Re: Internal organs homologous across phyla?
- From: "Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx>
- Date: Mon, 30 Apr 2007 01:23:04 -0400 (EDT)
"Lorentz" <drosen0000@xxxxxxxxx> wrote in message news:f103mg$dqu$1@xxxxxxxxxxxxxxxxxxxxxx
In which case you'd really define pineal gland as a thrid eye?In many vertebrates, including some amphibians, what in humans
but the lenses of the eye and
associated parts of the eye are clearly analogous.
is called a pineal gland really is a photosensitive organ. It is in
fact a sort of third eye, even with regards to function. The most
primitive vertebrates all have photosensitive pineal bodies. I am not
sure whether it uses rhodopsin, but I think so. However, in all the
extant vertebrates (even the very primitive ones) the photosensitive
pineal organ has a slightly more specialized function than the other
two eyes. In vertebrates, the pineal eye is almost exclusinvely used
for timing. The animals diurnal cycle is timed by the light response
of the pineal eye. Even in the most primitive living vertebrates, the
imaging function of the pineal eye seems to have mostly dissappeared.
In us, even though the photosensitive function of the pineal
gland is gone, it still seems to serve as a regulator for our diurnal
cycle. I think that the pineal gland is the gland that controls the
melatonin, which helps us sleep.
So yes. I consider the pineal organ highly homologous to our
eyes. No lenses, though.
I am not an expert. The stuff on uses of the human pineal gland
I found earlier this month while googling. However, my main source of
material on the more primitive pineal organs are from the book:
Kenneth V. Kardong, "Vertebrates," (McGraw Hill, 2002). ISBN
0-07-290956-0
Interesting. So is the rhodopsin in the human pineal gland functional -
in that it cycles between two configurations and that cycling is
essential to pineal function? Presumably, the cycling is no longer
driven directly by light, but then what does drive it? Nervous
impulses from the retina? Chemicals diffusing from the retina? It
seems that the evolutionary history of this one HAS to be interesting.
How does a mechanism which originally involved sensing light directly
evolve to one which senses light indirectly, yet still retains the
original light-sensing chemical? Kind of like the division of labor
between antenna complexes and reaction centers in photosynthesis, but
the distances involved are much greater.
.
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