Apoptosis and homeostasis in the eye

Although the human eye is only a few centimeters in diameter, it contains an extraordinary array of cells and tissues, some of which are found in no other organ (Figure 16–1). The eye is an anatomical extension of the brain and processes an enormously complex array of information that provides us with our most precious sense – our vision. The retinal ganglion cells process more than 500 electrical signals per second, which is roughly equivalent to 10⁹bits of computer information. The conversion of photons of light that enter the eye to crisp visual images in the brain is orchestrated by a diverse array of cells and tissues with vastly different properties and functions. Apoptosis and apoptosis-like processes contribute to the embryonic development of the mammalian eye in the womb and the long-term function of the visual axis from the time of birth to death. The eye, like other components of the central nervous system, is composed of cells that have limited and sometimes no capacity for regeneration. As a result, immune-mediated inflammation can lead to blindness. However, the fluids that fill the eye contain an extraordinary variety of immunosuppressive and anti-inflammatory molecules that control inflammation produced by elements of the innate and adaptive immune systems. Among these eye-derived factors are molecules that induce apoptosis of inflammatory cells. In addition, antigens that enter the eye elicit a unique deviation of the immune response that actively suppresses antigen-specific immune responses such as delayed-type hypersensitivity (DTH), which nonspecifically damages innocent bystander cells within the eye. Interestingly, apoptosis is intimately involved in the induction of this ocular immune deviation.