Friday, February 15, 2008

7.22 One, two, or three eyes

(Yang Jian, the 3-eyed god-general)

In Homer's Odyssey, Book 9, Mr Odysseus ran into Mr Polyphemus, a cyclops. The one-eyed giant ate a few of Odysseus's crew before losing his eye to Mr O's trickery. In Chinese mythology, The Investiture of the Gods (封神演義), Chapter 97, Yang Jian (楊戩) was born with three eyes; the third, sitting vertically on the forehead, can see beyond space and time.

While it is possible for a vertebrate embryo to develop only one eye, the three-eyed variety probably does not exist because this will require a fundamental re-structure of the cerebrum into three 1/3 brains. If we want to venture a guess, General Yang's third eye was most likely either a birthmark or a nevus that looked like an eye.

Cyclopes result from a problem with the prosencephalon which normally divides into right and left brains. Failing so, only one eye forms in the place where the nose is supposed to be. Cyclopes are mostly stillborns, so it was highly unusual for Polyphemus to even grow up, but then his Dad probably intervened (as far as life-support).

Actually, a close inspection reveals a complicated origin for each tissue in the eye. For those who for some reason must know, here is a quick summary:

Neuroectoderm: the retina, epithelium of the ciliary body/iris, and optic nerve
Surface ectoderm: the lens and corneal epithelium
The surrounding mesenchyme (neural crest origin): the sclera, the remainder of the cornea, the choroid, ciliary body/iris, and blood vessels

In the vertebrates, the development of the eye is through several invagination processes with precise timing. Any insults from microbes or toxins can of course cause mutations and congenital eye diseases. (See previous posts under Pediatrics.)

Here is something new and interesting: The development of the lens maybe controlled by the Six3 gene which in turn regulates Pax6, the master gene for eye development. The presence of gene product of Six3 does precede that of Pax6. In fact, problem with Six3 leads to holoprosencephaly. And the worse case? Yes, Mr Polyphemus the Cyclops and a kitten named Cy (see image below).

(Cy the kitten, the only living cyclops ever, even if for only one day - from

So Six3, in effect, initiates the whole cascade of gene activation in the development of the eye. We wonder why Poseidon did not see this coming.

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