LAUSR

(mammals). Narrowing of the midline is a universal feature of amniotes but the mechanisms are unknown. Our objectives were 1) to use the chicken embryo to tease out the extrinsic versus intrinsic factors and 2) to investigate the role of planar cell polarity (PCP) in mesenchymal cell behaviors. In-vivo results: Heads were scanned with an Optical Projection Scanner, the frontonasal mass was segmented out, landmarks applied and 3D geometric morphometrics carried out using MorphoJ. We found that the frontonasal mass (equivalent to the medial nasal prominences) narrows medio-laterally (ML) while increasing dorsoventrally (DV) and craniocaudally (CrCa). The most rapid changes occurred during a 12 hour period between stage 28 and 29. In vitro results: Faces were dissected and placed into organ culture with or without the eyes and forebrain. Another set was treated with a ROCK inhibitor. Significant narrowing occurred over 48h, independent of the eyes/brain. narrowing was blocked by the ROCKi. Time-lapse tracking of mesenchymal nuclei over 6h showed intercalation of cells in the CrCa axis. The ROCKi prevented cells from intercalating which correlates with the flattening observed in organ cultures. Conclusions. The data demonstrate that intrinsic mechanisms lead to convergent extension (CE) of the frontonasal mass (narrowing in one axis with growth occurring in the perpendicular axes). CE is linked to intercalation of cells in the CrCa axis. The molecular pathway is likely to involve planar cell polarity signaling via small GTPases. Live cell imaging techniques permit the measurement of cell behaviours in growing facial mesenchyme. Supported by CIHR doctoral award to AD (RN 0000183232) and CIHR operating grant to JMR (MOP-123536).