Biology 441, Spring 2013 Review for Second Hour Exam, part 4


Sketch the shapes, arrangements and relative sizes of chondrocytes in articular cartilages.

Sketch the shapes, arrangements and relative sizes of chondrocytes near parts of bones where bone formation is occurring.

If all the calcium phosphate is dissolved out of a bone, what sort of thing is left?

    A brittle object the same size and shape of the original bone?
    A flexible, rubbery object the same size and shape of the original bone? A shrunken object, smaller but otherwise the same shape as the original bone?
    An inflated, larger object, bigger, but approximately the same shape of the original bone?
    Something else? What?
    The result cannot be predicted?

If all the collagen were somehow digested or otherwise removed from a bone, which of the listed results should occur?

If all or many of the sulfate groups were somehow digested or otherwise removed from a CARTILAGE, which of the listed results should occur?

If all or most of the collagen were somehow digested or otherwise removed from a CARTILAGE, which of the listed results should occur? (If in doubt, explain your reasoning.)

How could any of these 4 changes in bones or cartilages (maybe) be of medical use?


Very difficult, but please think about this: Look at the changes in sizes and shapes of chondrocytes near where ossification is occurring. Which of the following could provide the directional driving force that elongates growing leg bones, arm bones, and ribs? (Based on what you know about electro-osmosis; based on what you can see about cell shapes, sizes and arrangements?)

    * The chondrocytes orient their mitotic divisions, so that these will push in the longitudinal direction?

    * The chondrocytes secrete more sulfonated sugar chains from those sides of the chondrocytes that face the directions of elongation?

    * The chondrocytes cause digestion of sulfonated sugar chains in the DIRECTIONS of elongation?

    * The chondrocytes cause digestion of sulfonated sugar chains in the LOCATIONS closest to where new bone is forming?

    * The chondrocytes cause digestion of collagen fibers that resist osmotic swelling of cartilage?

?? Which of these explanations are taught in medical schools? Which are asserted on 99% of informational web sites about endochondral ossification? ? Which of these explanations are logically possible?

What experiments can you invent to prove or disprove each of these theories?

* Can you invent some additional theories to explain how bones "grow" in length?


When you inflate a long balloon, does it grow?

Do nerve fibers grow? (Partly yes, partly no.) Please explain.

Do capillaries and other blood vessels GROW? (Partly yes, partly no.) Please explain.

Do cartilages GROW in length or width? (Partly yes, partly no.) Please explain.

Do bones GROW in length or width? (Partly yes, partly no.) Please explain.

Are the shapes of cartilages necessarily the result of more growth in some directions and/or locations? (Partly yes, partly no.) Please explain.


How is cell locomotion related to the paths of axons and dendrites in the nervous system?

Sensory nerve cell bodies are located where?

Motor nerve cell bodies are located where?

Postganglionic autonomic nerve cell bodies are located where?

Optic nerve cell bodies are located where? And are called by what name?

What physical process creates the wiring patterns (of nerve connections) inside the brain?

How was this discovered, and who discovered it? (Hint: Read the Wikipedia article on "tissue culture." But ignore what they say Roux; his tissues merely survived.)

What is chemotaxis?

What is haptotaxis? Why do tissue cells move up gradients of adhesiveness?

Suppose cells can measure or compare concentrations of a substance on different parts of their surface, how should they change the direction of their locomotion in order for chemotaxis to occur?

There are several possibilities:

    What if they turn toward their side where the detect the highest concentration of the chemical?

    What if they continue crawling in the same direction as long as the chemical concentration increases, but then turn if and when the chemical concentration decreases?

    What if they continue in a direction as long as the chemical concentration at their front end is higher than elsewhere, but turn if the concentration becomes higher on any other part of their surface?

    By what angle should they turn in order to achieve chemotaxis? (trick question)

    * By what angle should they turn in order to optimize chemotaxis? (In the sense of reaching their target soonest, and after moving the minimum average distance?)

    What if cells continue crawling, with random turns, as long as the concentration of a certain chemical is between a certain minimum and a certain maximum, and stop if the concentration gets above this maximum, and reverse direction if the concentration is below the minimum?

    Would you classify this as chemotaxis?

    Could you distinguish this from chemotaxis, if you watched cells behaving this way?

