During the development of many animal organs, mesenchymal cells co-ordinately polarize to form epithelial sheets or tubes. In vitro studies have suggested that the extracellular matrix component laminin functions as a polarity cue during this mesenchymal to epithelial transition. Here (p. 2050), Jeffrey Rasmussen and colleagues provide in vivo evidence for laminin’s involvement in polarization by studying the development of the C. elegans pharynx, an epithelial tube that forms from pharyngeal precursor cells (PPCs). The researchers show that cell fate regulators, including the transcription factor PHA-4, cause the PPCs to form a bilaterally symmetric, rectangular array of cells called the ‘double plate’. PPC polarization and apical PAR localisation begin in the double plate cells, which then undergo apical constriction to form a cylindrical cyst. Notably, laminin provides an essential cue that orients the apical localisation of the PAR-3 complex in the double plate but not in the developing C. elegans intestine. Thus, the researchers conclude, laminin is an early polarizing cue for some but not all epithelia.

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