Injecting horseradish peroxidase (HRP) into area 17 of tree shrews, Rockland and Lund (1982 Science 215 1532 – 1533) found connection-specific columnar staining superimposed on a Gaussian decrease of staining intensity with distance from the injection site. Such data are important when constructing models of neuronal nets. We have investigated in more detail the staining of different types of neurons after HRP injection into one orientation column of cat striate cortex. The intensity of staining was classified as dark, intermediate, or light. Cells with different staining intensities were found in all cortical layers and at different distances from the injection site of HRP. According to the shape of their somata and the organisation of their dendritic trees, the labelled cells were identified as pyramidal, small pyramidal, non pyramidal, or modified pyramidal (inverted and fusiform). The ratio of dark : light : intermediate cells within all identified morphological types was 2 : 1 : 1, except for the inverted pyramidal cells (1 : 1 : 1). Thus the intensity of staining depends not only on the physical uptake and transportation of HRP but also on the organisation of axonal terminals and neuron location in the functional structure of the cortex.
