Embryology Biology 441 Spring 2011 Albert Harris
Bard's Questions:(Based on the questions in Appendix #3 "Unanswered questions"
in Jonathan Bard's excellent, and still not outdated, book "Morphogenesis" Cambridge U. Press
My goal in paraphrasing Bard's list, and inserting comments, is not that you should memorize the list, but instead to give you a feeling for the progress that has been made by researchers over the past 20 years, and also the variety of questions for which answers have not yet been discovered and proved true.
On the final exam, one of the questions might ask you to chose your favorite one or two or three from this list of questions, and describe what is known, and what is not yet known, and what kind of evidence is most needed to answer these questions conclusively.
1a) Does cell traction cause mesenchyme to condense into somites?
1b) To stimulate differentiation into cartilage?
1c) To form papillae for feathers or hair?
1d) To form future nephrons (i.e. rudiments of kidney glomeruli)
Does cell traction serve any other roles in morphogenesis?
1e) Notice that he doesn't even consider formation of skeletal muscles, as caused by traction!
What mechanisms shape bones? Is it a self-assembly process?
And what does he mean by "self assembly"?
2a) That the processes of cartilage shaping and bone formation are autonomous?
(Cartilages dissected out of early embryos, and put in tissue culture medium, will form correct shape?)
2b) Or does he mean "self assembly" in the sense of being closely analogous to crystallization?)
Do the tensile forces present in early embryos serve functions in anatomical shape formation?
3a) Hydrostatic pressure (Inside the neural tube? Inside the eyeball? Other examples?0
3b) Microfilament contraction?
(Do you need both? Contractile tension in addition to hydrostatic pressure?)
4) How do wounded epithelia repair in early embryos?
( "when they are under tensile rather than compressive forces?")
5) Regarding the phenomenon of cell rearrangement
I interpret this as being in reference to the many examples (first discovered by Diane Fristrom) in which multicellular structures elongate or otherwise change shape, while their individual component cells remain "equidimensional", i.e. NOT-elongated, and approximately hexagonal.
This phenomenon occurs often. Personally, I believe it can be explained by rounding-up tendencies at the individual cell level, superimposed on larger-scale forces that reshape the masses of cells, of which the hexagonal cells are component units.
Most biologists who know about this phenomenon interpret it as evidence that adjustments of cell-cell adhesions must themselves be the causes of changes in organ shape (for example, in the elongation of imaginal disks, vertebrate limb buds, gastrulating sea urchins, and amphibian.
6) What determines the spacings between dermal condensations (papillae)?
Between branches in tubular epithelia?
7) Does sorting-out play any role/serve any function in morphogenesis?
(Personally, I believe this is the wrong way to think about it; and that it is better to ask whether the combinations of forces and properties that cause cell sorting also cause formation of normal anatomical structures, whether they also cause neurulation, also cause gastrulation, also cause somite formation, etc.)
8) What principles cause the shaping of the nervous system, including guidance of nerve axons to their correct locations of innervation?
(Ephrin A gradients seem to be a good example)
9) How is size invariance accomplished?
Driesch's phenomenon of embryonic regulation is an important special case of size invariance.
Another example is the ability of Dictyostelium slugs to develop into fruiting bodies with the same ratios of numbers of stalk cells to spore cell, the same ratios of width to height, etc.
Apart from cutting early embryos into halves (Driesch, and others), it is also observed that oocytes can differ in size, yet develop into normally proportioned animals.
Related to these questions is the failure of regulation to occur in Roux' "hot needle" experiment?<
(Maybe the Entelechy gets killed or frightened by the hot needle!)
Maybe Wolpert's theoretical gradients of morphogens are blocked by the effects of hot needles?
Not enough attention has been given to the hot needle effects. Because these experiments were wrongly interpreted back in the 1890s, and because their wrong interpretation was disproven by Driesch's experiments on splitting embryos into halves, nobody asks "What is the true explanation of the failure of regulation to occur in embryos in which some cells have been severely damaged, rather than removed?"
The morphogenetic role of the extracellular matrix:10) How is organized collagen laid down in tissues and what determines the axes of fibril elongation?
11) How are the tracks for contact guidance and haptotaxis laid down?
