After several days in the fridge, an otherwise fresh and unopened
(cellophane wrapped) head of iceberg lettuce started to turn
brown on the interior leaves.   Is it still OK to eat?

The outside leaves were still green and OK but I usually toss
the top two leaves anyway.  The head was still firm and crisp.
The brown color is NOT fungus or on the surface of the leaves.
It is "in" the leaves, kind of along the veins (brown streaks,
not big brown blotches) and mostly on the white or light green
portion.  The darker green portion of the leaves have much
less of the brown stuff.  All of the leaves are still nice and
crisp.

Anybody what is turning brown?  And what the brown is?  (Not
much sugar in lettuce, so I doubt that it's colorless sugars
turning brown / caramelizing.)

THANK YOU.