Vertebrate somites form via a sequential process of segmentation, which proceeds from anterior to posterior. In many species, anterior somites form more quickly than posterior ones, but whether this is functionally important, and how the switch in timing might be controlled, is unknown. Now (), Takaaki Matsui and co-workers address these issues in zebrafish, where the anterior four somites form every 20 minutes, while more posterior ones form every 30 minutes. They find that this difference in timing is not due to a change in how long a somite takes to form, but rather to the extent of overlap between segmentation periods. Mechanistically, retinoic acid signalling appears to be key to regulating the transition from fast-forming to slow-forming somites, acting via Ripply1 to regulate the clock gene her1. When RA signalling is impaired, there is a specific vertebral defect at the head-to-trunk linkage, indicating the functional importance of this anterior-posterior difference in the rate of somitogenesis.
