Pii: S0168-9525(98)01584-4

steps during the development of multicellular eukaryotic organisms. As embryonic cells are incorporated into the blastoderm epithelia they begin to display the hallmark characteristics of apical–basal polarity, and during successive stages of development, cell polarity continues to play a significant role, influencing patterns of cell division, shape change and movement. Therefore, developmental biologists have actively sought to elucidate the genetic programs that regulate the specification and maintenance of cell polarity. Whereas much work has addressed the determination of apical–basal polarity in epithelia, relatively little is known about the specification of polarity orthogonal to the apical–basal axis, known as planar polarity, or tissue polarity. Nevertheless, planar polarity is integral to the function of many tissue-systems, ranging from the specialized hair cells of the vertebrate ear to the dynamic cilia of the tracheal and reproductive tract epithelia1. Here, we review recent progress in elucidating mechanisms of planar polarity specification, with special attention to the role played by the Frizzled (FZ) family of serpentine membrane receptors and its downstream signal transducers. Because the role of FZ signaling in regulating cell polarity has been studied first and most thoroughly in Drosophila, we have concentrated on this system. In light of previous excellent reviews1–4, our discussion is focused on the most recent developments and emphasizes the remaining conceptual hurdles for a coherent model of polarity signaling. Drosophila FZ was first identified many years ago for its role in regulating planar polarity5. Subsequently, a large number of FZ homologs, including Drosophila FZ2, have been found in numerous organisms, where they have been proposed to function as cognate receptors for WNT signaling molecules6,7. WNTs, of which murine REVIEWS