Sequential structural response of lens epithelium to retina-conditioned medium.

When lens epithelium is cultured with retina-conditioned medium, many of the cells undergo fibre differentiation within 6-10 days. Here we report the temporal sequence of structural events that characterize this change in an organ culture model system. Within an hour of exposure to conditioned medium, some cells withdrew from the epithelial monolayer and began migrating over stationary cells still attached to the capsule. By 24 hr, migratory activity was largely responsible for causing the explant to become multilayered, increasing its thickness while at the same time reducing its surface area. Control cells attached to each other along their lateral boundaries through interdigitations of microvilli. After BRCM treatment, microvilli flattened out and the membranes had a crinkled appearance. Eventually, as the cultured cells developed into fibres, membranes straightened and developed knob and socket junctions and large numbers of their organelles underwent degradation within autophagic vacuoles. Nucleoli began to enlarge by 16 hr and by the time cells had been exposed for 24 hr, some nucleoli were enlarged to seven-fold their original area, as measured on electron micrographs. This nucleolar change was followed over the next few days by a gradual increase in cytoplasmic protein, and cells became plump or elongated. The ultrastructural changes that we observed in culture are similar to those that can be seen in the intact lens. Such fidelity of change indicates that this cultured lens explant system is an excellent model for experimental intervention and analysis of the processes involved in terminal lens fibre differentiation.