Abstract
Purpose: :
In order to understand the contribution of intraocular pressure (IOP) to primate fovea formation, the maturation of the ciliary body, trabecular meshwork and the canal of Schlemm were related to the onset of pit formation. In addition, the temporal relationship between changes in pit depth and the changing axial positions of inner retinal vessels across the retina was explored.
Methods: :
Measurements were made in paraffin sections through macaque retinas varying in age from 64 days postconception (dpc) to 11 years. Pit depth and foveal laminar deformations were measured in sections dewarped with finite element analysis. Inner nuclear (INL) deformations associated with overlying vessels were quantified in sections.
Results: :
A shallow pit appears at 115 dpc. Neither the canal of Schlemm nor the trabecular meshwork is apparent at, or before, 115 dpc, but the ciliary body was more mature at 115 dpc than it was at 64 dpc. Aqueous secretion may begin before the canal of Schlemm drainage system is mature suggesting that IOP may ramp up slowly. An adult–like trabecular meshwork and canal of Schlemm appear after birth and after the foveal pit reaches maximum depth. Many blood vessels across the retina, which are initially on the inner retinal surface, are progressively displaced outwardly into the ganglion cell layer (GCL) and the vessels also deform the INL. The number of such deformations increases slowly from 73–105 dpc and sharply increases thereafter until 164 dpc. The increasing number of these INL deformations coincides with the pit deepening. These data suggest that both the pit and the vascular–induced INL deformations are caused by IOP, but for different reasons. Vessels are more rigid than the retina and are therefore pushed into the retina by IOP. A pit forms because the avascular, foveal, retina is more deformable than the surrounding vascularized retina. Sections dewarped with finite element analysis revealed that the GCL and INL overlying the pit center bowed outwardly, toward the sclera, as the pit deepened. This outward deformation occurred first in the GCL and then in the INL, consistent with a force, IOP, progressively pushing the inner retina at the pit outwardly.
Conclusions: :
Vasculature on the inner retinal surface and the GCL and INL at the fovea all appear to be pushed simultaneously, outwardly, towards the sclera. These events are, therefore, coincident with formation of the foveal pit and occur while the main aqueous drainage system is immature. Outward force generated by IOP is the most parsimonious explanation for these events.
Keywords: anatomy • development • anterior chamber