Formation and distribution of chick lens proteins.

The agar diffusion technique of Ouchterlony was used to determine the time sequence in which the three main lens proteins arise during organogenesis and to examine the distribution of these proteins in the lenses of animals throughout the vertebrate series and in those tissues of the eye which possess the capacity to form a new lens upon removal of the original lens. Alpha crystallin is detected in the epithelial cells of the lens placode before the appearance of any other lens antigens. Since it is also present in the pigment layer of the retina and iris of 72hour-old embryos—that is, those tissues which have a capacity for regeneration of the lens— and is found in identical or partially identical form in the lenses of representative animals throughout the vertebrate series, it may be considered as an "ancient" lens protein which plays a key role in formation of the lens. Beta crystallin appears at the onset of differentiation and growth of the nuclear lens fibers and is characterized by the rapid formation of 4 closely related subfractions. Since it is found in identical or partially identical form only in the lenses of birds and reptiles, but not in those of mammals, amphibians, and fishes, it may be considered a specialized protein which in the course of evolution develops along divergent lines. Its highly differentiated form and rapid formation may make it a target for teratogenic factors, such as German measles virus, which causes cataract only when it acts during growth and differentiation of the nuclear lens -fibers—that is, the time of beta crystallin synthesis. Gamma crystallin—the last lens protein to arise during development—takes an intermediate position between alpha and beta crystallin in regard to its species-specific properties.