PECs simply by modification of culture conditions. A very high cell density in ascorbic acid-containing medium results in lens differentiation, while placing cells in the medium without both PTU and HUase but with ascorbic acid results in a rapid redifferentiation

Once a specialized cell has expressed its cell-type-specific genes, it and its daughter cells in general do not change to another cell type. However, a clear exception to this rule is observed in the process of lens regeneration in newt. New lens cells are transdifferentiated from pigmented epithelial cells (PECs) of dorsal margin of iris. Such dramatic change of phenotype from pigment cells to lens cells can also be observed in PECs of chickens and humans in certain culture conditions in vitro (for reviews; see Eguchi, 1979, 1986, 1988). To investigate how the cells stably expressing PEC phenotype change to the cells expressing lens-specific gene, we have tried to isolate PECand lens-specific genes and to analyze their expression in the process of lens transdifferentiation from PECs in vitro. Transdifferentiation of chicken PECs into lens cells is probably the least studied case owing to a unique culture system in which a massive, almost synchronous transition of cell types can be provoked by changing culture conditions (Itoh and Eguchi, 1986a,b). When PECs of 9-day-old chicken embryos were cultivated in the medium containing phenylthiourea (PTU) and testicular hyaluronidase (HUase), they actively proliferated in the absence of melanogenesis and eventually depigmented. These depigmented cells lose the property of contact inhibition of growth, do not evidently show the phenotypes of either PECs or lens cells, and have been designated as ‘dedifferentiated’ PECs (dePECs). Such dePECs readily differentiate to either lens cells or PECs simply by modification of culture conditions. A very high cell density in ascorbic acid-containing medium results in lens differentiation, while placing cells in the medium without both PTU and HUase but with ascorbic acid results in a rapid redifferentiation of PECs. Therefore, it is possible to obtain a large number of well-differentiated PECs, of cells in a dedifferentiated state (dePECs), of cells which transdifferentiate into lens cells (tLCs), or of redifferentiated PECs (rePECs) (Itoh and Eguchi, 1986b). Such cells are useful in the study of molecular events of the transdifferentiation patterns. In this study, the expression pattern of some representative genes specific to PEC and lens, and of proto-oncogenes will be described. Particular attention will be paid to the nature of dedifferentiated cells, which can differentiate in two directions and lose the property of contact inhibition of growth.