Specificity of corneal nerve positions during embryogenesis

Embryonic corneal innervation first involves pericorneal nerve ring formation, with nerves in specific positions, followed by innervation of the corneal stroma from the ring. Here we determine whether nerve bundles enter the cornea at specific locations along the ring and whether bundles enter the cornea at specific depths.

Whole mount embryonic quail corneas immunostained for nerves were scanned using confocal laser microscopy. Images were superimposed digitally in pseudo-colored pairs to detect similar positions of innervation, and then rotated stepwise to determine if degree of synchrony was decreased. Degrees of innervation of each corneal quadrant were quantified. Depths of stromal bundles innervating the cornea were determined by depth of focus analysis.

Superimposition of images indicated many nerve entry points in similar locations, suggesting specificity. However, stepwise rotations of one image above the other revealed that degree of positional synchrony remained constant, suggesting that nerves do not occur in specific locations, but rather simply at approximately equal distances around the cornea. Corneas from both left and right sides are innervated by similar numbers of nerve bundles (Left, 44+/-0.4; Right, 44+/-1.0), with the same number/quadrant (Left, 11+/-0.2; Right, 11+/-0.2). Nerves entering the stroma closest to Descemet's layer innervate either the entire cornea along that radius or only the central-peripheral and central cornea; those entering nearer Bowman's layer innervate only peripheral cornea.

Avian corneal nerve bundles enter along radii spaced at equal intervals along the pericorneal nerve ring, suggesting an innervation mechanism based on equal spacing between nerves. Nerve bundles from the nerve ring enter the stroma at depths correlated with their subsequent targets.