Sema7A is enriched in CA2 hippocampal neurons. In situ hybridization for Sema7A and plexinC1 on consecutive coronal sections of the hippocampus. Dorsal is to the top. (A, B) At E19, Sema7A is expressed in the dentate gyrus (DG) and CA1 region, whereas plexinC1 expression is restricted to the CA1 field. Sema7A and plexinC1 are also detected in the habenula (Hb) and thalamus (Th). (C-F) As development progresses, both Sema7A and plexinC1 can be detected in the DG, CA1 and CA3 fields. (E-I) Note that in the adult Sema7A signals are higher in the CA2 as compared to the adjacent CA1 and CA3 regions. Panels G and I are higher magnifications of the CA2 region in E and F, respectively. H shows a Nissl stain of the CA2 region (indicated by arrows in G-I) revealing its characteristic cellular organization. Small arrows in G indicate CA2 neurons, arrowheads indicate CA3 neurons. Note that CA2 neurons show a stronger Sema7A expression as compared to adjacent CA3 neurons. (J, K) CA2 neurons receive a particular prominent input from several subdivisions of the mammillary nucleus several of which display prominent Sema7A and plexinC1 labeling including the supramammillary nucleus (SuMM). 7A, Sema7A; C1, plexinC1; LM, lateral mammillary nucleus; MM, medial mammillary nucleus; Sb, subiculum. Scale bar 120 μm (A, B), 775 μm (C, D), 900 μm (E, F), 80 μm (G-I), and 420 μm (J, K).