Lens - Organogenesis of Eye and Limb
There is much experimental proof that the lens formation in many species is dependent on the induction by the optic vesicle while it contacts the presumptive lens ectoderm. If this contact is prevented the lens placode is not made in many amphibian species and also the chick. The induction from the optic vesicle is non- specific, since the vesicle of a different species when placed within the presumptive lens ectoderm induces a lens placode. For example, optic vesicle of Rana esculenta induces lens if grafted within Rana sylvatica head ectoderm. Though, in some other species, involving some amphibians, the optic vesicle does not appear to be the only tissue able of inducing lens. In xenopus and Rana fusca presumptive lens ectoderm makes lens even in the absence of optic vesicle. Experiments on salamander have shown that lens induction is a step wise process where the lens ectoderm comes under the effect of three successive inducers one after another.
It receives the first induction from pharyngeal endoderm that passes under it during gastrulation. Next presumptive lens ectoderm lies for several time over the presumptive heart mesoderm which gives the second inductive signal. Only after that neurulation the optic vesicle makes contact with it and provides the final inductive instruction or signal. Hence, even in the absence of the vesicle a lens may make as the ectoderm has already received two necessary signals from the endoderm and heart mesoderm. In salamander, this result could be obtained in 42% cases when optic vesicle was removed earlier than reaching the ectoderm. In the chick lens can be induced by a variety of tissues within experimental conditions and not by the optic vesicle alone.
Even in those cases where lens is generally formed only after optic vesicle touches the ectoderm assistance from other tissues appears to be essential This was illustrated by a series of in vitro experiments by Muthukkaruppan (1965). He cultured the presumptive lens ectoderm of mouse embryo on porous filter along with some mesenchyme or without it. Below the filter he located the optic vesicle in some experiments but not in others. A good lens developed only while the optic vesicle was present below the filter and mesenchyme on its upper surface with the lens ectoderm. Absence of either of these two resulted in ectoderm not making the lens. This and some experiments of similar type indicate that induction of lens may not needed actual physical contact of ectoderm with the optic vesicle but might be mediated by a diffusible substance of small molecules secreted by the vesicle. The nature of this substance is not known.