The Morphogenetic Effects of Exogenous Sex Steroid Hormones in the Planarian Girardia tigrina (Turbellaria, Tricladida)

Abstract

Abstract—Morphogenetic effects of two exogenous sex steroid hormones, testosterone and estradiol, during planarian regeneration have been studied. Anterior regenerants, which retained the head ganglion after cutting, exhibited the highest sensitivity to the introduced hormones. The influence of the studied steroids on the growth rate of their blastema was different: the regeneration process was stimulated by estradiol and inhibited by testosterone. In posterior regenerants, the inhibitory effect of testosterone was also present, while estradiol had hardly any influence on the recovery rate. Both hormones significantly increased mitotic activity in the anterior and posterior regions of the regenerants. The highest mitotic index values were observed in the regions that were most remote from the cutting zone and in those immediately adjacent to it. As a working hypothesis, it is proposed that specific effects of either hormone were related to the formation of specific hormone–receptor complexes and to the pattern of hormone receptor distribution in the proximal and distal regions of the planarian body.