The complexity of Hydra: homeostasis, morphogenesis, controls and integration.

Abstract

Research on the complexity of organisms has advances through the use of simple systems like Hydra. The present article review some concepts concerning organization of Hydra. The body of Hydra is a storage organ for structural cell used in the maintenance of tentacles and foot. Excell cell are reject in buds. Changes occur in the size of the animal, in the proportion of its cells involved in maintenance and budding, and in the areas of the animal occupied by cells moving toward terminal strutures and converging on the budding region. In normal and regenerating hydras the mesoglea could aid in directing cell movement, nerves, differentiated from interstitial cells, could release neurosecretory products with inductive activity. Constructive controls promote development in the victinities of head, foot, and budding region. Two repressive controls seem to operate; negative dominance operated through cell-to-cell contact by drawing cells toward existing dominant centers; inhibition, dependent on the action of specific diffusible inhibitors of head and foot regeneration, may operate by preventing the differentiation of interstitial cells. Results on experimentally elongated multiple-graft animals indicate that the budding region forms at a point of minimal negative dominance without respect to the presence of the inhibitors. The production of inhibitors throughout the animal is controlled by continuity with the respective terminal structures and is disrupted by graft borders in elongated animals. Continuity with the head controls the destruction of foot inhibitor throughout the animal: continuity with the foot controls the head inhibitor, chiefly in the peduncle. Other substances may be present in diffusion gradients; one produced by dividing cells may direct cell loss through bud and graft rejection.