Inhibition of Hypostome Formation and Polarity Reversal in Hydra

Abstract

REGENERATION of Hydra littoralis is usually strictly polarized in the sense that distal structures such as hypostome and tentacles always form at the distal cut surface whereas proximal structures such as the basal disk always form at the proximal cut surface. The physiological basis of this polarity is unknown, but most theories of hypostome regeneration imply that the hypostome forms at the high point of a gradient which runs from the hypostome to the basal disk (disto-proximally), and defines the polarity. The nature of this gradient is unknown but it is variously regarded as one of metabolic activity¹, growth substances², or inhibitor threshold³. Polarity need not necessarily involve a gradient. If, for example, the transfer of positional information⁴ were polarized in a disto-proximal direction, the distal end could be specified as that region which received no positional information from other cells. Another type of model could operate if each cell were polarized and the absence of cells at a cut surface were recognized⁵. Whether a cut surface is necessary for the regeneration of distal structures is a very old problem¹. Far less attention has been paid to the problem of basal disk regeneration and particularly whether it is dependent on the presence of a functioning hypostome.