Re: Internal organs homologous across phyla?
- From: dk@xxxxxxxxxxxxxxxxxxxxxxxxx (DK)
- Date: Mon, 30 Apr 2007 13:52:32 -0400 (EDT)
In article <f13ufo$22ch$1@xxxxxxxxxxxxxxxxxxx>, "Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote:
"Lorentz" <drosen0000@xxxxxxxxx> wrote in message
news:f103mg$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 do realize it's an argument about definitions and conventions (and
thus kind of pointless) but still.
Yes, pineal gland phototransduction cascade is basically identical
to retina. It is even likely that an eye and pineal gland have common
ancestor (e.g. light sensor in primitive chordates). Does it mean they
are homologous? Nope, I'd still contend only the analogy built
independently based on one common motif. Because it's a slippery
slope - once you've said an eye and pineal gland are the same thing,
next is an eye and a brain are, next a brain and nails are homologous
(after all, they both come from ectoderm) and the next you know,
everything is homologous to everything and "homologous" lost any
meaning.
An eye is an *organ*. And organ means a lot more than sharing
one common biochemical pathway (and using it differently).
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?
AFAIR, in cold-blooded animals the trigger is still light, just not
a lot of it. Also, there is such a thing as spontaneous (thermal)
rhodopsin isomerization, which is pretty substantial - on the order of
about 1e(-9) excited rhodopsins/sec (giving the rise to "dark
noise") - and that rate is highly dependent on lipid composition of
the membrane, providing for a plethora of regulatory possibilities.
I'd expect the pinopsin to be a noisy receptor with lower activation
threshold. Finally, pinealocytes' response is under tight control of
adrenergic system, so I can imagine there can be a feedback
reaching all the way down to the retinal isomerization.
DK
.
- References:
- Internal organs homologous across phyla?
- From: Paul Ciszek
- Re: Internal organs homologous across phyla?
- From: Lorentz
- Re: Internal organs homologous across phyla?
- From: Perplexed in Peoria
- Internal organs homologous across phyla?
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