Reducing intraocular pressure by intubation elicits precocious development and innervation of the embryonic chick cornea.

Growth of the embryonic chick cornea was directly related to, and coordinated with, overall eye growth. During normal development, the size of the embryonic chick cornea increased in three linear phases of diametric growth. Corneal diameter increased at a rate of 216 microns per day between embryonic day 4 (E4) and E7, 511 microns per day between E7 and E10, and 144 microns per day from E10 until after hatching. After the sustained release of intraocular pressure by intubation on E4, corneal diametric growth was reduced to a single phase of 122 microns per day. After intubation on E4, the mesenchyme surrounding the developing cornea was substantially thicker and the neural crest-derived corneal endothelium was established earlier. The primary corneal stroma of the intubated eye swelled and was precociously populated by neural crest-derived corneal fibroblasts. Thus, the timing of arrival of neural crest cells in the anterior segment and their contribution to the cornea were determined by the growth rate of the eye. Although the diameter of the cornea was substantially reduced after intubation, it was more densely populated by fibroblasts, resulting in a cornea that was substantially thicker than the control by E14. Prospective corneal nerves normally extend into the cornea proper on E11, concomitant with a decrease in its diametric growth rate. After intubation on E4, the perilimbal nerve ring was virtually complete by E5 and numerous nerves had extended throughout the E8 cornea. By E16, the cornea from the intubated eye contained a very high density of nerve fibers, possibly reflecting its reduced size. These data suggest that the primary corneal stroma does not permit nerve fiber extension and demonstrate that the timing of nerve fiber extension into the secondary corneal stroma is specified by the rate of oppositional diametric growth of the cornea.