Comparison of the effects of methallibure and thiourea on the testis, thyroid and adenohypophysis of the adult and juvenile guppy, Poecilla reticulata Peters.

Abstract

Thiourea and methallibure are equally effective in completely blocking the initiation of spematogenesis in juvenile guppies and also in inhibiting spermatogenesis in adults. Both compounds appear to act by blocking the activity of pituitary gonadotrophs which regress. Methallibure has antithyroid properties in adult guppies but has negligible effects in juveniles. On the other hand, the goitrogenic effects of thiourea are far more pronounced and affect both adults and juveniles. Whereas thiourea inhibits growth and causes an increase in granulation in the pituitary somatotrophs, methallibure causes a degranulation of the somatotrophs and a depression of the somatotroph cell index (in adults) and does not significantly affect the growth rate.