Development of spiny and aspiny neurons in the caudate nucleus of the dog during the first postnatal month

The adult and developmental morphology of spiny and aspiny neurons in the dog caudate nucleus was examined using the Golgi‐Kopsch technique. In the adult, three types each of spiny and aspiny neurons were identified based upon dendritic morphology and cell soma size. They corresponded in large part to those neurons described previously in the caudate nuclei of the rat, cat, and monkey. At birth, dendrites of spiny neurons possessed varicosities, filopodia, and thick proximal dendritic stumps—all characteristic of immaturity. Maturation of these processes involved the thinning of proximal dendrites, lengthening of dendritic shafts, and growth of dendritic spines. Although most of the dendritic maturation occurred during the first postnatal month, spine densities and dendritic lengths of spiny I neurons at 30 days were still less than those seen in the adult. Aspiny I neurons were also immature at birth but lacked the filopodia and thicker proximal dendrites that characterized immature spiny neurons. Aspiny dendritic development involved primarily the lengthening of dendritic processes; by 30 days the aspiny I neurons were indistinguishable from those seen in the adult. These results suggest that dendritic development of spiny I neurons may extend well past the end of the first postnatal month and that studies investigating functional development in the caudate nucleus should consider the relatively extended time period required for maturation of these primary synaptic sites.