Avian corneal nerves: co-distribution with collagen type IV and acquisition of substance P immunoreactivity.

Within the avian cornea collagen type IV is preferentially and characteristically localized to the epithelial and endothelial basement membranes. In the present paper, we demonstrate that collagen type IV also is present within the corneal stroma coincident with the development and distribution of corneal nerves indicating that intra-stromal fibers are associated with Schwann cells or an equivalent cell type. We also demonstrate intra-stromal fibers of collagen type IV orthogonal to the epithelial basement membrane. These novel structures are most prominent on the tenth day of development and become progressively less distinct until they are no longer detectable on the eighteenth day of development. Substance P immunoreactivity is prominently expressed by nerves innervating the epithelium. The first substance P immunoreactive nerves are detected on the twelfth day of development, concomitant with the initiation of epithelial innervation and not the extension of nerves through the stroma. Such nerve fibers become more numerous with progressive development and demonstrate extensive association with both basal and superficial epithelial cells. Thus, the avian cornea is richly supplied with substance P primary afferents. The expression of substance P immunoreactivity correlates directly with the initiation of innervation of the corneal epithelium.