Figure 2. Postganglionic parasympathetic axons are associated with intercalated ducts.
(A–C) Confocal stack showing that the basket endings of parasympathetic axon terminals (red, see also Figure 1C) are centered on the intercalated ducts and spread only sparsely to the nearby acinar cells (gland parenchyma labeled green with lectin- see Figure 1B). The direction of the salivary flow within the intercalated duct is illustrated by arrows. (D–F) Evidence showing that parasympathetic axons predominantly innervate intercalated ducts. The entire luminal surface was stained (with anti-ZO1, cyan, D, arrow shows major duct branches) and traced (green, E). The distal luminal surface could be divided into thin and thick regions that corresponds to interstitial spaces between the acinar cells and regions enclosed by intercalated duct cells, respectively (arrows in F point to thin regions) (see Figure 1A). Duct tracing superimposed on the axonal terminal arbors (red, F) shows the selective association of parasympathetic axons with intercalated ducts. (G) Labeling of four different parasympathetic axonal terminal arbors with different fluorescent colors (using Brainbow AAV injection) shows that individual baskets of different axons segregate onto non-overlapping regions (yellow green, dark orange, orange and green asterisks). Some apparent overlap is actually segregated in the depth axis (arrow). Scale bars: (A-C and F–G) 20 µm. (D–E) 50 µm.