Lens development: III. Relationship between the growth of the lens and the growth of the outer eye coat

Abstract

This investigation explored the relationships which exist between the growth of the lens and the growth of the outer eye coat (cornea plus sclera) during the development of the chick embryo eye. By introducing one end of a minute glass drainage tube through the eye wall at 4 days of incubation, vitreous substance was allowed to escape as it formed. Intubated eyes increased in overall size at rates which ranged from normal to very slow. Consequently, by 16 days of incubation the group of intubated eyes had achieved widely differing sizes, despite the fact that they were all of the same chronological age. In the first of two experiments, the total protein nitrogen per eye wall in the intubated eyes was proportional to the size which the eye had achieved by 16 days of incubation. This indicates that the smaller size of the outer eye coat reflected a depressed growth rate, and not simply a different spatial distribution of the same amount of substance, Furthermore, within the outer eye coat the growth rates of both the cellular portion, which was measured as total deoxyribonucleic acid per outer eye wall, and the extracellular portion, which was measured as hydroxyproline present in collagen per eye wall, were depressed. However, the amount of extracellular substance produced per cell (estimated as collagen/DNA) was constant regardless of the rate of growth of the outer eye wall. These results suggest that the enlarging vitreous body regulated the growth of the outer coat of the eye by controlling the number of cells in this layer rather than by controlling the amount of extracellular matrix produced by each cell. In a second experiment, similar to the first, the increase in size and the change in shape of lenses in intubated eyes were measured as a function of age. Such lenses grew normally and maintained normal shapes as development proceeded. Thus the growth of the lens was uncoupled from the growth of the outer eye coat. While the outer coat of the eye did not control the growth of the lens to harmonize with its own growth, the lens appeared to exert an indirect control over the growth of the outer coat of the eye in the following manner. Previous work had established that the lens must be present for vitreous substance to accumulate. The enlarging vitreous body, in its turn, helped control the growth of the outer coat of the eye.