What is the signal pathway for eye growth regulation?
          Studies in chick provide convincing evidence that the eye is not dependent on the brain for growth regulation; we have shown that eyes can emmetropize, even when the link with the brain via the optic nerve is severed although there is some indication that the brain is required for fine tuning. The hot news is that the expression of ZENK, an immediately early gene and a common marker for rapid cellular responses to external stimuli, increases and decreases with myopic and hyperopic defocus respectively in a subclass of retinal amacrine cells, the glucagonergic amacrine cells. ​

     The implication of this result is there are defocus selective pathways within the retina. How this selectivity property is conferred glucagonergic amacrine cells and how their signal is transferred to the two main effector tissues, the choroid and sclera, is yet another hot topic. The application of neurotoxins to selectively inactive subpopulations of retinal cells is one of the approaches being used to tease out the retinal circuitry involved.