La regeneració i l'homeòstasi en les planàries, un model clàssic de biologia del desenvolupament

Abstract

Planarian regeneration and homeostasis, a classic developmental biology model. Regeneration is the ability of the fully developed organism to replace fragments lost by traumatic amputation or degeneration. Regeneration of the new structures is achieved either by cell proliferation and de novo formation, or by remodelling of the pre-existing tissues. Planarians, which can regenerate a whole new organism from a small piece of their body, have attracted the interest of scientists throughout history. In 1814, Dalyell concluded that planarians could almost be called immortal under the edge of the knife. Planarian regeneration involves the generation of new tissue at the wound site via cell proliferation, producing a new undifferentiated tissue or blastema, and the remodelling of pre-existing tissues to restore proportions. Another astonishing property of planarians is their capacity to grow and shrink depending on food availability and temperature. Remarkably, during the whole growth/degrowth process planarians keep normal body proportions and functions, thanks to a homeostatic control. All that plasticity is based, at the cellular level, upon the presence of totipotent stem cells in a high proportion (20-30% of total cell number in an adult organism). Another basic property is the continuous activity of the morphogenetic mechanisms that usually appear only once during the development of other model systems. The application of new methodologies at the cellular, molecular and genetic level in the postgenomics era allows us to functionally analyse the universal developmental pathways and genes in a new scenario, planarian regeneration.