Fig. 10.

PSM ablations suggest the notochord cannot segment in the absence of the surrounding somites. A. Schematic diagram showing the PSM ablation procedure.(Left= dorsal view; right= sagittal view of steps 1–4 of ablation procedure). B–D. WMISH for Tbx6 (a marker of the PSM) in HH11–12 embryos fixed immediately after ablation, demonstrating successful ablation of the PSM from a portion of notochord. C. Transverse sections confirm the complete absence of Tbx6-positive cells in the ablated region (black star=remnants of ink used for contrast during ablation procedure). D. Transverse section shows that the PSM posterior to the ablated region was unaffected. E–G. WMISH for paraxis (marker of the somites) in embryos cultured overnight after PSM ablation, showing that PSM ablation led to a complete absence of somites in the ablated region. F. Transverse section confirming the absence of paraxis-positive cells in the ablated region, with lateral plate mesoderm moving medially to lie adjacent to the notochord. G. Transverse section showing that somites posterior to the ablated region form normally. H–I. Haematoxylin-stained coronal sections through notochord of a six day old embryo, four days after PSM ablation. The length of the ablated region is shown in two sections. H. Section rostral to ablated region. Notochord shows segmental pattern of swellings and constrictions that coincide with dark stripes in surrounding sclerotome which correspond to the future AF region. I. Section spanning ablated region (left) and further caudally (right). The area between black arrowheads shows absence of sclerotome and the notochord shows no signs of segmentation. Immediately anterior and posterior to this, the notochord is surrounded by apparently unsegmented sclerotome. Caudally, normal segmented sclerotome is seen surrounding the notochord and the segmental pattern of swellings and constrictions in the notochord resumes. J–K. Schematic diagram corresponding to sections H-I. NC = notochord, NT = neural tube, US-Scl = unsegmented sclerotome, LPM=lateral plate mesoderm, Black stars = dorsal root ganglia, grey arrows = stripes of high cell density in sclerotome corresponding to future AF.