Cortico-Cortical Connectivity: Measurements on Distant Axonal Arbours of Pyramidal Cells in the Mouse Neocortex

The axons of pyramidal cells have usually both local collaterals in the vicinity of their dendritic tree and distant terminal ramifications. Via their distant axonal arbour pyramidal cells connect to other brain regions or—in the majority of cases—to other regions in the cortex. In earlier work, we have made estimates on the length of axonal arbours of individual pyramidal cells, including both local and distant parts [1]. In the present study wemeasured the length of distant terminal arbours of pyramidal cells stained by extracellular injections with the anterograde tracer biotinylated dextran amine (BDA). We scrutinized our BDA-material for isolated axonal arbours which were far away from the injection site. The ramifications of 10 such terminal arbours (7 ipsi-, 3 contralateral to the injection site) were traced under a microscope connected to a computer system (Eutektit), and the diameters of the axonal arbours and the total length of fibres of each arbour within the section were measured. The axonal arbours had diameters between 0.12 mm and 0.35 mm. The measured lengths within a section of a thickness of 0.05 mm ranged between 0.87 mm and 2.19 mm, with an average of 1.55 mm. These measurements have to be corrected for the fibres not contained in the section. Such a correction can be based on the observation that the terminal ramifications measured in this study appear to be distributed in a cone-like volume and on the assumption of a symmetrical spread outside the section. Preliminary results indicate that such a correction increases the values to an average length of about 9 to 11 mm (including the ascending main axon). This is compatible with estimates of the total length of axon produced by an average pyramidal cell in the mouse cortex (10–40 mm) [1]. The data will be compared to measurements in the literature on other species