As described previously, the pattern of dendritic branching depends on the initial contact of filopodia with afferent axons. The hypothesis predicts that regions with numerous prospective presynaptic terminals will attract more growing dendrites. Researchers have used the developing mouse spinal cord to test this hypothesis. A computer-assisted three-dimensional reconstruction system has been used with Golgi's method preparations of mouse spinal cords. The relative dendritic lengths and densities at various zones in the spinal cord indicate that dendritic growth is initially primarily towards the marginal zone (because of synaptogenic presynaptic terminals). However, this biased distribution is lost as synapses form in the intermediate zone. This study is consistent with predictions of the synaptotropic hypothesis of dendritic branching.