Practice question

Hydra, freshwater cnidarian with diploblastic body and simple nerve net, epitomizes near-total regeneration due to perpetual activity of three stem cell lineages: ectodermal epithelial, endodermal epithelial, and interstitial multipotent stem cells capable of producing neurons, nematocytes, and gametes. Small fragments containing few hundred cells reorganize polarity and reform head with hypostome organizer and basal disc within days without scarring. Zebrafish regenerate selected organs, mammals and birds show limited repair due to fibrotic responses. Hydra's continuous Wnt3-driven head organizer activity and absence of complex adaptive immunity enable whole-body reconstitution from minimal tissue.