Differentiation pathways of ectodermal epithelial cells in hydra.

The differentiation pathways of ectodermal epithelial cells in hydra were investigated. We found that under steady state conditions the ectodermal epithelial cells of the foot, the foot mucous cells, and the ectodermal epithelial cells of the tentacles, the battery cells, differentiate from gastric ectodermal ephithelial stem cells. From stem cell to the terminally differentiated state, a single cell cycle is required. The cells undergo a final round of DNA replication, double their genome to 4 n and become arrested in the G2-phase of the cell cycle. The ectodermal ephithelial cells of the hypostome, which like the tentacle cells are part of the head structure, can also arise from gastric ectodermal epithelial stem cells, but do so only during head regeneration and budding. They differentiate from stem cell to hypostomal cell in a single cell cycle, but in contrast to foot mucous and battery cells they remain capable of cell proliferation. Due to this self-renewal potential, they do not require recruitment from the gastric stem-cell pool in steady-state animals.