Early studies on lens induction suggested that the optic vesicle, the precursor of the retina, was the primary inducer of the lens; however, more recent experiments with amphibians establish an important role for earlier inductive interactions between anterior neural plate and adjacent presumptive lens ectoderm in lens formation. We report here experiments assessing key inductive interactions in chicken embryos to see if features of amphibian systems are conserved in birds. We first examined the issue of specification of head ectoderm for a lens fate. A large region of head ectoderm, in addition to the presumptive lens ectoderm, is specified for a lens fate before the time of neural tube closure, well before the optic vesicle first contacts the presumptive lens ectoderm. This positive lens response was observed in cultures grown in a wide range of culture media. We also tested whether the optic vesicle can induce lenses in recombinant cultures with ectoderm and find that, at least with the ectodermal tissues we examined, it generally cannot induce a lens response. Finally, we addressed how lens potential is suppressed in non-lens head ectoderm and show an inhibitory role for head mesenchyme. This mesenchyme is infiltrated by neural crest cells in most regions of the head. Taken together, these results suggest that, as in amphibians, the optic vesicle cannot be solely responsible for lens induction in chicken embryos; other tissue interactions must send early signals required for lens specification, while inhibitory interactions from mesenchyme suppress lens-forming ability outside of the lens area.

Expression of many cell type-specific genes is correlated with a reduced level of cytosine methylation and some results argue that genetic programmes may be activated by a reduction in DNA methylation. During embryogenesis, however, when many genes are activated in specific cell lineages, it has not been demonstrated that they are hypomethylated prior to their expression. We have examined the timing of hypomethylation and gene activation during embryonic chick lens development for the two genes encoding delta-crystallin (the major lens-specific protein). We report here that while many of the CCGG sequences analysed become hypomethylated, most do not do so until 2 days after delta-crystallin is first synthesized. However, there is at least one site which is hypomethylated earlier, approximately when transcription is thought to commence. We conclude that hypomethylation in the delta-crystallin genes is probably not a simple process which activates transcription, although early hypomethylation events indicate obvious sites to be examined for a role in gene activation.