APOPTOSIS IN MAMMALIAN EYE DEVELOPMENT - LENS MORPHOGENESIS, VASCULARREGRESSION AND IMMUNE PRIVILEGE

Formation of the mammalian eye requires a complex series of tissue int eractions that result in an organ of exquisite sensory capability, The early steps in eye development involve extensive cell death associate d with morphogenesis, Later, suppression of programmed cell death is e ssential for tissue differentiation and in the adult, the immune privi leged status of the eye is maintained in part through factors that ind uce inflammatory cell apoptosis, Experimental evidence suggests that s uppression of apoptosis in cells of the lens lineage by fibroblast gro wth factors is one component of their action during lens morphogenesis , Fibroblast growth factors are also required for normal lens fiber-ce ll differentiation, This includes a degenerative step for organelles t hat is presumably an adaptation for the clearance of light scattering elements from the optic axis. The process of organelle degeneration ma y be related to apoptosis in a few of its features. Actively-induced a poptosis becomes important for eye development as the temporary ocular vasculatures regress. This too, is presumably an adaptation far the d isposal of cells that would disturb the passage of light to the retina , Ocular macrophages appear to be essential for the induction of apopt osis in the endothelial cells comprising the ocular vasculatures. In t he adult, inflammatory cells entering the eye are exposed to the pro-a poptotic agents transforming growth factor-beta(2) 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 vers atile system for studying the role of programmed cell death in lens de velopment, vascular regression and immune privilege.