Figure 6.
Dendritic branch points preferentially associate with PADREN1s, and multipolar and pyramidal cells have distinct PADREN lengths. *A*, *B*, Frequency histograms showing the relative position of branch points along PADREN1s (blue) and PADREN2s (green) at 8 DIV (*A*) and 10 DIV (*B*). Relative position was computed as the distance from the proximal PADREN edge to the branch point divided by the length of the PADREN containing the branch point. “Soma” icons confer proximal–distal orientation to shaft icons. Branch points significantly associate with PADREN1s (*p* = 1.80 × 10^{−6}, 8 DIV; *p* = 4.07 × 10^{−6}, 10 DIV; custom test; for details, see Materials and Methods). Branch positions form a bell-shaped distribution along PADREN1s, which mirrors a Gaussian distribution of immunostaining intensity along PADREN1s (*C*). Points represent average immunostaining as a function of relative position after standardization and normalization from six randomly selected PADREN1s that did not contain branch points. *D*, *E*, Examples of the association between PADREN1s and branch points in 10 DIV neurons. Scale bar, 20 μm. *F*, Scatter plot of average cellular PADREN2 length versus average cellular PADREN1 length for each of 78 10 DIV cells. Point style indicates arbor type classification (pyramidal, multipolar, or other plotted as triangles, crosses, and dots, respectively). The shafts of pyramidal cells were approximately twice as long as the shafts of multipolar cells (pyramidal, 202 ± 14 μm; multipolar, 110 ± 4 μm; *p* = 1.75 × 10^{−8}, *t* test). Showing PADREN dimensions and arbor types correspond, convex polygons enclosing pyramidal or multipolar cells delineate distinct PADREN dimension spaces. Both PADREN1s (*p* < 1 × 10^{−10}) and PADREN2s (*p* = 0.006) are longer in pyramidal cells than multipolar cells. The chance of uniformly distributed pyramidal and multipolar cell types falling into the PADREN space areas they occupy is *p* ≪ 0.005 in both cases (χ^{2} test, df = 1). Additionally, average PADREN1 lengths and average PADREN2 lengths are correlated (β_{1} = 0.50; *p*(β_{1} = 0) = 2 × 10^{−6}; simple linear regression; line not shown). *G*, Examples of arbor morphologies classified as pyramidal and multipolar. − det, Before the addition of detergent; + det, after the addition of detergent; AU, arbitrary units.