Amacrine cells: key players in growth regulation in the eye

The development and maintenance of emmetropia is largely controlled locally by the retina. This is accomplished by detecting and signaling whether images are in or out of focus hyperopically or myopically. The amacrine cells are most likely the key players of growth regulation, said William Stell, PhD, MD.

"The amacrine cells play critical roles in the processing of focus-defocus information by which growth is controlled," said Dr. Stell, of the University of Calgary, Alberta, Canada. "Photoreceptor-horizontal-bipolar cell assemblies provide amacrine cells with the means to do this, but are unlikely to signal focusing or defocusing directly."

The roles of dopamine and acetylcholine in the signaling process of the amacrine cells have been studied extensively, but glucagon is considered the key candidate for the focus-defocus messenger in chick retinas. Glucagon, however, has not been demonstrated or eliminated as having that role in mammalian retinas.

"What is unknown is where glucagon acts," he said. There are at least two pathways through which glucagon acts: one direct and one indirect.

"The rate of axial elongation of the eye is regulated visually by feedback to optimize the vision," Dr. Stell said. "The mechanisms are in the retina. The amacrine cell-derived messenger must regulate scleral growth and choroidal thickness via the retinal pigment epithelium (RPE).

"However, the effects of the RPE and the RPE-derived messenger that act upon the choroids and sclera are not yet known," Dr. Stell concluded.