In the Drosophila eye, ommatidia must align correctly. Here, polarity has to be established in tissue planes rather than across the organism.
Cell polarity is the asymmetrical organisation of components within a cell. Polarity has to be established in planes of tissues as well as across whole organisms. This is key for correct tissue organisation during development.
One example of planar polarity is in the Drosophila eye. Ommatidia, the 800 individual units of the fly eye, must point one way in the dorsal (upper) half and the opposite direction in the ventral (lower) half.
The line of segregation between the two halves of the eye is controlled by the expression of the protein Mirr. Mirr is a protein involved in cell-to-cell adhesion. If it is not expressed correctly, ommatidia will not align correctly in the plane.
This image was made in response to research by Helen McNeil.