Fig. 4. Decreased sympathetic innervation to target organs is accompanied by abnormalities in axon extension and terminal axon branching in Egr3^-/- mice. (A) In the submandibular gland and sublingual gland (broken outline) from Egr3^+/+:DlZ⁺ mice, lacZ histochemistry revealed robust sympathetic innervation. (B) Framed area in A: sympathetic axons branched into the distal lobules of the glands (arrowheads). (A') In Egr3^-/-:DlZ⁺ glands, there was a relative decrease in sympathetic innervation, consistent with sympathetic neuron loss. (B') Framed area in A': there was less complex axon branching and numerous axons that failed to extend to the distal lobules of the glands (arrowheads). (C) In trachea from Egr3^+/+:DlZ⁺ mice, sympathetic innervation entered along the dorsal midline and branched circumferentially to innervate smooth muscle and submucosal glands (arrowheads). (C') In trachea from Egr3^-/-:DlZ⁺ mice, however, sympathetic axon branching was consistently decreased and the branching of remaining axons was markedly diminished. (D,D') Sympathetic axons entered the splenic parenchyma along the splenic arteries (black arrowhead) and Egr3^+/+:DlZ⁺ spleens (D) the axons branched extensively after entering the organ parenchyma (white arrowheads). By contrast, in Egr3^-/-:DlZ⁺ mice (D'), sympathetic axons branched poorly as they entered the parenchyma (white arrowheads). Global sympathetic innervation to the spleen could be observed in Egr3^+/+:DlZ⁺ spleens (D, right), which showed a diffuse blue reaction product (arrow) that was consistently diminished in Egr3^-/-:DlZ⁺ spleens (D', right) (arrows). (Representative results from three adult mice for each genotype and organ analyzed; scale bars: 250 µm.)