Apoptosis in mammalian eye development: lens morphogenesis, vascular regression and immune privilege

Formation of the mammalian eye requires a complex series of tissue interactions that result in an organ of exquisite sensory capability. The early steps in eye development involve extensive cell death associated with morphogenesis. Later, suppression of programmed cell death is essential for tissue differentiation and in the adult, the immune privileged status of the eye is maintained in part through factors that induce inflammatory cell apoptosis. Experimental evidence suggests that suppression of apoptosis in cells of the lens lineage by fibroblast growth factors is one component of their action during lens morphogenesis. Fibroblast growth factors are also required for normal lens fiber-cell differentiation. This includes a degenerative step for organelles that is presumably an adaptation for the clearance of light scattering elements from the optic axis. The process of organelle degeneration may be related to apoptosis in a few of its features. Actively-induced apoptosis becomes important for eye development as the temporary ocular vasculatures regress. This too, is presumably an adaptation for the disposal of cells that would disturb the passage of light to the retina. Ocular macrophages appear to be essential for the induction of apoptosis in the endothelial cells comprising the ocular vasculatures. In the adult, inflammatory cells entering the eye are exposed to the pro-apoptotic agents transforming growth factor-beta2 and Fas ligand. The expression of these molecules in the eye, and their action in killing inflammatory cells, has evolved as a means of preventing inflammation and subsequent loss of vision. Thus, the eye offers a unique and versatile system for studying the role of programmed cell death in lens development, vascular regression and immune privilege.