posted February 27, 5:45 pm
a) You should be able to label each part of each illustration (including the color photographs), and tell what part(s) of the body and differentiated cell types develop from each part.
b) Conversely, you may be asked to make a drawing of any of the structures shown in any of the photographs or diagrams in this set of web pages. (without being shown the photograph)
"Draw the shapes and orientations of the component cells of a vertebrate lens"
c) "Sketch the paths of optic nerve fibers connecting to different parts of the optic tectum (in the brains of vertebrates other than mammals)
"Sketch the part of the developing eyeball where the axons of the optic nerve leave the lower side of the eyeball"
"Make a diagram of a 'neural projection'"
d) What sensory organs develop from ectodermal placodes?
e) Sketch the locations of the olfactory, otic and lens placodes?
f) Sketch the locations of the light-sensing nerves of the retina (rods and cones) relative to the nerve fibers that converge together to form the optic nerve.
In what sense is this geometric arrangement different from what should have been produced by "Intelligent Design"?
g) What is the relation of the blind spot to the optic nerve and to the choroid fissure?
h) Propose a hypothesis capable of explaining how the "growth cones" (crawling tips) of the optic nerve fibers are guided toward the "blind spot" where the optic nerve leaves the back of the eyeball.
i) Adjust your nerve guiding hypothesis to enable it to predict (accurately) that optic nerve fibers radiate centrifugally away from the fovea, in addition to converging centripetally toward the base of the choroid fissure.
j) What part of the optic tectum is innervated by the upper right part of the left eye? etc.
k) Where is the optic chiasm? Please sketch alternative paths of nerve fibers through the optic chiasm. Explain why these paths are a challenge for any theory of nerve guidance to explain.
l) Compare the paths of nerve fibers through the optic chiasm in vertebrates that don't normally have stereo vision, as compared with vertebrates that do have stereo vision.
m) Describe the abnormality of eye-to-brain nerve fiber connections in albino mutants of mammals that normally have stereo vision. (White rats; Siamese cats)
n) Describe and draw the optic nerve connections in frog tadpoles, during the time when they can see, but their retinas and optic tecta are still growing larger. What is surprising about these connections?
o) Describe the radioactive labeling experiments that seemed to confirm that adhesion gradients on the tectal surface were the means by which optic nerve axons are guided to the appropriate parts of the optic tectum (i.e. the parts of the tectum where the axons form synapses)?
p) Explain how the true explanation of those experimental results can also be explained by concentration gradients of pairs of certain cell surface proteins ("eph proteins" and "Ephrin proteins") that selectively bind to each other and induce detachment of cell-cell adhesions.
Hint: Instead of gradients of adhesion proteins, the real gradients are of DE-adhesion proteins.
q) Do you understand the difference between a diffusion gradient versus a concentration gradient of a protein or other chemical whose molecules are fixed in location?
Which of these two kinds of gradients would be disturbed by blood flow?
r) Is stimulation of detachments of adhesions the same thing as repulsion?
(Hint: No; but the textbook we previously used equated the two. If you look at the videos of fibroblasts crawling on adhesion gradients, and also the videos or nerve axons crawling ("axons in culture"), they can help you visualize why stimulation of detachment of extensions from cells would appear almost indistinguishable from weakening of cell adhesions.
s) If axons from the anterior half of an animals retina were prevented from reaching the optic tectum,
which of the following experimental results would you expect to occur?
* The axons from the posterior half of the retina would innervate the anterior half of the optic tectum (i.e. the same locations that they would normally have innervated, if the anterior retinal cells had not been destroyed or otherwise prevented from reaching the optic tectum)
** The axons from the posterior half of the retina would spread out their innervation across the entire optic tectum.
The later result (spreading out of innervation) has been reported to occur. Suggest two possible explanations.
(Hint: One of them could be based on the phenomenon mentioned in question "n" above. The other interpretations could be based on gradients of adhesion, or gradients of proteins that induce detachment of cell-cell adhesions).
t) What would you expect to happen if if optic nerve fibers from three different eyeballs were surgically guided to innervate the same optic tectum, (Intermingle? Divide the tectum into thirds, with each eye innervating only its third? Axons of only one of the three eyes would spread out over the tectum, keeping the others out? Something else?
u) What would happen if you had concentration gradients of two proteins ("X" on the surfaces of optic nerve retinal cells, and axons that extend from them, and "Y" on the surface of cells of the optic tectum) (really called "ephs" and "ephrins" of course)
Also suppose that detachment of adhesions is stimulated by binding of X molecules on axons and Y molecules on tectal cells.
Do you see why having a larger population density of X molecules would cause axons to be more sensitive to detaching in response to contact with Y molecules on the tectum? Explain why?
v) Why would axons crawling up a concentration gradient of Y molecules be induced to detach sooner, at the low end of the gradient of the Y substance? Explain.
A computer simulation of this would be a big help.
Don't panic if you have trouble visualizing how concentration gradients of eph and ephrin can cause optic nerve fibers to spread out into neural projections.
w) Try to visualize why such a neural projection would be caused to be upside down and backwards as a result of the higher concentration sides of the eph and ephrin gradients both being on the same side. (Look at my diagram on the web page)
x) Incidentally, another function of ephs and ephrins is to prevent arteries from connecting directly to veins (forming anastomoses) without a capillary network in between. Try to figure out how to produce this separation, for examples by arteries having lots of eph proteins and veins having lots of ephrin proteins.
Incidentally, one of the main discoverers of this phenomenon took this course when she was a graduate student in this department.
y) Suppose that cells from a certain liver cancer secreted a hormone (erythropoietin) that stimulated formation of more blood cells and another protein that induced detachment of either arteries or veins,
suggest an acronym name for this cancer-produced protein.
z) Guess how eph proteins were discovered, originally.
Guess how nerve growth factor, epidermal growth factor, most fibroblast growth factors, and many other important proteins got discovered.