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Re: Scales, hair, integumentary structure relationships?

--- On Fri, 6/13/08, Erik Boehm <erikboehm07@yahoo.com> wrote:

> I was not aware rat tails had scales, so I guess my
> question now goes to the homology of these tail scales and
> reptilian scales. Also how homologous are the tail scales
> to the hair. Birds have scales and feathers, but the scales
> and feathers are homolohous.


Mammalian scales are not homologous with reptilian scales. Reptilian and avian 
scales are composed of a mixture of Î & Ã-keratins. Mammal scales are 
composed strictly of Î-keratins. In fact, Ã-keratin is unique and probably 
autopomorphic to sauropsids. In mammals, the scales are composed of 
agglutinated hairs. To make up for the much softer nature of Î-keratin, 
mammalian scales (though probably not rat scales, as they appear to be very 
soft) also contain high concentrations of trichohyalin and other, high sulphur, 


> I'm not sure what hairs being between scales has to do
> with the ancestry of each structure- I guess I need to
> know more about the nature of hair and scale structure in
> rats if I'm missing something.
> I know genes involved in hair growth also have homologs in
> birds that affect feather growth, example:
> http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T39-438V3JY-4&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b7854da5e8bc36789fe582f8a3000473
> "Sox18 encodes a transcription factor known to be
> important for the development of blood vessels and hair
> follicles in mice."
> "we have isolated and characterized Sox18 in chickens.
> cSox18 shows a high degree of sequence homology to both the
> mouse and human orthologues"
> "In situ hybridization analyses showed expression in
> the developing vasculature and feather follicles,
> consistent with reported expression in the mouse embryo.
> "
>  The gene isn't just active in hair/feather
> development - so it doesn't neccesarily imply a
> very close homology yet.
> btw Molecular biology is what I studied in college, not
> palentology.

I certainly wouldn't be surprised to find that the mammalian Î-keratin family 
was homologous with the sauropsid one.

For a nice rundown of epidermal appendage evo/devo, I recommend reading: 

WU, P., Hou, L., Plikus, M., Hughes, M., Scehneti, J., Suksaweang, S., 
Wideltiz, R.B., Jian, T.X., Chuong, C.H. 2004. Evo-Devo of Amniote Integuments 
and Appendages. Int. J. Dev. Biol. 48: 249-270


> I wonder what value there would be to looking at the
> various alpha- and beta- keratin sequences, and try to
> determine a rough time frame for their divergence (does it
> roughly co-incide with the synapsid/reptile split?)
> What would be the probable reason for loosing "fish
> scales" around the time of the amphibian/amniote split
> - to aid in respiration through skin - seems to make sense
> for amphibians. Did early amniotes likely respire partly
> through skin? (implying a very amphibian like basal
> amniote). Are ossified fish like scales significantly
> heavier? Might scale loss be a weight saving feature?


They are all possible. I think the need for flexibility also probably played a 
role in dermal scale loss.