Cells are polarised in the plane of the sheet and this shows when they make oriented structures such as cilia or hairs.  The literature in this small field is so inconclusive and complex that most are put off from reading it.

Dr Peter Lawrence and Dr José Casal have written a "primer" to explain the elegant and simple mechanism that evolution has produced to polarise cells. They explain their hypothesis that polarity is cued by the direction of slope of a multicellular gradient and how each cell becomes molecularly polarised. The mechanism involves a comparison between each cell and its neighbours.

Read more in their new paper: Planar cell polarity: two genetic systems use one mechanism to read gradients. Peter A. Lawrence, José Casal,Development 2018, 145:dev168229 doi: 10.1242/dev.168229