Permissive and directive interactions in lens induction.

The interactive events leading to lens formation and the developmental potentialities of the presumptive lens ectoderm were examined in vitro. The presumptive lens ectoderm of both mouse and chick embryos was capable of forming a lens even when isolated from the optic vesicle before the two tissues reach the stage of close association. This lens-forming bias can be released with favourable culture conditions and by various heterotypic mesenchymes. The same permissive, unspecific conditions or heterotypic tissues failed to trigger lens formation in trunk ectoderm. The directive effect of the optic vesicle was demonstrated in experiments where it was grown in contact with the trunk ectoderm. The latter developed distinct lentoid bodies synthesizing lens proteins. The origin of the lentoid was confirmed in interspecies combination of chick and quail tissues. It is concluded that lens formation is governed by a series of interactive events consisting of both directive and permissive influences.