BIOL 303 Lecture Notes - Lecture 8: Optic Vesicle, Neurogenic Placodes, Paracrine Signalling
14 May 2018
School
Department
Course
Professor
Biol 303 Jan 25th – Lecture 6
Eyes – the vertebrate eye
• First thing: formation of two bumps in what will become the brain
• These bumps are called the optic vesicles
• The brain is technically a tube, with bulges
• Two of the earliest bumps to be discovered were the optic vesicles
• You have two epithelia interacting with each other (neuro ectoderm and epidermis)
• Once it starts interacting with the epidermis, the epithelium becomes the lens
• The inner cup becomes the neural retina
• The outer coating of the retina becomes the lens
• We will look at the signals that make this happen
Movie – the vertebrate eye
• Labelled the cell membranes with GFP – with a membrane tether of some sort
• This shows the transformation of the eye structure to become a fully formed organ
• The optic vesicle collapses
• The optic cup pulls in epidermic tissue that becomes the spherical lens
The lens placode
• Where the epidermis starts thickening to become the cells that are going to form the
lens
• Correlation: the lens placode always forms where the optic vesicle touches the head
ectoderm / the upper epithelia
• Hypothesis: optic
• Necessity Experiment: remove the optic vesicle, see that no induction of placode or eye
formed/ no lens formed
• Sufficiency experiment: Ectopic optic vesicle causes induction of an eye in the head –
put the optic vesicle against ectoderm in other parts of the body and see if induction
happes it doest ork
• Competence: only the head ectoderm is competent to form the lens placode
• Specificity: other tissues cannot induce lens placode formation
We want to know this signal might be
• What about the overlying ectoderm makes it competent to form the lens?
Molecular basis of eye induction
• F
Distinguishing between paracrine and juxtacrine signalling
• Co-culture experiment
• A big frog embryo, take the optic vesicle out and put it near things (especially the head
ectoderm) to see what happens/ if induction happens
find more resources at oneclass.com
find more resources at oneclass.com
• Now try putting them beside each other but put a filter between them which will
prevent juxtacrine signalling but will allow paracrine signalling – will tell you which type
of signalling is necessary for induction
• Assess lens induction – we find that lens tissue forms even with the filter, so the signal
must be paracrine
Why is head ectoderm competent?
• Ectoderm being neuralized – sigal fro edoder estalishes a les area i the
ectoderm
• Otx2 expressed in presumptive lens ectoderm – anterior neural plate and cardiac
mesoderm induce (Pax6 expression)
• **
Pax6 gene – the eye gene
• Called a master regulator gene
• All Pax genes are involved in developmental patterning
• Pax6: a paired box gene, a transcription factor, key regulator in eye and brain
development
• Pax6 is involved in lens induction (but how do we know?)
Knockout of Pax6 in mice and fish
• End up with mice with no eyes
• Pax6 is necessary for eye formation
• Two genuses of fish: one species lives at the surface, the other species lives in
completely dark caves
• The cave dwelling fish does not have Pax6, they only have Pax2
• Surface dwelling fish – you have huge domains of Pax6 expression and Pax2 + Pct2 as
well
Insect eyes
• Completely different from human eyes – theirs are more light-sensing, their vision is
based on a completely different principle
• Insects have many small eyes
• Insects still use Pax6
• Eyes have evolved many times and Pax6 is always involved – camera-type, compound,
mirror-type eyes all have Pax6
• Plaaria dot hae eyes ut they hae photo-sensors that let them move relative to
light – and there is Pax6 in these sensors
• In flies, wherever there is Pax6 expression, there will be eyes – if you have Pax6 in the
adoe, a eye ill for ut it ot hae the eural oetiity to see
• These are ectopic eyes
In vertebrates, is Pax6 involved in induction or competence?
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Why is head ectoderm competent: ectoderm being neuralized sig(cid:374)al fro(cid:373) e(cid:374)doder(cid:373) esta(cid:271)lishes a (cid:858)le(cid:374)s area(cid:859) i(cid:374) the ectoderm, otx2 expressed in presumptive lens ectoderm anterior neural plate and cardiac mesoderm induce (pax6 expression) Insect eyes: completely different from human eyes theirs are more light-sensing, their vision is based on a completely different principle. In flies, wherever there is pax6 expression, there will be eyes if you have pax6 in the a(cid:271)do(cid:373)e(cid:374), a(cid:374) eye (cid:449)ill for(cid:373) (cid:271)ut it (cid:449)o(cid:374)(cid:859)t ha(cid:448)e the (cid:374)eural (cid:272)o(cid:374)(cid:374)e(cid:272)ti(cid:448)ity to see: these are ectopic eyes. Pax6 as a competence factor: pax6 is one of the factors that binds directly to the crystalline gene leading to the expression of these genes. Many inductive interactions are conversations: the diagra(cid:373) (cid:449)here there(cid:859)s i(cid:374)du(cid:272)tio(cid:374) (cid:1005) a(cid:374)d (cid:1006) arro(cid:449)s (cid:271)et(cid:449)ee(cid:374) the di(cid:448)isio(cid:374)s: this is sequential induction. Quiz i got 1 correct, 2 wrong (guessed d), 3 wrong (guessed c), and 4 incorrect (i said a)
