Development of the primate area of high acuity. 1. Use of finite element analysis models to identify mechanical variables affecting pit formation.

Most primate retinas have an area dedicated for high visual acuity called the fovea centralis. Little is known about specific mechanisms that drive development of this complex central retinal specialization. The primate area of high acuity (AHA) is characterized by the presence of a pit that displaces the inner retinal layers. Virtual engineering models were analyzed with finite element analysis (FEA) to identify mechanical mechanisms potentially critical for pit formation. Our hypothesis is that the pit emerges within the AHA because it contains an avascular zone (AZ). The absence of blood vessels makes the tissue within the AZ more elastic and malleable than the surrounding vascularized retina. Models evaluated the contribution to pit formation of varying elasticity ratios between the AZ and surrounding retina, AZ shape, and width. The separate and interactive effects of two mechanical variables, intraocular pressure (IOP) and ocular growth-induced retinal stretch, on pit formation were also evaluated. Either stretch or IOP alone produced a pit when applied to a FEA model having a highly elastic AZ surrounded by a less elastic region. Pit depth and width increased when the elasticity ratio increased, but a pit could not be generated in models lacking differential elasticity. IOP alone produced a deeper pit than did stretch alone and the deepest pit resulted from the combined effects of IOP and stretch. These models predict that the pit in the AHA is formed because an absence of vasculature makes the inner retinal tissue of the AZ very deformable. Once a differential elasticity gradient is established, pit formation can be driven by either IOP or ocular growth-induced retinal stretch.