Centrioles and the basal bodies that organize cilia and flagella represent two versions of the same organelle. Based around a radially symmetric array of nine microtubule triplets, their structure has been picked apart by numerous groups for some forty years. Using transmission electron microscopy to analyse serial sections of isolated basal bodies from Chlamydomonas reinhardtii, Stefan Geimer and Michael Melkonian have now generated a high-resolution map of the organelle (see ). Surprisingly, they have identified a novel feature – the `acorn' – that had previously gone unnoticed. This filamentous structure is attached to the inner wall of the microtubule shaft at its distal end and is present in both the mature basal body and the nascent probasal body. It imposes inherent rotational asymmetry on the basal body. This is particularly important because the acorn might thus provide the basis for asymmetric attachment of additional structures during basal body maturation. As such it might ultimately define the direction of flagellar beating and underpin cellular asymmetry in general.
