Initiation of corneal differentiation prior to cornea-lens association

Summary

By day 3 of development, embryonic chick corneal epithelium is actively producing much of the extracellular matrix (ECM) which contributes to the formation of the primary corneal stroma. While evidence suggests that the lens influences subsequent development of the cornea, the role of the lens in initiating primary stroma production by corneal epithelium remains unclear. Therefore, corneal development was studied from 48 h of incubation, when corneal epithelial cells lie circumferentially adjacent to the lens placode, until 65 h of development, when the epithelium nearly covers the newly invaginated lens. Examination of the ECM underlying corneal epithelium at 60h of development with the transmission electron microscope reveals 5–6 layers of finely fibrillar striated collagen (10–15 nm diameter) and unstriated fibrillar material embedded in clusters of electron dense granules (11–14nm diameter). The granules stain positively with ruthenium red and are sensitive to hyaluronidases, enzymes specific for glycosaminoglycans. This pattern of epithelial ECM deposition extends from the attachment stalk of the invaginating lens radially to a region just distal to the outer edge of the optic cup. Here the ECM under the surface ectoderm (overlying head mesenchyme) has only 2–3 plies of collagen and fewer evenly dispersed dense granules. Apparently, the corneal epithelium has already begun production of ECM prior to complete lens detachment in areas considerably distal to the underlying lens. In fact, even at 48h of development, the corneal epithelium accumulates subjacent ECM as it lies next to the margin of the optic cup, long before its supposed “inductive” tissue interaction with lens.

This work was supported in part by a seed grant from the University of Southern California No. RR-05356 and by NIH Grant DE 04613