(And, from a more modern perspective, what mechanism creates the concentration gradients of ephrin A?)
12) How do basal laminae adapt to the growth and to the changes in shape of their overlying epithelia?
13) What distinct roles do the different proteoglycans play in development? (or do they?)
Molecular mechanisms.14) What is the molecular basis of cell rearrangement? [ephrins seem to be major causes]
15) How do cells move, and what is the relationship between this and that for cell rearrangement?
16) What determines the sites on the cell membrane to which microfilaments adhere?
17) Do interactions between microfilaments and the cell membrane generate filopodia?
18) Can cell membranes expand and contract? [What happens when plasma membranes "bleb"?]
19) By what mechanisms do epithelial cells become columnar? [How do epithelial cells "palisade"?]
(Is this combination of cell elongation and tighter packing between neighboring cells caused by changes in the cells that are becoming columnar? Or is it more accurate to think of the process as something that is done to the cells by something else?
20) How do epithelia become polarized? (formation of the apical surface, versus the baso-lateral surface)
For example, if you dissect out any epithelium, and plate it out in culture with what had been its apical surface facing downward, against the substratum ("upside down", apical side down), then what is the mechanism that causes each cell either to shift its apical and basal surfaces, or form a new apical surface and resorb the previous one, or whatever process it is that (within hours) will cause the epithelium to have its apical surface facing upward, toward the culture medium and away from the substratum?
* A related phenomenon is that if you dissect out a strip of epithelium, and use weights to twist it so that one end has its previous apical surface facing downward toward the substratum, and the other end of this same strip of epithelial cells with its apical surface facing upward away from the substratum, then within an hour or two the epithelium will tear itself in two, with one part keeping its apical surface polarized upward, and the other section reversing its polarity, so that its new apical surface also faces upward.
* Guess what happens if you plate a sheet of epithelium between two Millipore filters?
The answer is that it splits into two epithelia, each with a basal surface oriented toward the closest filter, and each with apical surfaces polarized toward each other, in the middle. This is analogous to the splitting of lateral plate mesoderm to form two sheets with their apical sides facing each other.
This was discovered by Mary S. T., who deserves all credit; I wish she would publish this discovery and to whom I apologize for teaching her discovery to students in the course in which she was once my Teaching Assistant.
21) How are integrins and other cell adhesion proteins inserted and removed from particular locations in the plasma membranes of embryonic cells.
22) Does cell volume increase the probability that embryonic cells will enter the cell cycle and undergo mitosis? Yes! The ratio of volume per amount of DNA, apparently. But what is the mechanism of this control? How can a cell measure its own volume?
23) Do electric fields serve as normal guidance or orientation mechanism for embryonic cells? If so, for which differentiated cell types? Osteocytes and/or osteoclasts? Muscle cells?
If so, do the cells respond to electrophoretic displacement of membrane proteins, or to depolarization or to hyper-polarization of the resting potential differences in voltage between the inside and outside of their plasma membranes?
! Some questions about specific tissues and differentiated cell types.
24) What spatial cues halt the migration of neural crest cells?
25) How do neural crest cells form structures? (in those cases when they do).
26) What molecular mechanisms cause separation and then re-joining of pigment cells in the newt?
27) By what force are plastic latex spheres moved down the same pathways as neural crest cells migrate?
28) How does the developing pancreas become shaped in developing embryos, and especially what causes narrowing of the neck connecting the pancreas and the digestive tract?
29) Do optic neurons find their correct sites of connection on the tectum by sorting out on gradients of adhesion? NO; the true answer seems to be that they use gradients of Ephrin, which inhibit adhesion.
30) Does the inner surface of a sea urchin blastula have a gradient of adhesion?
31) How do the cells around the blastopore of amphibian embryos cause their own involution?
Do they become more adhesive?
Do their surfaces contract in some particular pattern?
By what methods can we reasonably hope to find conclusive answers to questions like this?
32) Finger prints? ="Friction ridges." What mechanism of cell contraction, or cell-cell adhesion, etc. causes these thickenings to develop in the skin of our hands and feet?
Their patterns are not exactly the same even in identical twins? What does that tell us about mechanism?
33) The structures found in some invertebrates that are called "epithelial feet"? Do they serve any function?