to yield two stem cell daughters. The principal goal of current experiments is to trace stem cell lineages in single GFP labeled clones in vivo over longer periods than is possible with DiI labeling. We will focus on the following questions: What factors influence stem cell decisions for different differentiated cell types (nerves, nematocytes, gland cells)? Is decision-making influenced by position in the polyp? Are stem cell decisions stochastic? Is there a programmed sequence of stem cell decisions?