Intraocular pressure-dependent and -independent phases of growth of the embryonic chick eye and cornea.

The pattern and relative rates of diametric growth of the avian eye and cornea are described throughout embryonic development. The effect of reduced intraocular pressure on eye and corneal diametric growth also was investigated. Between embryonic day 4 (E4) and 1 day posthatching, the eye undergoes two distinct phases of linear growth. The first phase (E4-10) is very rapid (1.193 mm/day). The second phase, after E10, is significantly slower (0.346 mm/day). By contrast, over the same developmental period, the cornea undergoes three distinct and sequential phases of linear growth. The second phase of corneal growth (E7-10) is the most rapid (0.429 mm/day) and separates two periods of slow growth (0.211 mm/day during E4-7 and 0.128 mm/day after E10). After the sustained release of intraocular pressure by intubation on E4, growth of both the eye and cornea is reduced significantly. Operated eyes grow at a rate of 0.356 mm/day (E4-10) and 0.155 mm/day (E10-16). Intubation reduces corneal growth to a single phase of 0.125 mm/day (E7-16). Thus, from E4-10 both the eye and cornea possess intrinsic growth potentials that are elevated significantly by intraocular pressure. After E10, the rate of growth of both the eye and the cornea is independent of intraocular pressure. Because both control and intubated eyes change their growth rate on E10, this transition also is independent of intraocular pressure. This contrasts with the cornea which, after intubation, shows no detectable variation in growth rate. Correlation of eye with corneal growth demonstrates an exponential relationship in the presence of intraocular pressure and an almost linear relationship after intubation.