Centrioles are arguably the most enigmatic cell organelles. In most animal cells, a pair of centrioles resides in the centrosome — a macromolecular complex that organizes the microtubule system (part of the cell's internal skeleton). The two centrioles, a mother and a daughter, “are positioned at right angles to each other”. This quote from a popular textbook epitomizes the idea of a fixed arrangement of the two bodies within the centrosome (see part a in the figure: at the top is an end-on view of one of the centrioles, while at the bottom is a longitudinal view of the other centriole). Deviations from a close apposition are considered exceptional.

figure 1

Figure 1

Not so, say Matthieu Piel and colleagues (J. Cell Biol. 149, 317–330; 2000). They have been following the behaviour of centrioles throughout the cell cycle (the time between successive cell divisions) in living cells. Centrioles are difficult to see by standard light microscopy, so Piel et al. made use of the fluorescent tag green fluorescent protein (GFP). They fused GFP to centrin, a small calcium-binding protein present in the lumen of the centriole. Fluorescence — a marker of the amount of GFP-tagged centrin — is always more intense in the mother centriole. So, the intensity of fluorescence seems to be indicative of the maturity of a centriole. This allowed the authors to show that a daughter centriole remains ‘immature’ until it duplicates and itself becomes a mother. The observation also indicates that one centriole may not be able to replace the other functionally, but this needs to be confirmed.

The most striking observation, however, is that the two centrioles exhibit dramatically different behaviour. For several hours after cell division, the daughter centriole roams extensively through the body of the cell, separating from the mother by many micrometres. Motility gradually subsides at the time of centriole duplication, the start of which coincides with the beginning of DNA replication in the nucleus. But these movements also take place — and are more pronounced — in the absence of nuclei. An example of these movements over a period of 20 minutes is shown in b and c of the figure. While the daughter (blue) runs free, the mother centriole (red) resides in the centre of the microtubule array ( d in the figure).

Splitting of the centriole pair has previously been observed under certain experimental conditions or in specialized cell types, but what the work of Piel et al. implies is that centriolar excursions are a normal event during a considerable portion of the cell cycle. Why one centriole should roam the cell after completion of cell division is unknown, as is the mechanism underpinning these movements. But the observations shake the myth of a stereotypical, right-angled orientation of centrioles, and suggest that these curious organelles might have some hitherto unsuspected functions associated with their wanderings.