The lens organizes the anterior segment: Specification of neural crest cell differentiation in the avian eye

During the development of the anterior segment of the eye, neural crest mes enchyme cells migrate between the lens and the corneal epithelium. These ce lls contribute to the structures lining the anterior chamber: the corneal e ndothelium and stroma, iris stroma, and trabecular meshwork. In the present study, removal of the lens or replacement of the lens with a cellulose bea d led to the formation a disorganized aggregate of mesenchymal cells beneat h the corneal epithelium. No recognizable corneal endothelium, corneal stro ma, iris stroma, or anterior chamber was found in these eyes. When the lens was replaced immediately after removal, a disorganized mass of mesenchymal cells again formed beneath the corneal epithelium. However, 2 days after s urgery, the corneal endothelium and the anterior chamber formed adjacent to the lens. When the lens was removed and replaced such that only a portion of its anterior epithelial cells faced the cornea, mesenchyme cells adjacen t to the lens epithelium differentiated into corneal endothelium. Mesenchym e cells adjacent to lens fibers did not form an endothelial layer. The cell adhesion molecule, N-cadherin, is expressed by corneal endothelial cells. When the lens was removed the mesenchyme cells that accumulated beneath the corneal epithelium did not express N-cadherin. Replacement of the lens imm ediately after removal led to the formation of an endothelial layer that ex pressed N-cadherin. Implantation of lens epithelia from older embryos showe d that the lens epithelium maintained the ability to support the expression of N-cadherin and the formation of the corneal endothelium until E15. This ability was lost by E18. These studies provide evidence that N-cadherin ex pression and the formation of the corneal endothelium are regulated by sign als from the lens. N-cadherin may be important for the mesenchymal-to-epith elial transformation that accompanies the formation of the corneal endothel ium. (C) 2000 Academic Press.