Morphometric analysis of granule cell dendrites in the mouse dentate gyrus.

We devised a computer program to analyze the dendritic geometry of dentate granule cells as seen in rapid Golgi impregnations from the mouse. Three dimensional coordinates were recorded by using a computer-assisted microscope. Geometric parameters are of two general types: (1) LINEAR parameters include the number of dendritic segments per branch order and their individual and aggregate lengths. (2) ANGULAR parameters define the spatial relationships of branch points and segment terminals with each other and with the axis of symmetry derived for all the dendrites. We find that values for linear parameters are highly variable and more susceptible to artifacts. Values for most angular parameters are more highly constrained and are presumably the best descriptors of the class-characteristic conical shape of granule cell dendrites. Additional features which are necessary to describe granule cell dendrites fully are: (1) Branching frequency is highest proximal to the cell soma, (2) deviant segments are kept "on course" to ensure axial symmetry, and (3) terminal segments end at the plane of the cortical surface. A critical analysis of the various parameters suggests the hypothesis that the characteristic and uniform geometry of granule cell dendrites is controlled largely by factors residing in the molecular layer where growth and differentiation are sustained. An additional finding of potential interest is that there are two subpopulations of granule cells with a twofold difference in spine density.