    Could this process serve the normal functions of chemotaxis? For leucocytes? For nerve growth cones?

    For future egg and sperm cells finding the future testis and ovary? For sperm cells finding egg cells?

    For muscle cells finding their correct site of attachment to bones?

    For capillaries vascularizing tissues which are not yet receiving enough oxygen?

    For the pronephric duct finding its correct path from the pronephros to the cloaca?

    For the ureter connecting to the metanephros?

    For the oviduct connecting to the vicinity of the ovary?

    For optic nerve fibers to find their way toward the blind spot?

Could endothelial cells use chemotaxis to vascularize tissues in an optimal pattern? (get enough oxygen everywhere with the minimal total length of capillary?

Could sensory and motor nerves use chemotaxis to guide them toward connection to the proper locations in the skin, or on muscles? What difficulties can you think of, for using this guidance mechanism? (I can think of at least two big ones.)

When your eye detects light, in what sequence do light rays reach the following structures and differentiated cell types: (Please arrange in the correct order):
*Ganglion cells; *Pigmented retinal cells; *Cornea; *Rod and Cone cells; *Lens; *Optic nerve fibers

Why do vertebrate eyes have"blind spots" (In terms of the sequences of embryonic cell folding and cell crawling?

What is a neural projection?

What are three examples of optic projections?

Which of these has been subject to the most intensive research?

Most nerve fibers from the right eye connect primarily to what part of the brain? (In chicken, frog, and fish embryos. It's a little different in mammals)

What are ephrin proteins?

Where are these proteins found to occur in concentration gradients?

What are ephrin receptor proteins?

To what proteins do do ephrin receptor proteins bind specifically?

In what cells are amounts of ephrin receptor proteins found to occur as a concentration gradient?

* Why do we need to have at least two different kinds of ephrins, each specific for binding to its own kind of ephrin receptor?

In what directions to the gradients of ephrins and ephrin receptors need to vary, relative to each other, in order to cause optic nerves to project in the actual geometric patterns that they do project?

Difficult, but please read carefully and give them some thought:

When Wolpert says that binding of ephrin to ephrin receptors "repels" cells from each other, what does he really mean?

In this metaphorical sense of the idea of repulsion of nerve growth cones, which nerves will be most strongly repelled by cells with any given amount of ephrin receptors?

For any given growth cone of the optic nerve, which cells of the optic tectum will "repel" it most strongly?

* Would you regard these effects as either chemotaxis or haptotaxis?

* Would an optic nerve be able to form a correct pattern of connection if it entered the tectum at the wrong location? Why or why not?

Do you think optic nerve cells and optic tectum cells continue to have ephrin and ephrin receptor gradients all during life, at least in salamanders?

Experiments were done grafting pieces of retina to the surface of the tectum: What result do you suppose the experimenters hoped (expected) to observe?

Why didn't they get any consistent pattern of connection, resembling a projection, do you suppose?

Can you invent some other experiments that scientists should have tried?

What pattern of connection is formed when two eyeballs (and two optic nerves) are forced to innervate the same side of the brain?

In principle, what OTHER patterns of connection might have developed, instead?

Suppose you had invented the theory that ganglion cells sent out growth cones at a sequence of specific times, and that these were 'attracted" chemotactically to one side of the tectum, with the ganglion cell growth cones competing with each other for connections to locations on the tectum, "first come first served", with each nerve fiber forming a permanent connection, blocking other optic nerve fibers from connecting there, and being blocked from connecting to other locations where other optic nerves had already connected.

    1) Could this timing mechanism, in principle, create a neural projection?

    2) Which experiments (double eye grafting, eye rotation, etc.) would have produced different results that they actually did produce, if retina-tectal projections really had been produced by this hypothetical timing mechanism?

    3) Are there some results that would have been the same?

What is interesting about the paths of optic nerve fibers in the optic chiasm?

Can you explain this pattern by ephins and ephrin receptors?

In species of animals that have stereo vision, what happens to some of the nerve growth cones in the optic tectum (that is different from what the other optic nerve fibers do)?

* What could be the mechanism of this?

* Why do you suppose that those nerve fibers that turn back to the same side of the brain wait until they have gone all the way to the chiasm before turning 90 degrees?

In fact, half retinas spread out over the whole tectum. Would you have expected that? What does it tell you about causal mechanisms?


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