Figure 4. Pruning of postganglionic parasympathetic axonal arbors during postnatal development.
(A) Top left: A single P21 postganglionic axon (Thy1 YFP-H, yellow) superimposed on an image of the entire gland (grey). This region is magnified in the top right image. The axonal arbor occupies only a small area of the gland. Middle: A further magnified view shows the axon’s projection and its cell soma (lacking any dendrites, arrow). Bottom: The boxed region (yellow) shows that the arbor is composed of dense basket shaped terminal branches (*’s show three), plus a few unbranched non-basket endings (arrowhead). (B) Top: In contrast to the arbors at older ages, at P1 single axons innervate a larger proportion of the gland (yellow). Middle: Shown is the entire terminal arbor of the axon and it soma that is in this case located a far distance away on the salivary duct. Bottom: The boxed region shows a magnified view of part of the terminal arbor. Unlike older mice, the parasympathetic arbors at this age show no baskets but instead diffuse axon branches that cover considerable amounts of the gland’s territory (C–E): Quantitative analysis of axonal arbor size as a function of age in many samples collected during the first three weeks of postnatal life. The sample size required to acquire sufficient statistical power is based upon the initial observation that normalized axonal arbor size is about three times larger in neonates compared to that in adults. (C) The absolute axonal arbor size gradually decreased from P1 to P10, followed by a gradual increase to reach the adult sizes. (D) During the same period, the size of the gland grew progressively. To accommodate sample variations, the calculated gland size based on the fitted curve is used in subsequent analysis. (E) Normalizing axonal arbor size against the gland size indicates that the axonal arbor size relative to the gland size decreases from P1 to P10 and then stabilizes thereafter.