In recent years there have been rapid advances in the growth and differentiation of mammalian cells in culture. This has led to increasing use of such in vitro systems in a wide variety of studies on fundamental aspects of cell structure and function, including normal growth and metabolism, mechanisms of differentiation and oncogenesis, mechanisms of protein and membrane synthesis and cell polarity. Recent advances in our ability to grow cells, including human cells, on permeable supports, to generate confluent cellular barriers with the morphological polarity corresponding to their in vivo...