Tissue Polarity in the Retina

Abstract

In multicellular organisms, most tissues derived from epithelial cell sheets form highly organized structures that are not only polarized in the apicalbasolateral axis but also display a polarization within the plane of the epithelium (Fig. 1). The function of many organs or tissues requires this additional axis of polarity within the epithelium, namely a uniform polarity of single cells or multicellular units within the plane of the epithelium. This type of polarization of cells is usually referred to as epithelial planar polarity, planar cell polarity, or as mostly used in Drosophila,tissue polarity. Such polarization is evident in most epidermal structures (e.g., the ordered appearance of scales in fish or feathers in birds), in neuroepithelia (e.g., the inner ear epithelium, where the stereocilia bundles are aligned for normal sensitivity to sound) as well as in internal organs (e.g., in the oviduct, with the cilia allowing directional transport of an egg). Another very good example is the exocuticle in insects, as all the respective tissues are derived from the single cell layer epithelial imaginal discs (Adler 1992; Gubb 1993; Eaton 1997). Similarly, the Drosophila retina, as it is derived from a single cell layer epithelium, the eye imaginal disc, displays a tissue or planar polarity (Dietrich 1909). It is an intriguing problem how cells that are hundreds of cell diameters apart adopt the same polarity in the plane.