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As I compare the morphologic differences between mouse models and the human disease for which they are analogous, I have become interested in what difference interspecies size has on molecular function. For example, the mouse colon has a diameter of 1 mm, the human 75mm it seems to me there should be many genetic/transcription differences between species just as a pup-tent (analogous to a mouse) requires a much different support structure than a 10 story building (analogous to a human) yet I find little about this in the literature, can someone help or show me where this has been addressed? Thanks!

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    $\begingroup$ It would be useful if the title question was more revealing of what is actually being asked $\endgroup$
    – rg255
    Apr 18 '16 at 12:27
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The renowned biologist JBS Haldane was concerned with the problem of size and wrote about it in his essay of "On being the right size". Two situations he discusses may be of interest.

The first is that large animals are not just scaled-up versions of smaller ones. He calculates the weight-bearing ability of bones to show that the increase in cross-section of bone in a scaled-up person could not accommodate the increase in "weight per square inch". The conclusion here, I think, is that a different type of structure is required to deal with the weight of a larger animal. In the sort of molecular genetic terms that did not then exist, one would need a different program of transcription.

A second situation relates to the size of eyes and the size and number of rods and cones they contain. Here he argues that there is little intrinsic difference between the cells that make up the eyes of mice and men (this is 1926), despite the difference in number (and hence eye size). In this case the gene transcription would be expected to be similar.

He also talks about the modification of lungs and gut to provide a required increase in surface area in larger animals. This would seem similar to the case you raise. Is this more of the same or a fundamental difference? I leave it to you to decide.

The original essay is out of copyright and can be found on the web. However you might also be interested in a printed collection that includes other of Haldane’s essays as well.

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  • $\begingroup$ @MattDMo Point taken. I've rewritten my answer entirely. $\endgroup$
    – David
    Apr 18 '16 at 10:00
  • $\begingroup$ looks good! Could you include a link to the online version of the essay as well? $\endgroup$
    – MattDMo
    Apr 18 '16 at 14:09
  • $\begingroup$ @MattDMo I've done that now. $\endgroup$
    – David
    Apr 18 '16 at 15:56

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