Tag Archives: vision

Growing Eyes in the Lab

[div class=attrib]From Nature:[end-div]

A stem-cell biologist has had an eye-opening success in his latest effort to mimic mammalian organ development in vitro. 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 embryonic stem-cells only into two-dimensional sheets. But over the past four years, Sasai has used mouse embryonic 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 cells.

Familiar patterns
The various parts of the human optic cup grew in mostly the same order as those in the mouse optic cup. This reconfirms a biological lesson: the cues for this complex formation come from inside the cell, rather than relying on external triggers.

In Sasai’s experiment, retinal precursor cells spontaneously formed a ball of epithelial tissue cells and then bulged outwards to form a bubble called an eye vesicle. That pliable structure then folded back on itself to form a pouch, creating the optic cup with an outer wall (the retinal epithelium) and an inner wall comprising layers of retinal cells including photoreceptors, bipolar cells and ganglion cells. “This resolves a long debate,” says Sasai, over whether the development of the optic cup is driven by internal or external cues.

There were some subtle differences in the timing of the developmental processes of the human and mouse optic cups. But the biggest difference was the size: the human optic cup had more than twice the diameter and ten times the volume of that of the mouse. “It’s large and thick,” says Sasai. The ratios, similar to those seen in development of the structure in vivo, are significant. “The fact that size is cell-intrinsic is tremendously interesting,” says Martin Pera, a stem-cell biologist at the University of Southern California, Los Angeles.

[div class=attrib]Read the entire article after the jump.[end-div]

[div class=attrib]Image courtesy of Discover Magazine.[end-div]

The Movies in Our Eyes

[div class=attrib]From Scientific American:[end-div]

The retina processes information much morethan anyone has ever imagined, sending a dozen different movies to the brain.

We take our astonishing visual capabilities so much for granted that few of us ever stop to consider how we actually see. For decades, scientists have likened our visual-processing machinery to a television camera: the eye’s lens focuses incoming light onto an array of photoreceptors in the retina. These light detectors magically convert those photons into electrical signals that are sent along the optic nerve to the brain for processing. But recent experiments by the two of us and others indicate that this analogy is inadequate. The retina actually performs a significant amount of preprocessing right inside the eye and then sends a series of partial representations to the brain for interpretation.

We came to this surprising conclusion after investigating the retinas of rabbits, which are remarkably similar to those in humans. (Our work with salamanders has led to similar results.) The retina, it appears, is a tiny crescent of brain matter that has been brought out to the periphery to gain more direct access to the world. How does the retina construct the representations it sends? What do they “look” like when they reach the brain’s visual centers? How do they convey the vast richness of the real world? Do they impart meaning, helping the brain to analyze a scene? These are just some of the compelling questions the work has begun to answer.

[div class=attrib]More from theSource here.[end-div]