Hydra’s well-touted simplicity conceals complexity in the regulation of growth, morphogenesis, body size, and budding rates. Rates of cell division change proportionally with rates of feeding animals, but higher rates of cell division do not lead to increased cell population size or cell density. The results of grafting experiments demonstrate that the size of the hydra’s mesoglea (extracellular material: ECM) regulates the animal’s body size, while cell loss through sequestering cells in budding primordia regulates cell population size. Buds form when bud primordia (modules) are filled with cells from adjacent gastric and budding regions. The number of primordia filled at a time determines the budding rate. The size of bud primordia and the duration of development vary with temperature (larger and longer at colder temperatures), but the size of tentacle primordia in buds and regenerating animals are comparable. Bud morphogenesis depends on interactions between epithelial (epithelialmuscular and gastrodermal digestive cells) and interstitial cells that may have evolved through cooperation and competition among multicellular and amoeboid ancestors joined in symbiogents. Hydra and its buds may reflect properties resembling tumors and metastases.
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