Lens - organogenesis of eye and limb, Biology

Assignment Help:

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.

 


Related Discussions:- Lens - organogenesis of eye and limb

Diabetes mellitus, Diabetes mellitus, Types I and II is a disorder regard...

Diabetes mellitus, Types I and II is a disorder regarding the defects in insulin action. Type I diabetes is characterized by an inadequate insulin secretion; Type II diabetes is

Explain ventricular septal defect in details, Explain Ventricular Septal De...

Explain Ventricular Septal Defect in details ? Indications of surgery :  Some VSDs close spontaneously or become smaller in size. This has to be taken into consideratio

Explain the sporangiophores features of rhizopus, Explain the Sporangiophor...

Explain the Sporangiophores features of Rhizopus? 1. Sporangiophores swell at the tip into a spherical knob like structure called sporangium. It has two zones. - Central dom

What is the trace element, What is a trace element and how are they importa...

What is a trace element and how are they important in organisms, give an example.

What is phenotypic ratio, The F2 phenotypic ratio from a dihybrid cross wit...

The F2 phenotypic ratio from a dihybrid cross with epistasis was 9:3:4. If instead of a dihybrid cross, a double heterozygote was test crossed (crossed to a homozygous recessive in

Structures in the Dermis, list of the structures you would expect to find i...

list of the structures you would expect to find in the dermis.

Human respiratory system - nasal chamber, NASA L CHAMBER - External...

NASA L CHAMBER - External nares are 2 in number, i.e., dirhynous. Situated at lower side of nose. Open into nasal chamber. Nasal chamber opens into pharynx by internal n

What are respiratory pigments, Q. What are respiratory pigments? What is th...

Q. What are respiratory pigments? What is the respiratory pigment present in some arthropods? Which is the analogous molecule in humans? Respiratory pigments are molecules able

Isomerization of citrate, Isomerization of  citrate Isomerization of ...

Isomerization of  citrate Isomerization of  citrate:   In  this  step,  citrate is isomerized to isocitrate by aconitase which has iron-sulphur centre as  its prosthetic  grou

Define investigative techniques in nutritional biochemistry, Define Investi...

Define Investigative Techniques in Nutritional Biochemistry? It is an important component of this manual as it focuses on providing an understanding of biochemical methods, tec

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd