The eye is a famous example of a supposedly irreducibly complex structure, due to its many elaborate and interlocking parts, seemingly all dependent upon one another.
It is frequently cited by intelligent design and creationism advocates as an example of irreducible complexity. In an often misquoted passage from On the Origin of Species, Charles Darwin appears to acknowledge the eye's development as a difficulty for his theory.
Fast forward to today and a stem-cell biologist has had an eye-opening success. Yoshiki Sasai of the RIKEN Center for Developmental Biology (CBD) in Kobe, Japan, has grown the precursor of a human eye in the lab.
The structure, called an optic cup, is 550 micrometres in diameter and contains multiple layers of retinal cells including photoreceptors. The achievement has raised hopes that doctors may one day be able to repair damaged eyes in the clinic. But for researchers at the annual meeting of the International Society for Stem Cell Research in Yokohama, Japan, where Sasai presented the findings this week, the most exciting thing is that the optic cup developed its structure without guidance from Sasai and his team.
“The morphology is the truly extraordinary thing,” says Austin Smith, director of the Centre for Stem Cell Research at the University of Cambridge, UK.
Until recently, stem-cell biologists had been able to grow stem-cells only into two-dimensional sheets. But over the past four years, Sasai has used mouse stem cells to grow well-organized, three-dimensional cerebral-cortex1, pituitary-gland2 and optic-cup3 tissue.
His latest result marks the first time that anyone has managed a similar feat using human stem cells.
“This resolves a long debate,” says Sasai, over whether the development of the optic cup is driven by internal or external cues.
The achievement could make a big difference in the clinic. Scientists have had increasing success in transplanting stem cells: last month, a group at University College London showed that a transplant of stem-cell derived photoreceptors could rescue vision in mice. But the transplant involved only rod-shaped receptors, not cone-shaped ones, and would leave the recipient seeing fuzzy images. Sasai’s organically layered structure offers hope that integrated photoreceptor tissue could one day be transplanted.