The development and pattern of innervation of the avian cornea

Abstract The involvement of nerves in the development of the avian cornea is poorly understood, primarily because the demonstration of corneal nerves has proved to be elusive. In the present study, the development of corneal innervation is demonstrated by the application of a modified Bodian staining technique (J. Lewis, 1978, Zoon , 6, 175–179) . On the 6th day of embryonic development, numerous large fascicles of axons are observed arriving at the ventrotemporal aspect of the cornea, within the periocular mesenchyme. These fascicles subdivide into two distinct groups which migrate both ventrally and, more extensively, dorsally around the cornea. Progressive migration of nerve fascicles around the cornea occurs through the 7th and 8th days of development, and by the 10th day the cornea is ensheathed within a ring of nerves. Concomitant with ring formation, nerves are observed leaving the main nerve fascicles and migrating toward the cornea. Numerous nerve processes, which enter through the mid-stroma, are observed migrating toward the center of the 12th-day cornea. Innervation of the epithelium is detected on the 12th day, beginning at the periphery and increasing dramatically with development. Innervation of the epithelium is almost complete on the 16th day and penetration of nerves into the central stroma occurs on the 18th day of development. On the 16th day, the basal epithelial cells begin to demonstrate silver-staining properties. The levels of this staining increase with development, and in the hatchling the squamous cells demonstrate a characteristic silver-staining pattern. Innervation of the corneal endothelium is not observed. These results indicate that the avian cornea and its epithelium become innervated over the same developmental period in which the major transition from corneal opacity to transparency is achieved.