THE DEVELOPMENT AND PATTERN OF INNERVATION OF THE AVIAN CORNEA

The involvement of nerves in the development of the avian cornea is poorly understood, primarily because the demonstration of corneal nerves has been elusive. The development of corneal innervation was demonstrated by the application of a modified Bodian staining technique. On the 6th day of embryonic development, numerous large fascicles of axons were observed arriving at the ventrotemporal aspect of the cornea, within the periocular mesenchyme. These fascicles subdivided into 2 distinct groups which migrated both ventrally and, more extensively, dorsally around the cornea. Progressive migration of nerve fascicles around the cornea occurred through the 7th and 8th days of development and by the 10th day the cornea was ensheathed within a ring of nerves. Concomitant with ring formation, nerves were observed leaving the main nerve fascicles and migrating toward the cornea. Numerous nerve processes, which enter through the midstroma, were observed migrating toward the center of the 12th-day cornea. Innervation of the epithelium was detected on the 12th day, beginning at the periphery and increasing dramatically with development. Innervation of the epithelium was almost complete on the 16th day and penetration of nerves into the central stroma occurred on the 18th day of development. On the 16th day, the basal epithelial cells began to demonstrate Ag-staining properties. The levels of this staining increased with development and in the hatchling the squamous cells demonstrated a characteristic Ag-staining pattern. Innervation of the corneal endothelium was not observed. The avian cornea and its epithelium apparently become innervated over the same developmental period in which the major transition from corneal opacity to transparency is achieved.