Why do hairs and cilia grow out in one particular direction?​

The Lawrence Group, has been trying to answer this question and has recently published a paper in Open Biology which explores and tests different models and hypotheses. 

Peter Lawrence explains that cells in developing animals detect their orientation and use this information to point cellular outgrowths such as hairs or cilia. This paper is part of a worldwide effort to study this phenomenon, known as planar cell polarity (PCP). Here we use the fruitfly to probe the mechanisms of PCP. We test and partially confirm a model that explains the orientations of larval denticles, some pointing backwards and some pointing forwards, in the segment. We also investigate a hypothesis of others that each cell uses the polarity of microtubules to orient itself; our new evidence does not support the hypothesis.

Pietra Stefano, Ng KangBo, Lawrence Peter A. and Casal José, 2020 Planar cell polarity in the larval epidermis of Drosophila and the role of microtubules. Open Biol.10200290, https://doi.org/10.1098/rsob.200290