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RE: pineal gland

Michael Teuton wrote:

> The pineal gland is too deep a structure to receive photons directly
> in humans.  I doubt that it is in other animals, but this is just an
> educated guess.  Someone correct me if I'm wrong.  I just don't have
> time to research this right now.

Some of the research in the lab I'm currently affiliated with is
focussed on the pineal gland, so I'll comment.  It's true that it's
highly unlikely for there to be any direct response to light by the
pineal gland of humans, but many other animals have light sensitive
cells in the middle of their heads.  In squamates they are expressed
as a parietal eye visible on the top of the head (it looks like a
small scale, but it really is a tiny eye which subserves functions
different from those subserved by the "lateral" eyes -- other people
would just call the lateral eyes "eyes" :-).  In frogs it is expressed
as the frontal organ which isn't quite an eye but does respond
directly to light (as has been shown electrophysiologically).  The
frontal organ directly innervates the pineal gland.  Salamanders have
an apparent homologue of the frontal organ called the parapineal

In any case, there is a small but growing amount of evidence that
pineal photoreceptors are responsible not only for judging daylengths
etc. but also for detecting the Earth's magnetic field and possibly
polarized skylight -- both useful for navigation.  That's the focus of
the relevant studies in the lab I'm in now.

SFAIK, irrespective of those uses, most all animals (including
mammals) are at least indirectly sensitive to light in their pineal
glands (via retinal inputs through the suprachiasmatic nucleus when
others don't exist).  Pineal glands are used by mammals to determine
photoperiods in order to lock their reproductive cycles into the
seasons of the year.  I think I expressed this reservation when Betty
first brought the subject up two years ago -- I'm not sure it makes a
lot of sense to talk of pineal glands that aren't fully functional.
Pineal glands (and associated organs) may well do different things in
different animals, but what would a fully functional pineal gland be?
I encourage Betty (or anyone else) to keep looking into it, but keep
in mind that pineal glands may be like tetrapod forelimbs.  What sort
of forelimb would you consider to be "fully functional"?

Mickey Rowe     (mrowe@indiana.edu)