Modulation of the shape of epithelial lens cells in vitro directed by a retinal extract factor. A model of interconversions and the role of actin filaments and fibronectin.

We have shown previously [1] that bovine epithelial lens cells can be stimulated to divide and elongate by a retinal extract (RE). In this report we show that the morphological response to the stimulatory factor is directly related to the target-cell shape, and we describe how the cell shape can be modulated into morphologically different types. If the cells are grown continuously from the explant in the presence of the RE factor, they keep a typical regular pavement-like epithelial shape (type I), even after serial passages. If the same cells are cultured in the absence of the factor, they become extremely irregular in shape and enlarge enormously (type II), and during serial passage elongate spontaneously to a fibroblast-like pattern. However, when type II cells are stimulated by RE, they elongate dramatically into type III cells as described in [1], provided they are stimulated at the optimal cell density. We show that the transformation of one type to another is directly under the control of RE, and we demonstrate that the changes in cell morphology are accompanied by alterations in cytoplasmic actin filaments. Type I cells contain few microfilaments, while type II cells display actin-tropomyosin polygonal fibre networks that reform during conversion to type III cells and then to elongated stress fibres. The change from type I to type II cells is also accompanied by massive accumulation of surface-associated fibronectin. We conclude that factors obtained directly from the eye have a direct ability to control morphology and proliferation of ocular cells like lens cells perhaps by modulation of cellular adhesiveness mediated by surface fibronectin and reorganization of cytoplasmic actin-based filaments.