Question

Explain the following observations. Be very specific with your answer.

1. When the optic cup is transplanted under head ectoderm, a lens forms.  But when the optic cup is transplanted under flank ectoderm, no lens forms.
2. When a piece of filter paper is inserted between head ectoderm and the optic cup, no lens forms.
3. Noggin secreting cells placed in lateral plate mesoderm causes somites to form there.
4. In embryos that are slug-/-, neural crest cells fail to migrate.
5. During chick development, if blood vessels are inhibited from forming, bone development is significantly delayed.
6. Portions of the notochord when transplanted above the somite causes what normally forms dermatome and myotome to form sclerotome.
7. Antibodies that prevent NT3 from being active prevents dermatome differentiation.
8. Swallow -/- drosophila embryos form two anterior ends.
9. Polar lobe removal in some species of snails results in embryos with no mesoderm.
10. Injecting nanos mRNA into the future anterior end creates a drosophila embryo with two tails.
11. A TCF3 mutation where TCF3 is missing its b-catenin binding domain, get no dorsal axis forming.
12. Injection of siamois into the ventral vegetal cells causes a secondary axis to form
13. Inject antibodies to dickkopf into embryos  results in an embryo with a deformed head – essentially no forebrain.
14. Injection of excess b-catenin into the ventral side of an oocyte, causes a secondary axis to form.

Answer 1

Ans:

• During embryonic development of amphibians, it is believed that optic vesicle (which in turn form optic cup) is solely responsible for lens induction. There are various transcription factors sequential activated during lens induction. When the optic cup is transplanted head ectoderm, they are activated sequentially so that lens is formed. These transcription factors involve Otx2, Pax6, and Lens1. They are in general expressed in already differentiated lens ectoderm and further expression of Sox, Maf, Prox1 assist in initiating the differentiation of lens. But when the optic cup is transplanted under chick flank ectoderm, there would be no coordinated expression of these genes and it will result in no formation of the lens. The chick flank ectoderm is meant for the expression of the ridge which in turn promotes external limbs.
• If researchers would place a piece of filter paper between the head ectoderm and optic cup, the communication between expressed transcription factors may be interrupted. Due to insufficient concentration, they will not reach lens ectoderm and as a result, the lens will not form. Their co-ordinated expression is the only key for sequential activation and leading to lens formation.
• When noggin-secretory cells are placed in lateral plate mesoderm, there would be an expression of the Paraxis gene which produces transcription factor which is responsible for the development of somite-like structures.
• The expression of the slug gene in an embryo is very essential for the development of neural crest and this gene is further regulated by the YY1 gene. In frogs, the migration as well as the specification of neural crest cells during embryonic development is essentially regulated by the functioning of the slug gene. If a double mutation in this gene occurs, there would be no migration of these cells.
• The inhibition of blood vessel formation, as well as the generation of the vascular system, leading to the impairment of tibial growth plates of chicken. It results in the death of cells secreting cartilages and is the reason why there is a significant delay in bone formation.
• When portions of the notochord are transplanted above the somites, it will lead to the formation of sclerotome. It is due to the fact the sonic hedgehog, a paracrine factor secreted from the notochord and the neural tube floor plate.
• The antibodies which are responsible for the blockage of neurotrophin-3 or NT-3 are used to prevent epithelial dermatome conversion into mesenchyme. It is essential to mention that dermatome cells differentiate in response to NT-3 and if one raises antibodies against NT-3 they will obviously prevent the differentiation of dermatome.