Overexpression of GAP-43 increases the size of mossy fiber terminal fields. (a) In G-Phos mice we observed an increment in the size of the IIPMF (here designated by its termination zone in CA3b, SO + SP) that was restricted to the rostral levels of the hippocampus (arrow) and a shift in the distribution of Timm’s reactivity along the CA3 pyramidal cells near the border of CA2 (boxed area in a). Quantitatively, the increment in Timm’s staining in G-Phos mice appeared to be due to a lengthening of the IIPMF, primarily within the rostral-septal CA3b (in b) and an outgrowth of the dorsolateral SL in the caudal-septal CA3a (in c). WT, wild type; G-Phos, transgenic GAP-43 overexpression mouse; SL, stratum lucidum.