Fig. 1. Sema3A and Nrp1 coexpression in spinal motoneurons. (A) In situ hybridization of Sema3A mRNA on brachial and thoracic transverse sections of chick spinal cord from HH18-26 embryos. Lim3 and Islet1/2 immunostainings performed on adjacent sections are provided to show that Sema3A expression coincides with the medial motor column (MMCm: black arrowhead) and not with the lateral motor column (LMC: white arrowhead) at HH24. In situ hybridization and immunostaining of Nrp1 (NP1) show that both MMCm (black arrowhead) and LMC (white arrowhead) neurons express Nrp1 (NP1). (B) Schematic of the Sema3A expression pattern (gray) at HH24 and HH26 in spinal motoneurons and in peripheral tissues, in parallel with motor projections at the brachial level. sc, spinal cord; dm, dermamyotome; drg, dorsal root ganglion; nt, notochord; ao, aorta. Sema3A expression is observed at HH24 in MMCm neurons forming the dorsal ramus, but not in LMC neurons extending to the ventral ramus, and is then expanded at HH25-26 in most motoneurons. Scale bars: 100 µm.